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WHO global
air quality
guidelines
Particulate matter (PM2.5 and PM10),
ozone, nitrogen dioxide, sulfur dioxide
and carbon monoxide
WHO global
air quality
guidelines
Particulate matter (PM2.5 and PM10),
ozone, nitrogen dioxide, sulfur dioxide
and carbon monoxide
WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide,
sulfur dioxide and carbon monoxide.
ISBN 978-92-4-003422-8 (electronic version)
ISBN 978-92-4-003421-1 (print version)
© World Health Organization 2021
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Cover image: Pellegrini
Contents
Foreword…………………………………………………………………………………………………………………..v
Acknowledgements………………………………………………………………………………………………….. vi
Glossary…………………………………………………………………………………………………………………… ix
Abbreviations………………………………………………………………………………………………………….. xii
Executive summary………………………………………………………………………………………………… xiv
1. Introduction………………………………………………………………………………………………….. 1
1.1 Objectives of the guidelines……………………………………………………………………………3
1.2 Target audience…………………………………………………………………………………………….4
1.3 Background and rationale for updated guidelines……………………………………………4
1.4 WHO guidelines relating to air quality…………………………………………………………… 21
2. Guideline development process ……………………………………………………………………25
2.1 Introduction………………………………………………………………………………………………..26
2.2 Groups involved in and general procedures of guideline development…………..27
2.3 Determining the scope of the guidelines and formulation of review questions…….. 30
2.4 Systematic review of the evidence…………………………………………………………….. 49
2.5 From evidence to recommendations…………………………………………………………….58
3. Recommendations on classical air pollutants …………………………………………………73
3.1 Introduction………………………………………………………………………………………………..74
3.2 PM2.5………………………………………………………………………………………………………….74
3.3 PM10…………………………………………………………………………………………………………..89
3.4 Ozone………………………………………………………………………………………………………..97
3.5 Nitrogen dioxide………………………………………………………………………………………… 111
3.6 Sulfur dioxide…………………………………………………………………………………………… 125
3.7 Carbon monoxide………………………………………………………………………………………130
3.8 Summary of recommended air quality guideline levels and interim targets…..134
3.9 Supporting burden of disease calculations…………………………………………………139
4. Good practice statements about other PM types……………………………………………145
4.1 Introduction ………………………………………………………………………………………………146
4.2 Black carbon/elemental carbon………………………………………………………………….146
4.3 Ultrafine particles…………………………………………………………………………………….. 151
4.4 Sand and dust storms……………………………………………………………………………….154
iii
5. Dissemination of the guidelines ………………………………………………………………….. 167
5.1 Tools and approaches to raise awareness of the guidelines…………………………168
5.2 Risk communication…………………………………………………………………………………..169
5.3 Advocacy and engagement of stakeholders………………………………………………. 170
6. Implementation of the guidelines ……………………………………………………………….. 173
6.1 Significance of the guidelines: an evidence-informed decision support tool… 174
6.2 Assessment of national needs and capacity-building…………………………………. 176
6.3 Moving from guidelines to air quality standards…………………………………………. 177
6.4 Air quality management…………………………………………………………………………….182
6.5 Methodological guidance for health risk assessment of air pollution……………186
6.6 Role of the health sector……………………………………………………………………………189
6.7 Intersectoral and multistakeholder cooperation………………………………………….190
7. Monitoring and evaluation of the guidelines………………………………………………….. 195
7.1 Tracking the implementation of the guidelines……………………………………………..196
7.2 Assessing population exposure to ambient pollution……………………………………196
7.3 Health benefits from implementation of the guidelines……………………………….. 197
8. Future research needs………………………………………………………………………………..199
9. Updating the guidelines…………………………………………………………………………….. 205
References………………………………………………………………………………………………….. 209
Annex 1. Groups engaged during the development of the guidelines…………………. 235
Annex 2. Assessment of conflict of interest……………………………………………………. 253
Annex 3. Summaries of systematic reviews of evidence informing the air
quality guideline levels…………………………………………………………………………………..257
iv
Foreword
Clean air is fundamental to health. Compared to 15 years ago, when the previous edition
of these guidelines was published, there is now a much stronger body of evidence to
show how air pollution affects different aspects of health at even lower concentrations
than previously understood. But here’s what hasn’t changed: every year, exposure to
air pollution is still estimated to cause millions of deaths and the loss of healthy years
of life. The burden of disease attributable to air pollution is now estimated to be on
a par with other major global health risks such as unhealthy diets and tobacco smoking.
In 2015, the World Health Assembly adopted a landmark resolution on air quality and
health, recognizing air pollution as a risk factor for noncommunicable diseases such
as ischaemic heart disease, stroke, chronic obstructive pulmonary disease, asthma
and cancer, and the economic toll they take. The global nature of the challenge calls
for an enhanced global response.
These guidelines, taking into account the latest body of evidence on the health impacts
of different air pollutants, are a key step in that global response. The next step is for
policy-makers around the world to use these guidelines to inform evidence-based
legislation and policies to improve air quality and reduce the unacceptable health
burden that results from air pollution.
We are immensely grateful to all the scientists, colleagues and partners around
the world who have contributed time and resources to the development of these
guidelines. As with all WHO guidelines, a global group of experts has derived the
new recommendations based on a robust and comprehensive review of the scientific
literature, while adhering to a rigorously defined methodology. This process was
overseen by a steering group hosted and coordinated by the WHO European Centre
for Environment and Health.
Although the burden of air pollution is heterogeneous, its impact is ubiquitous.
These guidelines come at a time of unprecedented challenges, in the face of the
ongoing COVID-19 pandemic and the existential threat of climate change. Addressing
air pollution will contribute to, and benefit from, the global fight against climate
change, and must be a key part of the global recovery, as prescribed by the WHO
Manifesto for a healthy recovery from COVID-19.
A guideline is just a tool. What matters is that countries and partners use it to improve
air quality and health globally. The health sector must play a key role in monitoring
health risks from air pollution, synthesizing the evidence, providing the tools and
resources to support decision-making, and raising awareness of the impacts of air
pollution on health and the available policy options. But this is not a job for one sector
alone; it will take sustained political commitment and bold action and cooperation
from many sectors and stakeholders. The payoff is cleaner air and better health for
generations to come.
Dr Tedros Adhanom Ghebreyesus
Dr Hans Henri P. Kluge
WHO Director-General
WHO Regional Director for Europe
v
Acknowledgements
WHO thanks all members of the steering group, guideline development group,
systematic review team and external review group for their invaluable contributions
in the guideline development process.
WHO, through its European Centre for Environment and Health, coordinated the
development of these guidelines. The work was coordinated by Román Pérez
Velasco and Dorota Jarosińska, under the overall supervision of Francesca
Racioppi, Head of the European Centre for Environment and Health and Nino
Berdzuli, Director of the Division of Country Health Programmes, WHO Regional
Office for Europe. María Neira, Director of the Department of Environment, Climate
Change and Health, WHO headquarters provided invaluable advice and support
throughout the process.
Members of the WHO steering group were Heather Adair-Rohani, Magaran Monzon
Bagayoko, Carlos Dora, Sophie Gumy, Mohd Nasir Hassan, Marie-Eve Héroux,
Dorota Jarosińska, Rok Ho Kim, Dana Loomis, Mazen Malkawi, Guy Mbayo,
Pierpaolo Mudu, Lesley Jayne Onyon, Elizabet Paunović, Genandrialine Peralta,
Román Pérez Velasco, Nathalie Röbbel, Agnes Soares da Silva, Nadia Vilahur
Chiaraviglio and Hanna Yang (see Annex 1, Table A1.1 for membership periods
and affiliations).
Members of the guideline development group were Marwan Al-Dimashki,
Emmanuel K.-E. Appoh, Kalpana Balakrishnan, Michael Brauer, Bert Brunekreef,
Aaron J. Cohen, Francesco Forastiere, Lu Fu, Sarath K. Guttikunda, Mohammad
Sadegh Hassanvand, Marie-Eve Héroux, Wei Huang, Haidong Kan, Nguyen Thi
Kim Oanh, Michał Krzyżanowski (co-chair), Nino Künzli, Thomas J. Luben, Lidia
Morawska (co-chair), Kaye Patdu, Pippa Powell, Horacio Riojas-Rodríguez,
Jonathan Samet, Martin Williams (co-chair), Caradee Y. Wright, Xia Wan and
André Zuber (see Annex 1, Table A1.2 for membership periods and affiliations).
The systematic review team consisted of the following experts: Richard Atkinson,
Ariel Bardach, Jie Chen, Agustín Ciapponi, Wei-jie Guan, Gerard Hoek, Peijue
Huangfu, Mei Jiang, Kuan Ken Lee, Hua-liang Lin, Mark R. Miller, Nicholas L. Mills,
Pablo Orellano, Nancy Quaranta, Julieta Reynoso, Anoop S.V. Shah, Nicholas Spath
and Xue-yan Zheng (see Annex 1, Table A1.3 for affiliations).
The external review group was composed of the following individual members,
who participated at various stages of the guideline development process:
vi
Samir Afandiyev, Mohammad Alolayan, Richard Ballaman, Jill Baumgartner,
Hanna Boogaard, David M. Broday, Richard T. Burnett, Jacob Burns, Flemming
Cassee, Evan Coffman, Séverine Deguen, Sagnik Dey, Dimitris Evangelopoulos,
Mamadou Fall, Neal Fann, Daniela Fecht, Julia Fussell, Davina Ghersi, Otto
Hänninen, Barbara Hoffmann, Michael Holland, Yun-Chul Hong, Bin Jalaludin,
Meltem Kutlar Joss, Juleen Lam, Kin Bong Hubert Lam, Puji Lestari, Morton
Lippmann, Sylvia Medina, Rajen Naidoo, Mark J. Nieuwenhuijsen, Jeongim Park,
Rita Pavasini, Annette Peters, Vincent-Henri Peuch, C. Arden Pope III, Reginald
Quansah, Xavier Querol Carceller, Matteo Redaelli, Eva Rehfuess, Alexander
Romanov, Anumita Roychowdhury, Jason Sacks, Paulo Saldiva, Najat Saliba,
Andreia C. Santos, Jeremy Sarnat, Paul T.J. Scheepers, Srijan Lal Shrestha, Mónica
Silva González, Kirk R. Smith, Massimo Stafoggia, David M. Stieb, Jordi Sunyer,
Duncan C. Thomas, George D. Thurston, Linwei Tian, Aurelio Tobías Garces, Rita
Van Dingenen, Sotiris Vardoulakis, Giovanni Viegi, Kuku Voyi, Heather Walton, Paul
Whaley and Takashi Yorifuji (see Annex 1, Table A1.5 for affiliations).
The following stakeholder organizations were also part of the external review
group and, in particular, provided comments on the draft guideline document:
Abu Dhabi Global Environmental Data Initiative, African Centre for Clean Air,
Association for Emissions Control by Catalyst, Clean Air Asia, ClientEarth,
Concawe, European Environment Agency, European Environmental Bureau,
European Federation of Allergy and Airways Diseases Patients’ Associations,
European Respiratory Society, Health and Environment Alliance, International
Society for Environmental Epidemiology, International Transport Forum and South
Asia Co-operative Environment Programme (see Annex 1, Table A1.6 for details).
Representatives from the European Commission observed the meetings of the
guideline development group (Frauke Hoss in 2016 and Thomas Henrichs in
2018–2020).
Special thanks are extended to the guideline development group co-chairs,
the experts who contributed to several methodological working groups:
Bert Brunekreef, Aaron J. Cohen, Francesco Forastiere, Gerard Hoek, Michał
Krzyżanowski, Nino Künzli, Lidia Morawska, Xavier Querol Carceller, Jonathan
Samet, Massimo Stafoggia, Aurelio Tobías Garces, Martin Williams and Caradee
Y. Wright (see Annex 1, Table A1.7 for details), and the external methodologists
Rebecca Morgan and Jos Verbeek (see Annex 1, Table A1.4 for details).
In addition, WHO would like to express special gratitude to the members of
the guideline development group who largely drafted the guideline document:
Michael Brauer, Bert Brunekreef, Aaron J. Cohen, Francesco Forastiere, MarieEve Héroux, Wei Huang, Michał Krzyżanowski, Nino Künzli, Thomas J. Luben,
vii
Lidia Morawska, Pippa Powell, Jonathan Samet, Martin Williams and Caradee
Y. Wright. The guideline development group worked in collaboration with the
members of the WHO Secretariat – Dorota Jarosińska, Pierpaolo Mudu and
Román Pérez Velasco to finalize the draft; their contributions are also acknowledged.
The draft was reviewed and inputs were provided by all members of the guideline
development group and of the external review group. Special thanks are
extended to Bert Brunekreef for his continued support in finalizing this task.
WHO is grateful to the authors of the Multi-Country Multi-City study, led by
professor Antonio Gasparrini of the London School of Hygiene and Tropical
Medicine, United Kingdom, for making available detailed air pollution data for
the calculation of ratios of higher percentiles of daily concentrations to annual
means, in support of establishing the short-term air quality guideline levels.
Martin Williams (co-chair, guideline development group) and Kirk R. Smith
(external review group member) sadly passed away in 2020 before the publication
of this document. WHO would like to express our heartfelt appreciation for their
leadership and dedicated support to WHO and, in particular, to the development
of air quality guidelines throughout the years and of this edition.
WHO gratefully acknowledges funding and in-kind contributions from the
European Commission (Directorate-General for Environment), Federal Ministry
for the Environment, Nature Conservation and Nuclear Safety (Germany), Federal
Ministry of Health (Germany), Government of the Republic of Korea, Federal
Office for the Environment (Switzerland) and the United States Environmental
Protection Agency.
viii
Glossary
Abatement. The reduction or elimination of pollution, which involves either
legislative measures or technological procedures, or both.
Accountability research. Assessment of the effectiveness of interventions.
Knowledge gained from such assessments can provide valuable feedback for
improving regulatory or other action.
AirQ+. A software tool for health risk assessment of air pollution that looks at
the effects of short-term changes in air pollution (based on risk estimates from
time-series studies) and of long-term exposures (using the life-tables approach
and based on risk estimates from cohort studies).
Air quality guidelines. A series of WHO publications that provide evidenceinformed, non-binding recommendations for protecting public health from the
adverse effects of air pollutants by eliminating or reducing exposure to hazardous
air pollutants and by guiding national and local authorities in their risk management
decisions. The current volume is the latest issue of the series.
Air quality guideline level. A particular form of a guideline recommendation
consisting of a numerical value expressed as a concentration of a pollutant in
the air and linked to an averaging time. It is assumed that adverse health effects
do not occur or are minimal below this concentration level. For the purposes of
this document, a long-term air quality guideline level is defined as the lowest
exposure level of an air pollutant above which the guideline development group
is confident that there is an increase in adverse health effects; the short-term air
quality guideline level is defined as a high percentile of the distribution of daily
values, for example the 99th percentiles equivalent to three to four days a year
exceeding this value.
Air quality standard. A given level of an air pollutant (for example, a concentration
or deposition level) that is adopted by a regulatory authority as enforceable. Unlike
an air quality guideline level, a number of elements in addition to the effect-based
level and averaging time must be specified in the formulation of an air quality
standard. These elements include:
■
■
■
measurement technique and strategy
data handling procedures (including quality assurance/quality control)
statistics used to derive, from the measurements, the value to be compared
with the standard.
ix
The numerical value of a standard may also include a permitted number of
exceedances of a certain numerical value in a given time period.
Ambient air pollution. Air pollution in the outdoor environment, that is, in outdoor
air, but which can enter or be present in indoor environments.
Averaging time. For the purposes of this document, the duration of the exposure
with a given mean concentration associated with certain health effects.
Black carbon. An operationally defined term that describes carbon as measured
by light absorption. As such, it is not the same as elemental carbon, which is
usually monitored with thermal-optical methods.
Concentration–response function. A statistical function or model based on the
results of epidemiological studies to estimate the relative risk from air pollution for
a disease or health outcome (e.g. premature death, heart attack, asthma attack,
emergency room visit, hospital admission) in a population per unit concentration
of an air pollutant.
Dust storm (or sand storm). A mix of dust and/or sand particles that has been
elevated to great heights by a strong, turbulent wind and can travel great distances
and reduce visibility. Dust or sand readily penetrates into buildings, results in
severe soiling and may also cause considerable erosion. The particles are usually
lifted to greater heights in a dust storm than in a sand storm.
Good practice statement. A statement formulated when a guideline development
group is confident that a large body of diverse evidence, which is hard to
synthesize, indicates that the desirable effects of a particular course of action
far outweigh its undesirable effects. In other words, there is high certainty that
implementing a measure would be beneficial, without the need for conducting
numerous systematic reviews and detailed assessments of evidence.
Hot spot. For the purposes of this document, an area where air pollution levels
are higher than the average levels in the local environment.
Household fuel combustion. Air pollution generated by the inefficient combustion
of fuels in the household environment that results in household air pollution and
contributes to local ambient air pollution.
x
Integrated exposure–response function. Models that combine exposure and
risk data for different sources of combustion-related pollution, such as outdoor
air, second-hand tobacco smoke, active smoking and household air pollution.
Interim target. An air pollutant concentration associated with a specific decrease
of health risk. Interim targets serve as incremental steps in the progressive
reduction of air pollution towards the air quality guideline levels and are intended
for use in areas where air pollution is high. In other words, they are air pollutant
levels that are higher than the air quality guideline levels, but which authorities
in highly polluted areas can use to develop pollution reduction policies that are
achievable within realistic time frames. The interim targets should be regarded
as steps towards ultimately achieving air quality guideline levels, rather than as
end targets.
Particulate matter. A mixture of solid and liquid particles in the air that are small
enough not to settle out on to the Earth’s surface under the influence of gravity,
classified by aerodynamic diameter.
Ultrafine particle. Particles of an aerodynamic diameter less than or equal to
0.1 μm (that is, 100 nm). Owing to their small mass, their concentrations are most
commonly measured and expressed in terms of particle number concentration
per unit volume of air (for example, number of particles per cm3).
xi
Abbreviations
AAQS
ambient air quality standards
ACTRIS

Aerosol, Clouds and Trace Gases Research
Infrastructure
APM
anthropogenic particulate matter
AQG level
air quality guideline level
BC/EC

black carbon or elemental carbon (an indicator
of airborne soot-like carbon)
BenMAP-CE

Environmental Benefits Mapping and Analysis
Program – Community Edition
CanCHEC

Canadian Census Health and
Environment Cohort
CCAC
Climate and Clean Air Coalition
CEN
European Committee for Standardization
CI
confidence interval
CO
carbon monoxide
COMEAP
Committee on the Medical Effects of Air Pollutants
COPD
chronic obstructive pulmonary disease
CRF
concentration–response function
EEA
European Environment Agency
ERG
external review group
EU
European Union
FAO

Food and Agriculture Organization of the
United Nations
GBD
Global Burden of Disease (study)
GDG
guideline development group
Global update 2005

Air quality guidelines – global update 2005. Particulate
matter, ozone, nitrogen dioxide and sulfur dioxide
GRADE

Grading of Recommendations Assessment,
Development and Evaluation
HEI
Health Effects Institute
HR
hazard ratio
ICD-10
International Statistical Classification of
Diseases and Related Health Problems, 10th edition

IHD
xii
ischaemic heart disease
ISA
(US EPA) Integrated Science Assessment
MCC
Multi-Country Multi-City
NCD

noncommunicable disease
NDPM
net dust particulate matter
NO2
nitrogen dioxide
O3
ozone
PECOS

population, exposure, comparator, outcome and
study design
PM
particulate matter
PM2.5

particulate matter, where particles have an
aerodynamic diameter equal to or less than 2.5 μm
PM10

particulate matter, where particles have an
aerodynamic diameter equal to or less than 10 μm
PNC
particle number concentration
ppb
parts per billion
ppm
parts per million
RBPM
regional background particulate matter
REVIHAAP

Review of evidence on health aspects of air pollution
(project)
RoB
risk of bias
RR
relative risk
SDG
Sustainable Development Goal
SDS
sand and dust storms
SDS-WAS

Sand and Dust Storm Warning Advisory and
Assessment System
SO2
sulfur dioxide
Swiss TPH
Swiss Tropical and Public Health Institute
UFP
ultrafine particles
UN
United Nations
UNECE
United Nations Economic Commission for Europe
UNEP
United Nations Environment Programme
US EPA
United States Environmental Protection Agency
VOC
volatile organic compound
WMO
World Meteorological Organization
xiii
Executive summary
The global burden of disease associated with air pollution exposure exacts
a massive toll on human health worldwide: exposure to air pollution is estimated
to cause millions of deaths and lost years of healthy life annually. The burden
of disease attributable to air pollution is now estimated to be on a par with
other major global health risks such as unhealthy diet and tobacco smoking,
and air pollution is now recognized as the single biggest environmental threat to
human health.
Despite some notable improvements in air quality, the global toll in deaths and
lost years of healthy life has barely declined since the 1990s. While air quality
has markedly improved in high-income countries over this period, it has generally
deteriorated in most low- and middle-income countries, in step with large-scale
urbanization and economic development. In addition, the global prevalence of
noncommunicable diseases (NCDs) as a result of population ageing and lifestyle
changes has grown rapidly, and NCDs are now the leading causes of death and
disability worldwide. NCDs comprise a broad range of diseases affecting the
cardiovascular, neurological, respiratory and other organ systems. Air pollution
increases morbidity and mortality from cardiovascular and respiratory disease
and from lung cancer, with increasing evidence of effects on other organ systems.
The burden of disease resulting from air pollution also imposes a significant
economic burden. As a result, governments worldwide are seeking to improve air
quality and reduce the public health burden and costs associated with air pollution.
Since 1987, WHO has periodically issued health-based air quality guidelines to
assist governments and civil society to reduce human exposure to air pollution and
its adverse effects. The WHO air quality guidelines were last published in 2006.
Air quality guidelines – global update 2005. Particulate matter, ozone, nitrogen
dioxide and sulfur dioxide (WHO Regional Office for Europe, 2006) provided
health-based guideline levels for the major health-damaging air pollutants,
including particulate matter (PM),1 ozone (O3), nitrogen dioxide (NO2) and sulfur
dioxide (SO2). Global update 2005 has had a significant impact on pollution
abatement policies all over the world. Its publication led to the first universal
frame of reference.
In various ways, these guidelines have stimulated authorities and civil society
alike to increase efforts to control and study harmful air pollution exposures.
1
That is, PM2.5 (particles with an aerodynamic diameter of ≤ 2.5 μm) and PM10 (particles with an aerodynamic diameter
of ≤ 10 μm).
xiv
In response to this growing awareness, the Sixty-eighth World Health Assembly
adopted resolution WHA68.8, Health and the environment: addressing the
health impact of air pollution, which was endorsed by 194 Member States in
2015 (WHO,2015). This resolution stated the need to redouble efforts to
protect populations from the health risks posed by air pollution. In addition, the
United Nations (UN) Sustainable Development Goals (SDGs) were designed
to address the public health threat posed by air pollution via specific targets
to reduce air pollution exposure and the disease burden from household and
ambient exposure.
More than 15 years have passed since the publication of Global update 2005.
In that time there has been a marked increase in evidence on the adverse
health effects of air pollution, built on advances in air pollution measurement
and exposure assessment and an expanded global database of air pollution
measurements (discussed in Chapter 1). New epidemiological studies have
documented the adverse health effects of exposure to high levels of air pollution
in low- and middle-income countries, and studies in high-income countries with
relatively clean air have reported adverse effects at much lower levels than had
previously been studied.
In view of the many scientific advances and the global role played by the WHO air
quality guidelines, this update was begun in 2016.
Objectives
The overall objective of the updated global guidelines is to offer quantitative
health-based recommendations for air quality management, expressed as long- or
short-term concentrations for a number of key air pollutants. Exceedance of
the air quality guideline (AQG) levels is associated with important risks to public
health. These guidelines are not legally binding standards; however, they do
provide WHO Member States with an evidence-informed tool that they can use to
inform legislation and policy. Ultimately, the goal of these guidelines is to provide
guidance to help reduce levels of air pollutants in order to decrease the enormous
health burden resulting from exposure to air pollution worldwide.
Specific objectives are the following.
■
Provide evidence-informed recommendations in the form of AQG levels,
including an indication of the shape of the concentration–response function
in relation to critical health outcomes, for PM2.5, PM10, ozone, nitrogen dioxide,
sulfur dioxide and carbon monoxide for relevant averaging times.
xv
These pollutants were chosen because of their worldwide importance.
However, this choice does not imply that other air pollutants are irrelevant.
■
■
Provide interim targets to guide reduction efforts towards the ultimate and
timely achievement of the AQG levels for countries that substantially exceed
these levels.
Provide qualitative statements on good practices for the management of
certain types of PM (i.e. black carbon or elemental carbon (BC/EC),2 ultrafine
particles (UFP), and particles originating from sand and dust storms (SDS))
for which the available information is insufficient to derive AQG levels but
indicates risk.
Methods used to develop the guidelines
The guidelines were formulated by following a rigorous process involving several
groups with defined roles and responsibilities (Chapter 2). In particular, the
different steps in the development of the AQG levels included:
■
■
■
■
a determination of the scope of the guidelines and formulation of systematic
review questions;
a systematic review of the evidence and meta-analyses of quantitative effect
estimates to inform updating of the AQG levels;
an assessment of the level of certainty of the bodies of evidence resulting from
systematic reviews for the pollutants; and
the identification of AQG levels, that is, the lowest levels of exposure for which
there is evidence of adverse health effects.
In addition, the 2005 air quality interim targets were updated to guide the
implementation of the new AQG levels, and good practice statements were
formulated to support the management of the specific types of PM of concern.
Interim targets are air pollutant levels that are higher than the AQG levels, but
which authorities in highly polluted areas can use to develop pollution reduction
policies that are achievable within realistic time frames. Therefore, the interim
targets should be regarded as steps towards the ultimate achievement of AQG
levels in the future, rather than as end targets. The number and numerical values of
the interim targets are pollutant specific, and are justified in the relevant sections
of Chapter 3.
The process and methods for developing these guidelines are described in detail
in Chapter 2.
2 An indicator of airborne soot-like carbon.
xvi
The systematic reviews that informed the formulation of AQG levels and other
related evidence discussed during the process are available in a special issue
of Environment International, entitled Update of the WHO global air quality
guidelines: systematic reviews (Whaley et al., 2021).
Recommendations on classical air pollutants
In this guideline update, recommendations on AQG levels are formulated, together
with interim targets, for the following pollutants: PM2.5, PM10, ozone, nitrogen
dioxide, sulfur dioxide and carbon monoxide (Table 0.1). The evidence-informed
derivation of each AQG level and an indication of the reduction in health risk
associated with the achievement of consecutive interim targets can be found
in Chapter 3. Only evidence assessed as having high or moderate certainty of
an association between a pollutant and a specific health outcome was used to
define the recommended AQG levels, and all recommendations are classified
as strong according to the adapted Grading of Recommendations Assessment,
Development and Evaluation (GRADE) approach (discussed in Chapter 2).
Table 0.1. Recommended AQG levels and interim targets
Pollutant
Averaging time
Interim target
AQG level
1
2
3
4
Annual
35
25
15
10
5
24-houra
75
50
37.5
25
15
Annual
70
50
30
20
15
24-houra
150
100
75
50
45
Peak seasonb
100
70
–
–
60
8-houra
160
120
–
–
100
Annual
40
30
20
–
10
24-houra
120
50
–
–
25
SO2, µg/m3
24-houra
125
50
–
–
40
CO, mg/m3
24-houra
7
–
–
–
4
PM2.5, µg/m3
PM10, µg/m3
O3, µg/m3
NO2, µg/m3
ᵃ 99th percentile (i.e. 3–4 exceedance days per year).
ᵇ Average of daily maximum 8-hour mean O3 concentration in the six consecutive months with the highest six-month
running-average O3 concentration.
xvii
It is important to note that the air quality guidelines recommended in previous
WHO air quality guidelines for pollutants and those averaging times not covered
in this update remain valid. This includes the short averaging times for nitrogen
dioxide, sulfur dioxide and carbon monoxide that were included in Global update
2005 and indoor air quality guidelines from 2010 (and not re-evaluated in this
update). Table 0.2 shows existing air quality guidelines for nitrogen dioxide,
sulfur dioxide and carbon monoxide with short averaging times. The reader is
referred to previous volumes of air quality guidelines – Air quality guidelines for
Europe (WHO Regional Office for Europe, 1987), Air quality guidelines for Europe,
2nd edition (WHO Regional Office for Europe, 2000a); and WHO guidelines for
indoor air quality: selected pollutants (WHO Regional Office for Europe, 2010) – for
other pollutants that are not covered in this 2021 update.
Table 0.2. Air quality guidelines for nitrogen dioxide, sulfur dioxide and
carbon monoxide (short averaging times) that were not re-evaluated
and remain valid
Pollutant
Averaging time
Air quality guidelines that remain valid
NO2, µg/m3
1-hour
200
SO2, µg/m3
10-minute
500
CO, mg/m3
8-hour
10
1-hour
35
15-minute
100
Good practice statements about other PM types
As yet, insufficient data are available to provide recommendations for AQG
levels and interim targets for specific types of PM, notably BC/EC, UFP and SDS.
However, due to health concerns related to these pollutants, actions to enhance
further research on their risks and approaches for mitigation are warranted.
Good practice statements for these pollutants are summarized in Table 0.3.
The full text of and rationales for the statements can be found in Chapter 4.
xviii
Table 0.3. Summary of good practice statements
Good practice statements
BC/EC
1.
2.
3.
UFP
1.
2.
3.
4.
SDS
1.
2.
3.
4.
5.
Make systematic measurements of black carbon and/or elemental carbon.
Such measurements should not replace or reduce existing monitoring of those
pollutants for which guidelines currently exist.
Undertake the production of emission inventories, exposure assessments and
source apportionment for BC/EC.
Take measures to reduce BC/EC emissions from within the relevant jurisdiction
and, where appropriate, develop standards (or targets) for ambient BC/EC
concentrations.
Quantify ambient UFP in terms of PNC for a size range with a lower limit of
≤ 10 nm and no restriction on the upper limit.
Expand the common air quality monitoring strategy by integrating UFP
monitoring into the existing air quality monitoring. Include size-segregated
real-time PNC measurements at selected air monitoring stations in addition to
and simultaneously with other airborne pollutants and characteristics of PM.
Distinguish between low and high PNC to guide decisions on the priorities of
UFP source emission control. Low PNC can be considered 10 000 particles/cm3
(24-hour mean) or 20 000 particles/cm3 (1-hour mean).
Utilize emerging science and technology to advance approaches to the
assessment of exposure to UFP for their application in epidemiological
studies and UFP management.
Maintain suitable air quality management and dust forecasting programmes.
These should include early warning systems and short-term air pollution action
plans to alert the population to stay indoors and take personal measures to
minimize exposure and subsequent short-term health effects during SDS
incidents with high levels of PM.
Maintain suitable air quality monitoring programmes and reporting procedures,
including source apportionment activities to quantify and characterize PM
composition and the percentage contribution of SDS to the overall ambient
concentration of PM. This will enable local authorities to target local PM
emissions from anthropogenic and natural sources for reduction.
Conduct epidemiological studies, including those addressing the long-term
effects of SDS, and research activities aimed at better understanding the
toxicity of the different types of PM. Such studies are especially recommended
for areas where there is a lack of sufficient knowledge and information about
the health risk due to frequent exposure to SDS.
Implement wind erosion control through the carefully planned expansion of
green spaces that considers and is adjusted to the contextual ecosystem
conditions. This calls for regional collaboration among countries in the regions
affected by SDS to combat desertification and carefully manage green areas.
Clean the streets in those urban areas characterized by a relatively high
population density and low rainfall to prevent resuspension by road traffic
as a short-term measure after intense SDS episodes with high dust
deposition rates.
PNC: particle number concentration.
xix
The settings to which these guidelines apply
The present guidelines are applicable to both outdoor and indoor environments
globally. Thus, they cover all settings where people spend time. However, as in
previous editions, these guidelines do not cover occupational settings, owing to
the specific characteristics of the relevant exposures and risk reduction policies
and to potential differences in population susceptibility of the adult workforce
in comparison with the general population.
What these guidelines do not address
These guidelines do not include recommendations about pollutant mixtures or
the combined effects of pollutant exposures. In everyday life, people are exposed
to a mixture of air pollutants that varies in space and time. WHO acknowledges
the need to develop comprehensive models to quantify the effects of multiple
exposures on human health. However, as the main body of evidence on air
quality and health still focuses on the impact of single markers of ambient air
pollution on the risk of adverse health outcomes, the current guidelines provide
recommendations for each air pollutant individually. Achievement of the AQG
levels for all these pollutants is necessary to minimize the health risk of the
exposure.
Furthermore, these guidelines do not address specific recommendations on
policies and interventions because these are largely context specific: what might
be effective in one setting might not work in another. Lastly, individual-level
interventions, such as the use of personal respiratory protection (e.g. masks,
respirators, air purifiers) or behavioural measures, are addressed in another
document, Personal interventions and risk communication on air pollution
(WHO, 2020a).
Target audience
The WHO global air quality guidelines aim to protect populations from the adverse
effects of air pollution. They are designed to serve as a global reference for
assessing whether, and how much, exposure of a population (including particularly
vulnerable and/or susceptible subgroups) to various levels of the considered
air pollutants results in health concerns. The guidelines are a critical tool for the
following three main groups of users:
■
xx
policy-makers, lawmakers and technical experts operating at the local, national
and international levels who are responsible for developing and implementing
regulations and standards for air quality, air pollution control, urban planning
and other policy areas;
■
■
national and local authorities and nongovernmental organizations, civil society
organizations and advocacy groups, such as patients, citizen groups, industrial
stakeholders and environmental organizations; and
academics, health and environmental impact assessment practitioners, and
researchers in the broad field of air pollution.
These groups are the targets of the information, education and communication
strategies outlined in Chapter 5. The strategies, and the tools to implement
them, will be essential to ensure that these global guidelines are widely
disseminated and considered in policy and planning decisions. In addition,
these groups are addressed in Chapter 6, on implementation of the guidelines.
This includes the aspects involved in developing air quality standards based on
the recommendations and general risk management principles, which are built
on decades of experience.
Implementation of the guidelines
While achievement of the AQG levels should be the ultimate goal of actions to
implement the guidelines, this might be a difficult task for many countries and
regions struggling with high air pollution levels. Therefore, gradual progress in
improving air quality, marked by the achievement of interim targets, should be
considered a critical indicator of improving health conditions for populations.
Key institutional and technical tools supported by human capacity-building are
necessary to achieve this goal. Implementation of the guidelines requires the
existence and operation of air pollution monitoring systems; public access to air
quality data; legally binding, globally harmonized air quality standards; and air
quality management systems. Policy decisions to set priorities for action will profit
from the health risk assessment of air pollution.
While actions to reduce air pollution require cooperation among various sectors
and stakeholders, health sector involvement is crucial for raising awareness of the
impacts of air pollution on health and, thus, the economy, and for ensuring that
protecting health strongly figures in policy discussions. Monitoring and evaluation
are equally crucial to ensure that guidelines are implemented; they are addressed
in Chapter 7.
Currently, the accumulated evidence is sufficient to justify actions to reduce
population exposure to key air pollutants, not only in particular countries or regions
but on a global scale. Nevertheless, uncertainties and knowledge gaps remain.
Future research (discussed in Chapter 8) will further strengthen the scientific
evidence base for making decisions on clean air policy worldwide.
xxi
1
Introduction
1. Introduction
1
The WHO air quality guidelines were last published in 2006: Air quality guidelines –
global update 2005. Particulate matter, ozone, nitrogen dioxide and sulfur dioxide
(hereafter referred to as Global update 2005) (WHO Regional Office for Europe,
2006). Since they were issued, air pollution has become recognized as the single
biggest environmental threat to human health based on its notable contribution
to disease burden. This is particularly true for PM (both PM2.5, i.e. particles with
an aerodynamic diameter equal to or less than 2.5 μm, and PM10, i.e. particles
with an aerodynamic diameter of equal to or less than 10 μm). However, other
commonly measured air pollutants such as ozone (O3), nitrogen dioxide (NO2),
sulfur dioxide (SO2) and carbon monoxide (CO) are also of concern, as are other
components of air pollution.
The burden of disease associated with both ambient and household air pollution
exposure is large and growing. The growth is partly due to increases in exposures
in low- and middle-income countries,3 but is in part also due to the rapidly
increasing prevalence of NCDs worldwide as a result of population ageing and
lifestyle changes. Air pollution especially increases morbidity and mortality
from the noncommunicable cardiovascular and respiratory diseases that are
the major causes of global mortality; it also increases the disease burden from
lower respiratory tract infections and preterm birth and other causes of death
in children and infants, which remain a major cause of the disease burden in
low- and middle-income countries. Although air quality has improved gradually in
high-income countries in the past decades, pollutant concentrations still exceed
the levels published in Global update 2005 for several pollutants in many areas.
Air quality has generally deteriorated in most low- and middle-income countries,
in step with large-scale urbanization and economic development that has largely
relied on the burning of fossil fuels. Disparities in air pollution exposure are,
therefore, increasing worldwide.
Science advances and, since the 2005 air quality guidelines were established,
many new studies have continued to document the adverse health effects of air
pollution. During this time, enormous advances have also occurred in measuring
levels and trends in ground-level air pollution concentrations. In particular, the use
of satellite remote sensing instruments in combination with advanced chemical
transport models and ground-based measurements has substantially improved
the understanding of worldwide pollution levels and trends. Studies conducted
in low- and middle-income countries where concentrations are high are of great
importance; however, equally important are studies in very clean areas, which
answer important questions on the effects of low-level exposures and the
evaluation of thresholds.
3 Country income groupings of low, lower-middle, upper-middle and high are determined by the World Bank based on
gross national income per capita (World Bank, 2021).
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WHO GLOBAL AIR QUALITY GUIDELINES
These studies provide critical information on the benefits that might be expected
if air pollution levels were reduced worldwide. In view of these many advances,
revision of Global update 2005 was both timely and necessary. This revision
benefited from thousands of new studies and from following the rigorous
process for developing guidelines outlined in the WHO handbook for guideline
development, 2nd edition (WHO, 2014a).
Global update 2005 has had a significant impact on abatement policies all over
the world. Its publication led to the first universal frame of reference. In various
ways, the air quality guidelines have stimulated authorities and civil society alike
to increase efforts to control harmful air pollution exposures. Major challenges still
exist, however, and it is hoped that this update of the WHO air quality guidelines
will continue to inspire and guide pollution reduction policies all over the world.
1.1 Objectives of the guidelines
The overall objective of these guidelines is to offer quantitative health-based
recommendations for air quality, expressed as long- or short-term concentrations
of a number of key air pollutants. Exceedance of the air quality guideline levels
(hereafter referred to as AQG levels) is associated with important risks to public
health. These guidelines are not legally binding standards; however, they do
provide countries with an evidence-informed tool, which they can use to inform
legislation and policy. In addition, the air quality guidelines will be a key component
to support air quality policies globally and the development of standards, clean air
policies and other tools for air quality management. Ultimately, the goal of these
guidelines is to provide guidance to help reduce levels of air pollutants in order
to decrease the enormous worldwide health burden resulting from exposure to
air pollution.
Specifically, the objectives of these guidelines are the following.
■
■
Provide evidence-informed recommendations in the form of AQG levels,
including an indication of the shape of the concentration–response function
(CRF) in relation to critical health outcomes, for PM2.5, PM10, nitrogen dioxide,
ozone, sulfur dioxide and carbon monoxide for relevant averaging time periods.
These pollutants were chosen in the process described in section 2.3 because
of their worldwide importance. This choice does not imply that other air
pollutants are irrelevant.
Provide interim targets to guide reduction efforts towards the ultimate and
timely achievement of the AQG levels for those countries that substantially
exceed the AQG levels.
INTRODUCTION
3
■
Provide qualitative statements on good practices for the management of
certain types of PM – that is, BC/EC, UFP and particles originating from SDS
– for which the available information is insufficient to derive AQG levels but
indicates risk.
1.2 Target audience
The WHO guidelines to protect populations from the adverse effects of air pollution
are designed to serve as a global reference for an audience of different groups
of end-users, including those involved in policy-making, research and advocacy.
Broadly, three main groups can be identified:
■
■
■
policy-makers, lawmakers and technical experts at the local, national and
international levels who are responsible for developing and implementing
regulations and standards for air quality, air pollution control, urban planning
and other policy areas;
national and local authorities and nongovernmental organizations, civil society
organizations and advocacy groups, such as patients, citizen groups, industrial
stakeholders and environmental organizations; and
academics, health and environmental impact assessment practitioners and
researchers in the broad field of air pollution.
1.3 Background and rationale for updated guidelines
An update of the global WHO air quality guidelines was required for several reasons.
More than 15 years have passed since the publication of Global update 2005 and
in the intervening years knowledge about the exposure of human populations, the
adverse health effects of this exposure and the public health threat that it poses
has seen a marked increase. Insight into global concentrations of some pollutants
such as PM, ozone and nitrogen dioxide has increased dramatically (section 1.3.1).
This is also true for insights in sources of emissions (section 1.3.2) and in the
contribution of air pollutants to the global burden of disease (section 1.3.3). Much
has been learned about the importance of addressing health inequities related
to air pollution and of protecting vulnerable groups in society (section 1.3.4).
Enormous advances have occurred since the early 2000s in measuring levels
and trends in ground-level air pollution concentrations, and section 1.3.5 provides
a summary of some major trends and achievements. Finally, there have been
significant advances in the worldwide adoption of the air quality guidelines
presented in Global update 2005 (section 1.3.6), and mitigating air pollution has
become more central in WHO and UN activities related to achieving the UN SDGs
(section 1.3.7).
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WHO GLOBAL AIR QUALITY GUIDELINES
1.3.1 Global concentrations and trends
Measurement of air pollutant concentrations at fixed-location monitoring sites has
been the traditional approach used for air quality management, for assessment
of trends and to estimate exposure for epidemiological analyses. However,
despite growth in the numbers of monitoring locations globally, even for the
most commonly monitored pollutants, coverage is inadequate – that is, it is often
restricted to major cities – to accurately estimate exposure in the many different
places where people live. There are two major gaps.
The first is a lack of monitoring in many countries of the world and inadequate
monitoring in rural areas or outside of major cities in many countries. Although
there is increasing coverage of PM monitoring, coverage for other pollutants such
as ozone, nitrogen dioxide and sulfur dioxide is less extensive. The second gap
relates to inadequate monitoring to characterize the spatial variation in specific
air pollutants within cities. In particular, this holds for concentrations of pollutants
such as nitrogen dioxide and black carbon and UFP (diameter of ≤ 0.1 µm; or
broader quasi-UFP, as discussed in section 4.3 on UFP), which may vary by an
order of magnitude over just a few hundred metres (Karner, Eisinger & Niemeier,
2010). Since 2010, there has been a dramatic improvement in the combination
of satellite data retrievals and chemical transport models with land-use information
and ground measurements to estimate concentrations globally, which have been
used to address the first gap (Shaddick et al., 2018; Brauer et al., 2012, 2016;
Larkin et al., 2017; de Hoogh et al., 2016; Novotny et al., 2011; Hystad et al., 2011;
Knibbs et al., 2014; Chang et al., 2019). To address the second gap, land-use
regression models (Hoek et al., 2008) have been used increasingly – these models
capture within-city variability, as discussed for example for UFP (Morawska et
al., 2008), and have been scaled up to the global context for nitrogen dioxide
(Larkin et al., 2017).
Although in many countries, regional and local authorities maintain accessible
databases of air quality measurements, the only global databases are the
WHO Global Ambient Air Quality Database and OpenAQ. The WHO Global Ambient
Air Quality Database provides information on the annual average concentrations
of PM10 and PM2.5 for specific cities based on available measurements (including
averages from multiple monitors within a single city, where these are available)
(WHO, 2021a). OpenAQ is a non-profit-making effort to maintain an open-source
database of aggregated current and archived air quality data gathered in real
time from government agencies (OpenAQ, 2021). Despite the progress made in
monitoring and in data access, many publicly funded agencies still do not provide
easy access to data.
INTRODUCTION
5
Exposure to air pollutants is heavily dependent on their ambient concentrations.
Ambient PM2.5 concentrations vary substantially between and within regions of
the world. Importantly, more than 90% of the global population in 2019 lived in
areas where concentrations exceeded the 2005 WHO air quality guideline of
10 µg/m3. In 2019 annual population-weighted PM2.5 concentrations were highest
in the WHO South-East Asia Region, followed by the WHO Eastern Mediterranean
Region. Elevated concentrations were also observed in some western African
countries, largely due to the impact of Saharan dust. Windblown desert dust
sometimes contributes to very high exposures to coarse particles larger than
2.5 µm or10 µm in diameter. This is a prominent issue in many arid areas in the
Middle East, northern Africa, the Gobi desert and elsewhere.
Many of the countries with the lowest national PM2.5 exposure levels were either
in the WHO Region of the Americas or parts of the WHO European Region.
Population-weighted PM2.5 concentrations averaged 7 μg/m3 or less in these
countries. Trends in PM2.5 indicate a relatively stable population-weighted
global mean concentration, which reflects both decreases in exposure in the
WHO European Region, the WHO Region of the Americas and the WHO Western
Pacific Region but increases elsewhere.
Population-weighted ozone concentrations vary less dramatically than is the case
for PM2.5, for example ranging from 30–50 µg/m3, mostly in small island nations,
to 120–140 µg/m3 in Asia and the Middle East. Among the world’s most populous
countries in southern Asia, population-weighted seasonal ozone concentrations
range up to approximately 130 µg/m3. Concentrations in African mega-cities are
also likely to be high but there is still comparatively little documentation.
Trends in ozone at a regional scale show little change over time, although
decreases within North America and Europe and increases in the Middle East
and much of Asia are apparent.
The patterns of ambient nitrogen dioxide concentrations are quite different from
those of PM2.5 and ozone, with the highest population-weighted concentrations
in eastern Asia, the Middle East, North America and much of Europe, reflecting
mobile sources (Larkin et al., 2017; Achakulwisut et al., 2019). In addition, nitrogen
dioxide displays a distinct urban–rural gradient, with higher concentrations in
more densely populated urban areas. This pattern contrasts distinctly from
that of ozone, which displays higher concentrations downwind of urban
areas, and PM2.5, which is more homogeneous regionally due to its longer
atmospheric lifetime and diversity of (urban, rural and regional) sources. Trends in
population-weighted nitrogen dioxide concentrations (for 1992–2012) indicated
6
WHO GLOBAL AIR QUALITY GUIDELINES
sharp decreases (-4.7%/year) in high-income North American countries and
somewhat lesser decreases in western Europe (-2.5%/year) and high-income
Asia–Pacific countries (-2.1%/year). In contrast, population-weighted nitrogen
dioxide concentrations increased dramatically during this period in eastern Asia
at a rate of 6.7%/year. Judging from satellite observations, concentrations in
Africa seem to be generally low, with some evidence of increases in northern
Africa and stable or slightly decreasing levels elsewhere (Geddes et al., 2016).
However, there are few actual monitoring data on small-scale spatial variability
within mega-cities in Africa.
1.3.2 Sources of emissions and exposure
Air pollution originates from numerous sources of emission, both natural and
anthropogenic, with the latter becoming globally dominant since the beginning
of industrialization. The process of combustion is the greatest contributor to air
pollution, in particular, the combustion of fossil fuels and biomass to generate
energy. In indoor environments, the use of polluting fuels in unvented heating
and cooking stoves, tobacco combustion and combustion for other purposes,
such as cultural or religious practices are also important. Fossil and biomass fuel
burning for domestic heating is also an important source of outdoor air pollution
in many parts of the world.
Outdoor combustion sources include land, air and water transportation; industry
and power generation; and biomass burning, which includes controlled and
uncontrolled forest and savannah fires and agricultural waste burning, as well
as waste burning in urban areas. Other sources and processes contributing to
outdoor pollution are the resuspension of surface dust and construction activities.
Long-range atmospheric transport of pollutants from distant sources contributes
to local pollution, particularly urban air pollution. Some of the pollutants are emitted
directly by combustion sources as primary pollutants (with elemental carbon as the
main constituent of PM), and some are formed in the air as secondary pollutants
(such as nitrates, sulfates and organic carbon) through complex physicochemical
processes involving gaseous precursors originating from combustion sources,
agriculture (ammonia), other anthropogenic processes and natural processes
such as biogenic emissions.
Comprehensive reviews of sources and concentrations of major outdoor air
pollutants have been published by the United States Environmental Protection
Agency (US EPA) (2010, 2016, 2017, 2019a, 2020). The European Environment
Agency (EEA) every year produces a comprehensive report on air quality in
Europe; the latest one from 2020 (EEA, 2020).
INTRODUCTION
7
In indoor environments, pollution is also generated by combustion sources,
mainly cooking and heating with polluting fuels such as coal, wood or dung; and
using candles, incense and kerosene lamps (e.g. for light or religious practices).
Tobacco smoking is also a significant source of indoor pollution. Non-combustion
sources and processes also have a significant impact on indoor air pollution,
particularly those that generate volatile and semi-volatile organic compounds
(VOCs) and/or ozone. These include the renovation of houses, usage of consumer
products (e.g. cleaning products and insecticides) and operation of electric
devices such as laser printers. Dust resuspension due to human movement is
another significant source in some indoor environments, particularly in schools.
However, indoor air pollution is generated not only from indoor sources but also
from outdoor air pollutants that are brought indoors in the processes of ventilation
and penetration through the building envelope. In indoor environments without
indoor sources of pollution, pollutants from outdoors are the main cause of indoor
air pollution. Exposure is then further influenced by indoor decay, which is very
fast for substances such as ozone (which is very reactive) and very slow for
substances such as carbon monoxide (which is fairly inert).
Airborne pollutants originating from the sources and processes listed above
include PM (measured as PM2.5, PM10 and UFP), gaseous pollutants (including
ammonia (NH3), carbon monoxide, nitrogen dioxide, sulfur dioxide and ozone)
and organic air pollutants. PM is partly formed in the atmosphere through
chemical reactions that produce inorganic nitrates and sulfates, as well as
organic compounds summarized as organic carbon. Other airborne pollutants not
discussed in this document include radon and its decay products, and biological
agents. WHO has developed dedicated air quality guidelines for these and for
other selected pollutants, dampness and mould, and household fuel combustion
(WHO, 2014b; WHO Regional Office for Europe, 2009, 2010).
The spatial and temporal concentration of pollutants in outdoor air varies according
to the spatial distribution of the sources and their pattern of operation (e.g. daily
or seasonal), the characteristics of the pollutants and their dynamics (dispersion,
deposition, interaction with other pollutants), and meteorological conditions.
In urban environments, some pollutants are distributed more homogeneously than
others; for example, PM2.5 concentration has much less spatial variation compared
with the concentration of UFP or gases directly emitted by local combustion
sources. Importantly, spatial variation determines to what extent ambient
concentrations measured at a single fixed site reflect the outdoor concentrations
at other sites in the area. Temporal variation is a very important feature of ambient
air pollution.
8
WHO GLOBAL AIR QUALITY GUIDELINES
Emissions often have specific and predictable temporal patterns (e.g. weekdays
versus weekends). Most importantly, however, meteorological conditions are very
strong determinants of temporal variations, and can have far larger effects than
the temporal variation in emission alone. Epidemiological research of short-term
health effects capitalizes on these short-term temporal variations in ambient
concentrations. It offers opportunities to investigate whether temporally varying
markers of health, including the number of adverse health events, correlate with
the temporal variation in ambient concentrations of pollutants.
In indoor environments, concentrations of pollutants originating from outdoor
air are influenced by their outdoor spatiotemporal patterns of concentration
and, in particular, by the proximity of the building to outdoor sources (e.g. busy
roads). Furthermore, indoor pollution concentrations depend on the amount of
air pollution penetrating from outdoors; this is dependent on the penetration
fraction, the ventilation rate and the decay rate. The penetration coefficient varies
for different particle size fractions and is highest for PM2.5. Finally, indoor pollution
concentrations depend on the temporal pattern of operation of outdoor sources
(e.g. traffic) but also on indoor sources (e.g. the daily cycle of cooking) and the
decay process (in the case of highly reactive gases such as ozone).
People are exposed to air pollution in all the microenvironments in which they
spend time, and the exposure puts them at risk. A microenvironment is defined
as a three-dimensional space in which the pollutant level is uniform at some
specified time. Exposure is a product of the pollutant concentration and the time
over which a person is in contact with that pollutant. Assessment of exposure
constitutes an element of risk assessment that is schematically represented as a
chain of events from emissions through air pollution concentrations, population
exposure, and body burden and pollutant dose at the organ or cellular level,
to health risk.
In some locations, pollutant concentrations are low but the overall contribution
to the exposure is high because of the longer time spent there (e.g. at home); in
other locations, pollutant concentrations are very high (e.g. at traffic hot spots),
and even short periods of time spent at such locations result in high exposures.
When concentration varies with time, the time-averaged concentration is used
for exposure calculation. For health risk assessment, exposures are defined on
different time domains as (i) lifetime exposure, which is the sum of exposures that
occurred in different environments – this is particularly important for carcinogenic
pollutants; (ii) long-term exposure, measured as a mean of one or several years;
and (iii) short-term exposure, measured over minutes to days.
INTRODUCTION
9
Considering indoor exposures is important because people spend most of their
time in various indoor environments, including home, workplace, school and
commuting (where the microenvironment is a bus, car or train). Indoors is also
where exposure predominantly occurs for vulnerable population groups, as sick
and older people may not venture outside much. Although the exposures occur
indoors, they are caused by both outdoor and indoor sources of emissions, since
outdoor pollutants penetrate indoors, as discussed above.
The most accurate assessment of the risk caused by total air pollution would
be based on the assessment of each individual’s personal exposure, which
would require pollution measurements in each microenvironment in which the
individual spends time and an accurate account of the time spent there (time–
activity diary). Yet, the most accurate assessment of exposure to ambient outdoor
pollution – which is subject to clean air policy-making – may not necessarily
be the measurement of personal exposure, unless the measured indicator of
pollution is clearly and solely of outdoor origin. Presently it is not possible to
measure all of the relevant pollutants in all microenvironments for each individual;
therefore, the approach to exposure assessment is pragmatically based on the
purpose of the assessment. For example, for studies on the long-term impact of
outdoor air pollution (chronic effects), data are typically sourced from a limited
number of monitors operating in some central outdoor locations. This has been
shown to effectively represent population exposure to outdoor pollutants that
are distributed more homogeneously, such as PM2.5 or ozone. More complicated
is exposure assessment for studies on the acute effects of air pollution (such as
mortality or hospital admissions), where spatiotemporal variations in pollution
need to be taken into account. However, for many pollutants, daily concentrations
are often very highly correlated temporally across rather large regions and, thus,
temporal variation may be well captured by single monitors.
Advanced methods of exposure assessment are available, including not only
ground base monitoring of pollution but also the use of satellite observations
and various modelling tools such as chemical transport models and land-use
regression models. Those modelling approaches have overcome some of the
former limitations of reliance on only a few monitoring stations to describe
population exposure in space and time.
1.3.3 Disease and economic burden
Air pollution is the leading environmental risk factor globally. WHO estimates
show that around 7 million deaths, mainly from noncommunicable diseases, are
attributable to the joint effects of ambient and household air pollution (WHO, 2018).
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Similar global assessments of ambient air pollution alone suggest between 4 million
and 9 million deaths annually and hundreds of millions of lost years of healthy life,
with the greatest attributable disease burden seen in low- and middle-income
countries (Burnett et al., 2018; GBD 2019 Risk Factors Collaborators, 2020; Vohra
et al., 2021; WHO, 2018). To date, strong evidence shows causal relationships
between PM2.5 air pollution exposure and all-cause mortality, as well as acute lower
respiratory infections, chronic obstructive pulmonary disease (COPD), ischaemic
heart disease (IHD), lung cancer and stroke (Cohen et al., 2017; WHO, 2018).
A growing body of evidence also suggests causal relationships for type II diabetes
and impacts on neonatal mortality from low birth weight and short gestation
(GBD 2019 Risk Factors Collaborators, 2020). Air pollution exposure may
increase the incidence of and mortality from a larger number of diseases than
those currently considered, such as Alzheimer’s and other neurological diseases
(Peters et al., 2019). The burden of disease attributable to air pollution is now
estimated to be competing with other major global health risks such as unhealthy
diet and tobacco smoking, and was in the top five out of 87 risk factors in the
global assessment (GBD 2019 Risk Factors Collaborators, 2020).
At the time of publishing these guidelines, global burden estimates are limited
to PM2.5 and ozone. Other common pollutants such as nitrogen dioxide and sulfur
dioxide are not yet included and, therefore, these figures based on exposure
to PM2.5 and ozone are likely to underestimate the full health toll from ambient air
pollution. For example, an analysis of the disease burden attributable to nitrogen
dioxide on one outcome, incident paediatric asthma, indicated that nitrogen
dioxide pollution was responsible for 13% of the burden (Achakulwisut et al., 2019).
With a spatial pattern quite different than that for PM2.5, exposure to nitrogen
dioxide resulted in a comparatively high burden in many high-income countries.
Air pollution also leads to health-related economic impacts. Such impacts arise
via two major pathways. The first, human health costs, are those related to the
incidence of disease and mortality and are estimated by a willingness-to-pay
approach. The second is due to lost labour productivity. In 2013 the World Bank
estimated a global economic impact of US$ 143 billion in lost labour income and
of US$ 3.55 trillion in welfare losses from exposure to PM2.5 (World Bank, 2016).
The welfare losses ranged from an equivalent of 1% of gross domestic product in
low-income countries to 5% in high-income countries not within the Organisation
of Economic Co-operation and Development. Apart from the health-related
burden, air pollution causes additional economic costs such as through its
impact on agricultural crops or through damage to buildings and infrastructure.
In addition, there are costs associated with air pollution-related climate change
and environmental degradation.
INTRODUCTION
11
Although some uncertainty surrounding the exact disease burden remains
(discussed in Chapter 8), it is clear that the global burden of disease associated
with air pollution takes a massive toll on human health and the economy worldwide:
exposure to air pollution is estimated to cause millions of deaths and lost years
of healthy life, as well as a loss of trillions of dollars annually. Air pollution is
now recognized as the single largest environmental threat to human health
and well-being.
1.3.4 Inequities and vulnerable and susceptible groups
As already discussed, air pollution from both ambient sources and household use
of polluting fuels is a recognized threat to human health, even at low exposures,
and causes increased mortality and morbidity worldwide.
This burden of disease is unevenly distributed, often disproportionately affecting
the most vulnerable and susceptible populations. The impact of air pollution can
be seen on vulnerable individuals with greater exposure levels and susceptible
individuals with chronic conditions (such as asthma, COPD, diabetes, heart failure
and IHD), as well as children and pregnant women.
According to WHO, health equity is the “the absence of unfair and avoidable
or remediable differences in health among population groups defined socially,
economically, demographically or geographically” (WHO, 2020b). Health
inequities, therefore, involve more than inequality with respect to health
determinants, access to the resources needed to improve and maintain health,
and health outcomes. They also entail a failure to avoid or overcome inequalities
that infringe on fairness and human rights norms.
The fact that this burden of disease and mortality is unevenly distributed also
impedes reduction of inequities and progress towards achieving full human rights
and the UN SDGs. Global efforts to reduce pollution levels will have a positive
impact on lowering inequity (Universal Declaration of Human Rights, Art. 1 and
Art. 2) and will promote the right of life and security by ensuring safe and healthy
environments (as stated in Art. 3) (UN, 1948).
Successful interventions are feasible, effective and compatible with economic
growth. However, only a few studies have looked at equity in health when
evaluating intervention delivery. In general, interventions that aim to reduce air
pollution in urban areas have a positive impact on air quality and mortality rates,
but the documented effect on equity is less straightforward. There is no evidence
on whether applied air pollution reduction interventions have reduced health
inequalities, since results from studies published to date have been mixed and
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not all interventions have had a positive distribution of health benefits. Indeed,
depending on the health outcome(s) under study and intervention type/study
design (simulations of air pollution concentrations or real interventions),
more vulnerable groups such as older persons and deprived households
were found to benefit more, equally or less than their socially better-off
counterparts. For an in-depth review of published studies until the early 2010s,
see Benmarhnia et al. (2014).
The largest inequities in air pollution exposure occur on the global rather
than the local scale. Indeed, countries with policy-driven improvements in air
quality have often seen particularly steep declines in pollution at hot spots since
the 1990s, whereas declines have been gradual in regions with already good
air quality. However, on a global scale, the steep decline in pollution in the
vast majority of high-income countries is paralleled by an unprecedented
increase in low- and middle-income countries. As documented by
Zhang et al. (2017), the model of globalized movements of goods with
inequities in emission and air quality standards contributes to inequity in
air quality (UNEP, 2020). Weak policies in low- and middle-income countries allow
pollution from the production of goods that are ultimately consumed in part in
high-income countries.
1.3.5 Progress on scientific evidence
There has been tremendous progress in the scientific understanding of the health
effects of air pollution since the early 2000s.
First of all, health effects of air pollution have now been studied in most
WHO regions; in contrast, almost all evidence underpinning Global update 2005
came from studies in Europe and North America. This is especially true for studies
of short-term effects on mortality and morbidity (Chen et al., 2017; Yang J et al.,
2020). However, quite a few studies of long-term effects have now also been
reported, especially from Asia and Oceania. These studies have generally found
relationships between air pollutants and ill-health that are qualitatively similar
to those in high-income countries, although the CRFs are sometimes quantitatively
different, with less steep relationships at high than at low concentrations (Yang
X et al., 2020; Hanigan et al., 2019).
Secondly, air pollution has now been implicated in the development or worsening
of several health conditions not considered in previous research. These
include, among others, asthma, diabetes, reproductive outcomes and several
neurocognitive end-points (Yang B-Y et al., 2020; Paul et al., 2019) (Thurston et
al., 2017).
INTRODUCTION
13
Thirdly, many studies have tried to identify which sources and/or physicochemical
characteristics of airborne PM contribute most greatly to toxicity. This is a
challenging area of research, given the great heterogeneity of airborne particles,
and a definitive set of particle characteristics has yet to be identified. However,
in its 2013 review of the evidence (WHO Regional Office for Europe, 2013a),
WHO did point out that a focus on primary combustion particles, secondary
inorganic aerosols and secondary organic aerosols was warranted (Thurston et
al., 2016b; US EPA, 2019a; Lippmann et al., 2013; Vedal et al., 2013).
Lastly, investigators have learned to collaborate on an unprecedented scale. Prior
to 2005, there were few examples of multicentre studies in the domain of timeseries studies investigating the short-term effects of air pollution; two notable
examples are the Air Pollution and Health, a European Approach (APHEA) studies in
Europe and the National Morbidity and Mortality Air Pollution Study (NMMAPS) in
the United States of America. These were followed after 2005 by the Air Pollution
and Health: A European And North American Approach (APHENA) study across
Europe, Canada and United States (Samoli et al., 2008); the ESCALA (Estudio de
Salud y Contaminación del Aire en Latinoamérica) study in Latin America (Romieu
et al., 2012); and the Public Health and Air Pollution in Asia (PAPA) study in Asia
(Wong et al., 2008) – all studies of short-term effects. A remarkable culmination
is the Multi-Country Multi-City (MCC) Collaborative Research Network (Chen et
al., 2021; Liu et al., 2019; Meng et al., 2021; Vicedo-Cabrera et al., 2020), which
combines multiyear data from 652 cities across the world in a single joint analysis
of the short-term effects of PM2.5, ozone, nitrogen dioxide and carbon monoxide,
among other studies. Large collaborations have also emerged in studies of longterm effects such as the European Study of Cohorts for Air Pollution Effects
(ESCAPE), which includes data from 36 different cohorts (Beelen et al., 2014).
Another example is the Global Exposure Mortality Model (GEMM), which includes
data from 41 cohorts from 16 countries across the globe (Burnett et al., 2018).
Finally, an ongoing collaboration is studying the long-term health effects of low
levels of air pollution in Europe (HEI, 2021), Canada and the United States (Brauer
et al., 2019; Dominici et al., 2019).
Collectively, these studies have considerably strengthened the evidence for health
effects of air pollution by increasing study power and using highly standardized
preplanned methods of data collection, analyses and reporting (Brauer et al.,
2019; Di et al., 2017a).
Methods of assessing exposure to air pollution have become much more refined.
In 2005 the annual air quality guideline for PM2.5 was largely based on results
from two studies, the Harvard Six Cities study (Dockery et al., 1993) and the
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WHO GLOBAL AIR QUALITY GUIDELINES
American Cancer Society Cancer Prevention Study II (Pope et al., 2002). In these
studies, exposure to PM2.5 was assessed from one or a few monitoring sites per
city. In addition, advanced chemical transport models, land-use regression
models, satellite observations and much more detailed ground-level monitoring
have formed the basis for very detailed assessment of exposure to PM2.5 (as
well as other pollutants) at very fine temporal and spatial scales. This has been
useful not only for population studies of health effects but also for estimating
the worldwide health impact of air pollution (Hammer et al., 2020; de Hoogh et
al., 2018).
These new methods of exposure assessment have facilitated studies of
nationwide populations, not only those living in cities but also those living in
rural areas where air pollution monitoring is sparse or even absent. Often, these
nationwide studies make use of administrative databases, which have increasingly
become automated. These include death registers, disease registers, census data
and population statistics. Such studies have the advantage of often including
large populations of millions or even tens of millions of subjects. In addition,
the data included are often more representative of underlying populations than
regular cohort studies. A disadvantage of such databases is that they usually do
not contain much information on potential confounding and modifying factors
such as smoking and diet. However, innovative solutions have been developed
to deal with this (e.g. survey results in Medicare and indirect adjustment for
covariates in Canadian census studies) (Crouse et al., 2015; Cesaroni et al., 2013).
Such databases usually also lack information from biological markers and
specimens and, thus, cannot shed light on biological pathways to explain the
observed associations.
Advances in statistical analyses techniques and conceptualization of causal
modelling in epidemiology have produced new insights into the robustness
of epidemiological associations between air pollutants and health effects.
Machine learning techniques are increasingly being applied to explain patterns
in complex exposure patterns. Most recently, large collaborative studies of the
so-called exposome (defined as the totality of exposure individuals experience
over their lives and how these exposures affect health) have started in an attempt
to understand the effects of lifelong exposures to complex environmental factors
on the development of health and disease throughout the life course. In such
studies, air pollution is regularly included as one of several sets of complex
environmental exposures and is combined with individual data, ranging from the
molecular, genetic or cellular level up to the level of social, cultural and lifestyle
data (Vrijheid et al., 2020).
INTRODUCTION
15
Decision-makers have increasingly asked for reliable estimates of the burden
of disease caused by air pollution as input for cost–benefit analyses of policy
alternatives and as a basis for risk communication. Since 2005, major steps forward
have been taken, especially by WHO and the Global Burden of Disease (GBD)
project. An innovative, integrated exposure–response function was developed,
integrating insights from studies on outdoor air pollution, on the health effects
of indoor exposure to household air pollution from solid fuel combustion and
environmental tobacco smoke, and on active smoking (Burnett et al., 2014). The
integrated exposure–response function formed the basis for the first-ever truly
global burden of disease estimate from exposure to PM2.5, ozone and household
air pollution from solid fuel burning, published in 2012 (Lim et al., 2012). These
estimates used the global exposure estimates mentioned in section 1.3.1 and
worldwide data on mortality and morbidity. They have been updated several
times as new exposure estimates became available, and the integrated exposure–
response function was updated based on new study findings (Cohen et al., 2017).
The latest version no longer includes studies on active smoking, for instance
(GBD 2019 Risk Factors Collaborators, 2020). Widely available software tools,
such as WHO AirQ+ (WHO Regional Office for Europe, 2021a) or the US EPA’s
Environmental Benefits Mapping and Analysis Program – Community Edition
(BenMAP-CE) (US EPA, 2021) facilitate similar analysis on a local (city, region,
country) level.
Decision-makers have also sought evidence that measures to reduce air
pollution actually produce health benefits. So-called accountability research
(i.e. assessment of the effectiveness of interventions) addresses the
consequences of policy interventions. An early example is a study from Dublin
suggesting that a ban on coal burning led to reduced mortality (Clancy et al., 2002;
Dockery et al., 2013). A nationwide study from the United States found that life
expectancy increased most in areas where fine particle concentrations decreased
the most (Pope, Ezzati & Dockery, 2009). A research programme on this subject,
developed by the United States-based Health Effects Institute (HEI), showed
promise, as well as pitfalls (Boogaard et al., 2017), while a Cochrane review on
interventions to reduce ambient air pollution and their effects on health concluded
that more research is needed in this area to reduce uncertainty (Burns et al., 2019).
Another issue of great interest to decision-makers is the that the co-benefits of
policies aimed at reducing greenhouse gases may also have adverse direct or
indirect health effects (e.g. methane, a powerful greenhouse gas and an ozone
precursor) or, conversely, that policies aimed at reducing health-relevant air
pollutants (such as black carbon) may also have climate forcing capabilities.
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WHO GLOBAL AIR QUALITY GUIDELINES
1.3.6 Adoption of the 2005 air quality guidelines worldwide
The first two editions of the air quality guidelines in 1987 and 2000 were successful
in providing guidance, mostly to European countries, and provided the basis
for the European Union (EU) legislation on air quality. Global update 2005 was
intended to be relevant to the diverse conditions within all WHO regions.
Evidence-informed guidance on air quality and associated health effects
is necessary so that countries can use this information in standard setting
and in providing information to the public. In 2012 a review of the processes
followed to establish national ambient air quality standards (AAQS) for PM10
and sulfur dioxide (24-hour average) in the period 2007–2008 concluded that
WHO air quality guidelines were the resource used most often to establish or
revise national standards by the relevant authorities (Vahlsing & Smith, 2012).
At that time, 91% of the countries that responded to a survey planned on using
Global update 2005 for future revision of their AAQS; however, this information
was only available for 96 countries. In collaboration with WHO, the Swiss Tropical
and Public Health Institute (Swiss TPH) has compiled information on the existence
of legally binding AAQS for all UN Member States for PM (PM2.5, PM10 and other
relevant types), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide
for different averaging times (both long and short term) (Kutlar Joss et al., 2017;
WHO, 2021b). This unique update of the current state of AAQS worldwide provides
a useful insight into the degree to which the 2005 air quality guidelines and
interim targets are used as a basis for legally binding and non-binding AAQS.
Information was identified for over 170 countries in the different WHO regions, of
which 53 did not define any standards (see Table 1.1). In general, standards for
short-term exposure were set more often than annual limit values. Levels varied
greatly by country and by air pollutant.
Daily mean standards for PM10 and sulfur dioxide (averaging time ≤ 24 hours) and
1-hour maximum values for nitrogen dioxide were most often defined. Although
compliance with WHO air quality guidelines was rather low, it was generally higher
for short-term than for long-term standards. Among all countries with standards
for 24-hour averaging times for PM2.5 and PM10, 21% and 46% met the air quality
guidelines, respectively. In contrast, only seven countries (2%) adopted the
WHO annual mean air quality guidelines for PM10 and PM2.5. In case of sulfur dioxide
(24 hours), only 7% of countries were in line with the air quality guidelines and
16% aligned their standard with the 1-hour guidelines for ozone. Adoption rates
were higher for nitrogen dioxide, sulfur dioxide (10-minute averaging time) and
carbon monoxide.
In addition, in the EU, WHO guidelines are referenced in the Ambient Air Quality
Directive (European Parliament & Council of the European Union, 2008),
INTRODUCTION
17
and several countries use/will use WHO air quality guidelines and/or interim targets
within existing and forthcoming legislation.
Analysis of the level of adoption of WHO air quality guidelines (see Table 1.1) shows
that many countries have guidelines or standards for at least one air pollutant;
however, there are many countries without standards or where information
is lacking. The gap between the WHO air quality guidelines and the levels
adopted in national regulations reflects the policy-making process. Whereas the
WHO guidelines are evidence-informed, health-oriented recommendations,
the process of developing legally binding regulations is driven by national
policy-makers and the willingness to set environmental standards. This process
involves different actors and may be influenced by a range of considerations.
Table 1.1. Adoption of WHO air quality guidelines in different regions
WHO region
Countries in
the region
(n)
Countries with
standards for
at least one
pollutant and
averaging time
Countries
without
standards
Countries
with no
information
n
%
n
%
n
%
African Region
47
17
36
21
45
9
19
Region of the Americas
35
20
57
13
37
2
6
South-East Asian Region
11
7
64
3
27
1
9
European Region
53
50
94
2
4
1
2
Eastern Mediterranean Region
21
11
52
1
5
9
43
Western Pacific Region
27
12
44
13
48
2
7
Total
194
117
60
53
27
24
12
Source: Kutlar Joss et al. (2017).
The difficulty of attaining the air quality guidelines for PM and other pollutants
was recognized in Global update 2005, and a series of interim targets were
set to provide milestones for countries on the way to achieving the air quality
guidelines. Interim targets were defined as air pollutant levels that are higher
than the air quality guidelines, but which authorities in highly polluted areas can
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WHO GLOBAL AIR QUALITY GUIDELINES
use to develop pollution reduction policies that are achievable within realistic
time frames. The interim targets should be regarded as steps towards ultimately
achieving air quality guidelines in the future, rather than as end targets. The
number and numerical values of the interim targets are pollutant specific and
they are justified in the relevant sections of Chapter 3.
1.3.7 Air pollution and health in the global agenda
World Health Assembly resolution and road map
In May 2015 the Sixty-eighth World Health Assembly adopted resolution WHA68.8,
Health and the environment: addressing the health impact of air pollution, which was
endorsed by 194 WHO Member States (WHO, 2015). This resolution stated the need
to redouble the efforts of Member States and WHO to protect populations from the
health risks posed by air pollution. Member States were urged to raise public and
stakeholder awareness on the impacts of air pollution on health; provide measures
to reduce or avoid exposure; facilitate relevant research; develop policy dialogue,
strengthen multisectoral cooperation at national, regional and international levels;
and take effective steps to reduce health inequities related to air pollution.
Specifically, the resolution recognized the role of the WHO air quality guidelines,
for both ambient and indoor air quality, in providing guidance and recommendations
for clean air that protect human health. It requested the Director-General to
strengthen WHO capacities in the field of air pollution and health through further
development and regular updating of the WHO air quality guidelines to facilitate
effective and efficient decision-making, and to provide support and guidance to
Member States in their efficient implementation. A road map for implementation
of this resolution on air pollution and health was presented at the Sixty-ninth
World Health Assembly and approved by Member States (WHO, 2016a).
UN Sustainable Development Agenda and other UN processes
The WHO air quality guidelines support the strategic priorities for NCDs
(UN, 2018a), as well as those established in the 2030 Agenda for Sustainable
Development, which was adopted at the United Nations Sustainable
Development Summit in 2015 (UN, 2015). These priorities emphasize the need
to strengthen national capacities to reduce modifiable risk factors, including
air pollution, for NCDs and to accelerate countries’ responses for their
prevention and control. The 17 SDGs contained in the Agenda present an
indicator framework for global monitoring and include 169 specific associated
targets (UN Statistics Division, 2020).
INTRODUCTION
19
These, in turn, are divided into indicators, thereby providing a tool for quantitative
assessment of achievement towards meeting the goals. This update of the WHO
air quality guidelines provides evidence-informed benchmarks on the health
impacts of air pollution, and will help assess the following air pollution-related SDG
indicators to inform the health trends associated with exposure to air pollution:
■
■
■
Indicator 3.9.1: Mortality rate attributed to household and ambient air pollution
Indicator 7.1.2: Percentage of population with primary reliance on clean fuels
and technology
Indicator 11.6.2: Annual mean levels of fine PM (population-weighted).
The health impacts of air pollution are a main driver for action by the environment
sector. The UN Environment Assembly adopted the following three resolutions
on the topic.
■
■
■
Resolution 1/7 from the United Nations Environment Programme (UNEP),
adopted at its first session in 2014 on Strengthening the role of the United
Nations Environment Programme in promoting air quality, highlights the effects
of air pollution, especially from a perspective of sustainable development. In
particular, it encourages governments to take cross-sectoral action to improve
air quality and formulate action plans while establishing (and implementing)
nationally determined air quality and emissions standards, taking into account
relevant information (e.g. WHO guidelines) (UNEP, 2014).
Additionally, the UN Environment Assembly presented a resolution, at its
second session in 2016, requesting the Executive Director to engage with
all relevant UN entities to promote a coordinated approach to combating the
challenges of SDS globally by supporting Member States in the identification
of relevant data and information gaps, best policy measures, and actions to
address the problem and by inviting them to intensify monitoring data collection
and knowledge sharin…