Ambient air pollution is a broader term used to describe air pollution in outdoor environments. Poor ambient air quality occurs when pollutants reach high enough concentrations to affect human health and/or the environment. Urban outdoor air pollution is a more specific term referring to the ambient air pollution experienced by populations living in urban areas, typically in or around cities.
Updated 2018
SECTION I: HEALTH IMPACTS OF AMBIENT AIR POLLUTION
Q.1 What is ambient air pollution versus urban outdoor air pollution?
Ambient air pollution is a broader term used to describe air pollution in outdoor environments. Poor ambient air quality occurs when pollutants reach high enough concentrations to affect human health and/or the environment.
Urban outdoor air pollution is a more specific term referring to the ambient air pollution experienced by populations living in urban areas, typically in or around cities.
Q.2 What are the health consequences of ambient air pollution?
Exposure to high levels of air pollution can cause a variety of adverse health outcomes. It increases the risk of respiratory infections, heart disease, stroke and lung cancer. Both short and long-term exposure to air pollutants have been associated with health impacts. More severe impacts affect people who are already ill. Children, the elderly and poor people are more susceptible. The most health-harmful pollutants - closely associated with excessive premature mortality - are fine PM2.5 particles that penetrate deep into lung passageways.
Although air quality in high-income countries has been generally improved over the last decades, the adverse health effects of particulate air pollution, even at relatively low levels, remain a global public health concern.
Q.3 What is particulate matter - PM2.5 and PM10?
Particulate matter, or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. Large concentrations of particulate matter are typically emitted by sources such as diesel vehicles, waste and crop burning, and coal-fired power plants.
Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the respiratory system.
Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and pose the greatest health risks. Because of their small size (approximately 1/30th the average width of a human hair), fine particles can lodge deeply into the lungs.
Q.4 How large is the disease burden associated with ambient air pollution?
Worldwide 4.2 million premature deaths are attributable to ambient air pollution in 2016. About 88% of these deaths occurs in low and middle-income countries.
Regional breakdown (low and middle-income countries)
- South East Asia: 1 332 000 deaths
- Western Pacific: 1 255 000 deaths
- Africa: 425 000 deaths
- Eastern Mediterranean: 319 000 deaths
- Europe:304 000 deaths
- Americas:164 000 deaths
Regional breakdown (high income countries)
- European: 205 000 deaths
- Americas: 95 000 deaths
- Western Pacific: 82 000 deaths
- Eastern Mediterranean: 17 000 deaths
Q.5 What information does WHO collect on ambient (outdoor) air pollution exposures?
WHO maintains a worldwide, public database on outdoor air pollution in its Global Health Observatory.
The database contains measured outdoor air pollution levels of PM2.5 and PM10 from over 4000 cities in 108 countries for the years 2010-2016. These are used as input for estimating mean annual exposures of both urban and rural populations to fine particulate matter. Links to the database and further information on how the data is collected can be found here.
In 2013, WHO formed a broad collaboration WMO, UN Environment, other major institutions and UN agencies, country representatives and researchers, called the Global Platform on Air Quality and Health (the Platform). The Platform meets on a regular basis to review the current evidence on air pollution and health including current epidemiological literature on air pollution related mortality and morbidity; methods for enhanced air quality and health monitoring as well as source apportionment; identifies research priorities and opportunities for collaboration across agencies and institutions.
Q.6 How is WHO responding to ambient air pollution effects on health?
WHO's main function is to identify and monitor those air pollutants with the greatest impact on people's health. This helps the WHO Member States to focus their actions on the most effective way to prevent or reduce health risks. WHO's task is to review and analyze the accumulated scientific evidence and use expert advice to draw conclusions on how much different air pollutants affect health as well as identify effective measures to reduce the air pollution burden.
Q.7 What steps should be taken to prevent the health impacts from ambient air pollution?
Governments can identify their main sources of ambient air pollution, and implement policies known to improve air quality and public health, such as: promotion of public transport, walking, and cycling (rather than transport relying on private motor vehicles); promotion of power plants that use clean and renewable fuels (e.g. not coal), and improvements in the energy efficiency of homes, commercial buildings and manufacturing.
Essential accompanying steps include increasing awareness about the high disease burden from ambient air pollution and its main sources, as well as highlighting the importance of taking action now to implement country-specific interventions. In addition, the use of effective monitoring to evaluate and communicate the impact of interventions is also an important tool in raising awareness. It can help drive policy action that brings benefits for health, climate and the environment.
The health sector itself has role to play at both the policy and patient level to prevent the health impacts from air pollution. Public health professionals working at the policy level can advocate for health benefits in policies that impact air pollution and health. At the clinician level, doctors, nurses and community health workers can talk to their patients about the health risks from air pollution, and the available interventions to protect oneself or minimize the impacts of exposure to high levels of air pollution (e.g. staying indoors, reducing exercise among vulnerable populations during acute episodes of high air pollution).
Q.8 WHO maintains that reducing ambient air pollution can lead to substantial reductions in mortality. Is this true and where might I find further information?
By reducing the annual mean ambient concentration of PM10 from 70 to 20 µg/m3, and PM2.5 from 35 to 10 µg/m3 , in line with the target levels by the 2005 WHO Air Quality Guidelines, a reduction of 15% of long-term mortality risk can be achieved (http://www.who.int/airpollution/publications/aqg2005/en/).
WHO estimates that 12.5% of deaths could be averted by improving air quality worldwide. Lower levels of air pollution will reduce the burden of respiratory and cardiovascular disease-related illnesses, health-care costs, and lost worker productivity due to illness, as well as increasing life expectancy among local populations.
In addition, actions that reduce ambient air pollution will also cut emissions of short-lived climate pollutants, particularly black carbon which is a major component of soot emissions from diesel vehicles, and other sources, as well as greenhouse gases (CO2) contributing to longer-term climate change impacts. Climate change produces a number of adverse effects on health. This includes those from drought and extreme weather events (e.g. windstorms, floods), such as water-borne and food-borne diseases. It also increases the prevalence of vector-borne diseases like dengue or malaria.
Q.9 What are the implications for public health?
Public health recognizes air pollution as an important determinant of health. Today this is especially the case in low and middle-income countries where exposure to air pollution is now higher than in higher income countries, and where mitigation measures led to reductions in exposure. There is significant inequality in the exposure to air pollution and related health risks: air pollution combines with other aspects of the social and physical environment to create a disproportional disease burden in the vulnerable and disadvantaged populations.
Exposure to air pollutants is largely beyond the control of individual and requires action by public authorities at the national, regional and even international levels.
The health sector can play a central role in leading a multisectoral approach to the prevention of exposure to air pollution. It can engage and support other relevant sectors (transport, housing, energy production and industry) in the development and implementation of long-term policies to reduce the risks of air pollution to health.
Q.10 Are there are other risk factors that could contribute to these deaths from outdoor air pollution?
Mortality from ischaemic heart disease and stroke are also affected by risk factors such as high blood pressure, unhealthy diet, lack of physical activity, smoking, and household air pollution. Some other risks for childhood pneumonia include suboptimal breastfeeding, underweight, second-hand smoke, and household air pollution. For lung cancer, and chronic obstructive pulmonary disease, active smoking and second-hand tobacco smoke are also main risk factors. These risk factors may contribute to deaths that are caused by ambient air pollution.
SECTION II: PUBLIC HEALTH POLICY FOR AMBIENT AIR POLLUTION
Q.1 What are public policies that can reduce the health impacts of ambient air pollution?
Reducing the public health impacts of ambient air pollution requires addressing the main sources of the air pollution, including inefficient fossil fuel combustion from motor vehicle transport, power generation, agriculture practices, waste and crop burning and inefficient energy use in building, homes and manufacturing.
Reducing the health effects from ambient air pollution requires action by public authorities at the national, regional and even international levels. Individuals can contribute to improving air quality by choosing cleaner options for transport, energy use, energy production, and waste disposal.
The public health sector can play a leading role in instigating a multisectoral approach to prevention of exposure to ambient air pollution, by engaging with and supporting the work of other sectors (i.e. transport, housing, energy, industry) to develop and implement long-term policies and programs aimed to reduce air pollution and improve health.
Q.2 What challenges do countries face, and what obstacles are preventing assistance in improving ambient air quality?
Enough knowledge exists internationally about the health effects of ambient air pollution but what often can limit policy development for the improvement of air quality, is the lack of access to information on levels of air pollutants and their main sources of pollutants.
Also, there is often a lack of awareness about the health burden of ambient air pollution. This can be due to little awareness of the international evidence from low, and middle-income countries linking ambient air pollution exposure and health, or due to a gap in information from air quality monitoring, or even due to an under-appreciation of the potential solutions and measures that can be taken to improve air quality.
Improving ambient air pollution is an inter-sectoral challenge. Improving air quality should be an important consideration in policy planning across different economic sectors (e.g. transport, energy, industry, urban development) to ensure the greatest benefits for health.
Inaddition, there is significant inequality in the exposure to air pollution and the related health risk: air pollution combines with other aspects of the social and physical environment, creating disproportional disease burden in populations with limited incomes and with minimal local resources to take action.
Q.3 Why are WHO country and regional estimates different from national or regional estimates (e.g. European ones)?
One of WHO's role is to monitor health trends and therefore there is a need to provide data that is internationally comparable. The methodology used should apply to all countries, irrespective of their levels of exposure, the health status of their population, and the amount of available exposure and health data. Therefore, the use of modelled data, which may differ from measured data (e.g. exposure to air pollution, health statistics) and which may almost inevitably lead to discrepancies with country published assessments.
First of all, the type of pollutant(s) considered in the analysis is crucial, and WHO is providing burden of disease estimates for outdoor air pollution from particulate matter (PM2.5) only, while countries may assess additional pollutants, such as ozone or nitrogen dioxide.
Second, the type of health outcomes (or diseases) included is key. WHO is using the comparative risk assessment method to derive its estimates and is therefore using cause-specific diseases. There are currently five diseases considered, e.g. lower respiratory infections, lung cancer, chronic obstructive pulmonary diseases, ischemic heart disease, and stroke. Some countries may use different or additional diseases or diseases groups (e.g. cardiopulmonary mortality) or all-cause, non-accidental mortality in their assessments, which may lead to significant differences in the estimates.
Moreover, there are three additional important ingredients needed to derive burden of disease estimates from air pollution in addition to the two mentioned above. These are:
1) Exposure: distribution of exposure levels within the population,
2) Exposure-response functions derived from the published literature,
3) Counterfactual or "ideal" exposure levels, that represents the exposure level at which minimal health impacts occur
Population exposure to air pollution from particulate matter used by WHO for its assessment are based on a global model using all available surface monitoring data, satellite observation and atmospheric transport models. One of the advantage of this approach is an ability to estimate risks in countries with no, or little, locally collected data. Exposure distributions produced by such models for larger regions agree well with the distributions from surface observations and are more reliable than estimates for smaller ones. With the progress of the methods and better availability of locally collected data (from air quality monitoring and emissions inventories), as well as with improvement of the spatial resolutions of atmospheric models and satellite data, national and sub-national estimates are increasingly becoming available. Countries, on the other end, use their own ground monitored data alone or combined with other locally available data in their exposure assessment, which can lead to different results than the global model.
WHO is using a set of integrated exposure response (IER) functions to derive its estimates. The IER combine the epidemiological evidence for outdoor air pollution, second-hand smoke, household air pollution and active smoking to estimate the level of disease risk (e.g. stroke) at different levels of PM2.5 concentrations, in order to fill the gaps in scientific evidence from outdoor air pollution at higher level of PM2.5. Countries or regional bodies however may use local epidemiological evidence, which may differ slightly.
As counterfactual level of exposure, WHO is currently using a cut-off defined as a uniform distribution - rather than a fixed value - between 2.4 and 5.9 ug/m3 based on the currently available data on the lowest value for which epidemiological studies are available. Different counterfactual may be used, like the lowest value for which data is available, or the natural level of exposure, or an exposure level under which no adverse health effects are expected to occur.
All these ingredients contribute to differing health estimates on the impacts of air pollution on health.
Q.4 How will reducing the number of cars in circulation in a large city reduce ambient air pollution and have a benefit on health and well-being?
Air pollution very often has multiple sources and acting on a single one may not bring enough benefits for health. Although most newer motor vehicles have more efficient engines and are using cleaner fuels, the absolute number of vehicles is still increasing in many cities worldwide -- and consequently so are levels of ambient air pollution. Indeed, in parts of Europe where stricter standards and regulations for vehicles have been enforced, ambient air pollution levels are stable or decreasing, yet air quality levels are still above the WHO AQG values. Per kilometer of travel, diesel vehicles also typically emit more particulate emissions than gasoline, gas-powered, or electric vehicles of comparable size and age -so that increased reliance on diesel vehicles in the vehicle fleet may be a contributing factor to health-harmful air pollution in many cities. Diesel emissions have also been defined by WHO's International Agency for Research on Cancer as a carcinogen.
Building cities around rapid public transport systems, complemented by dedicated walking and cycling networks are other ways to tackle the health impacts from ambient air pollution. This also tends to facilitate a "virtuous cycle" of more compact cities, more energy efficient housing, fewer private car trips, and thus fewer air pollution emissions overall. This helps minimize the health burden from ambient air pollution - as well as encouraging healthful active transport on safe walking and cycling networks, where people are at less risk of traffic injury.
However, it should be kept in mind that transport may be directly responsible for anywhere from 15 to 70% of urban ambient air pollution in urban areas, depending on the city, so a holistic approach involving energy, industry, and building sectors, together with the transport sector, is required to reduce the disease burden from ambient air pollution in an efficient way.
SECTION III - HEALTH IMPACTS FROM HOUSEHOLD AIR POLLUTION
Q.1 What is household air pollution?
Household air pollution (HAP) from the inefficient combustion of solid fuels (i.e. wood, coal, charcoal, crop waste, dung) and kerosene is the one of leading environmental risk factor for death and disability in the world, particularly among the poorest and marginalized populations, including women and children. It was previously described as indoor air pollution (IAP) but acknowledging that human exposure from the air pollution created by the household combustion of solid fuels and kerosene is not limited to the indoor environment, this risk factor has been renamed as HAP to more fully capture the health-based risks associated with it.
Q.2 What are the health impacts from exposure to household air pollution?
Exposure to household air pollution is associated with a wide range of child and adult disease outcomes, including respiratory conditions such as acute lower respiratory infections (e.g. pneumonia), chronic obstructive pulmonary disease, lung cancer, stroke and cardiovascular diseases.
Exposure has been linked with other health outcomes including other cancers (e.g. cervical), adverse pregnancy outcomes (e.g. low-birth weight), cataract (especially among women), cognitive impairment and tuberculosis.
Q.3 How large is the disease burden from household air pollution?
Household air pollution is the second largest environmental-health risk among low and middle-income countries, estimated to be responsible for 3,8 million deaths in 2016 (6,7% of total mortality).
Q.4 What are some of the major sources or causes of household air pollution?
The incomplete combustion of kerosene and solid fuels (i.e. wood, coal, charcoal, crop waste, dung) from the use of open fires or in poorly vented simple stoves for cooking, heating and lighting is one of the major sources of household air pollution.
The amount and relative proportion of toxic air pollutants generated by incomplete combustion of fuels in the home are dependent on a number of factors, including fuel type and moisture content, household ventilation, the behavior of the people using the stoves, and the stove technology. The emitted toxic pollutants include particles of varying sizes, carbon monoxide, volatile and semi-volatile organic compounds, and several others. Combustion of coal, in addition to the above pollutants, releases sulfur oxides, heavy metals such as arsenic, and fluorine which also have very negative consequences on health.
Q.5 Are there health effects from both short and long-term exposure to household air pollution?
Both long and short-term exposure to household air pollution has impacts on health. Long-term or chronic exposure over the lifetime increases the risk for non-communicable diseases like respiratory cancers, cardiovascular disease, stroke and chronic obstructive pulmonary disease. Shorter term exposure, days to months, can lead to more acute health outcomes like pneumonia and adverse pregnancy outcomes. Household members with pre-existing conditions like asthma and heart disease can see adverse health impacts from just hours of exposure to very high levels of household air pollution.
Q.6 How does WHO estimate the population exposed to household air pollution?
WHO uses the percentage of households in low and middle-income countries primarily cooking with solid fuels or kerosene as a proxy indicator for exposure to household air pollution. The percentage of households cooking with such ‘polluting fuels and technologies' combinations is estimated for a particular year using a statistical model based on household survey data from the WHO Household Energy database. This database contains compiled information on primary cooking fuel used and cooking practices from over 1100 nationally representative data sources like national census, national living standard and welfare surveys the USAID's Demographic Health Survey, UNICEF's Multiple Indicator Cluster Survey and the World Bank's Living Standards Measurement Survey. See the Global Health Observatory.
WHO's household air pollution exposure estimates have been a vital source of data for the tracking & monitoring of household energy use & health impacts for over a decade.
Q.7 Which regions and countries across the world are most affected?
WHO uses the percentage of households in low and middle-income countries primarily cooking with solid fuels and kerosene as a proxy indicator for exposure to household air pollution. According to recent estimates for 2016, exposure to household air pollution is most prevalent in the lower and middle-income countries (LMICs) of the African region where on average 83% of households primarily use polluting fuels for cooking. In LMICs of the Southeast Asia region, 59% of house¬holds mainly cook with solid fuels or kerosene and in other LMICs in other WHO regions, primary polluting fuel use ranges from 42% in the Western Pacific region, to 31% in the Eastern Mediterranean and < 15% in the Americas and Europe.
In some countries like Ethiopia and Rwanda, WHO estimates that over 95% of the population relies on solid fuels, suggesting that almost the entire population of these countries is exposed to household air pollution on a regular basis. WHO estimates that over 1 billion people each in China and India rely primarily on solid fuels for cooking.
Q.8 How is WHO responding to household air pollution effects on health?
Many efforts are underway to disseminate clean home energy solutions but there is a gap in our understanding of the most effective interventions at protecting health and data collection on the issue at both a national and global level. WHO is addressing these challenges through its normative guidance found in the WHO guidelines for indoor air quality: household fuel combustion work to support countries and other stakeholders to implement the WHO Guidelines for indoor air quality: household fuel combustion and building the capacity in country to achieve the SDG goal 7 target of aiming to achieve universal access to clean fuels and technologies by 2030.
Accordingly, WHO is developing a clean household energy solutions toolkit:
(i) improve the monitoring tools (e.g. national surveys) of home energy solutions & their health impacts,
(ii) enhance its global household energy database to include more indicators and tools for assessing health impacts, including data on the fuels and technologies used for space heating, lighting and to supplement cooking practices (i.e. fuel-stove stacking)
(iii) review and compile the scientific evidence on the health, safety (i.e. burns, poisonings) and livelihood impacts (i.e. time spent collecting fuel) of household air pollution,
(iv) work with countries to implement effective monitoring of air quality and its health impacts in both permanent and transitional household settings.
Q.9Are there are other risk factors that could contribute to these deaths from household air pollution?
Mortality from ischaemic heart disease and stroke are also affected by risk factors such as high blood pressure, unhealthy diet, lack of physical activity, smoking and ambient air pollution. Some other risks for pneumonia include active smoking, suboptimal breastfeeding, underweight, and second-hand smoke. For lung cancer, and chronic obstructive pulmonary disease, active smoking and second-hand tobacco smoke are also main risk factors.
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SECTION III WHO AIR QUALITY GUIDELINES AND OTHER INFORMATION RESOURCES
Q.1How do the WHO Air Quality Guidelines support the Member States?
The WHO Air Quality Guidelines (AQGs) inform policy-makers about the health impacts of air pollutants and provide appropriate targets for air quality that is safe for health. Countries can select among a broad range of policy options for the most appropriate to improve air quality and achieve a better protection of people's health.
The Guidelines are intended for a global audience. They have been developed to support actions for healthy air quality across different contexts. At the same time, they acknowledge the need for each country to set up its own air quality standards to protect the public health of their citizens based on local conditions.
WHO's Air Quality Guidelines are available at: http://www.who.int/airpollution/guidelines/en/ The WHO Air Quality Guidelines are currently under revision, and release of the update is planned for 2020.
Q.2Have there been any new guidelines on the health impact of ambient air pollution since the WHO's Air Quality Guidelines, Global Update 2005?
The Air Quality Guidelines, Global Update 2005 is still the most authoritative WHO document on air quality and health which provides a comprehensive review of the scientific evidence. The Guidelines set target outdoor concentrations for particulate matter (PM), nitrogen dioxide (NO2), sulfur dioxide (SO2) and ozone (O3) that would protect the large majority of individuals from the ill effects of air pollution on health. The guidelines can be accessed here.
The WHO REVIHAAP (Review of evidence on health aspects of air pollution) project was implemented in 2012-13 evaluated the newly accumulated evidence on health aspects of air pollution. Its results show that this new evidence supports the scientific conclusions of the WHO air quality guidelines, last updated in 2005, and indicates that the effects in some cases occur at air pollution concentrations lower than those serving to establish these guidelines. The report can be accessed here.
Q.3Are there are guidelines on the health impacts on indoor air quality or household air pollution?
Three sets of guidelines for indoor air quality have been published by WHO. They address health problems related to (1) dampness and mold (2) the selected pollutants common indoors and (3) household fuel combustion. These Indoor Air Quality Guidelines are available at: http://www.who.int/airpollution/guidelines/en/