Ambient (Outdoor) Air Pollution

Primary reference(s)

WHO, 2018. Ambient (outdoor) air pollution. World Health Organization (WHO). Accessed 26 October 2020.

Additional scientific description

Ambient (outdoor) air pollution is a major cause of death and disease globally. Long-term exposure to air pollution (over years or lifetimes) reduces life expectancy, mainly due to cardiovascular and respiratory diseases and lung cancer. The World Health Organisation (WHO) estimated that ambient air pollution caused 4.2 million premature deaths globally in 2016, of which 58% were due to ischaemic heart disease and strokes, 18% to chronic obstructive pulmonary disease and acute lower respiratory infections respectively, and 6% to lung cancer (WHO, 2018). Short-term exposure (over hours or days) to elevated levels of air pollution can also cause a range of health impacts, including effects on lung function, exacerbation of asthma, increases in respiratory and cardiovascular hospital admissions and mortality. Emerging evidence suggests that air pollution may also affect the brain with possible links to dementia and cognitive decline and may also have an effect on early life, such as low birth weight.

Ambient air pollution contains a range of pollutants (particles and gases) from a variety of sources, both natural and man-made (e.g., transport, industry, agriculture). Pollutants with the strongest evidence for public health concern, include particulate matter (PM), ozone (O3), nitrogen dioxide (NO2) and sulphur dioxide (SO2) (WHO, 2020).

‘Particulate matter’ is a generic term used to describe a complex mixture of solid and liquid particles of varying size, shape, and composition. Some particles are emitted directly (primary PM); others are formed in the atmosphere through complex chemical reactions (secondary PM). The composition of PM varies greatly and depends on many factors, such as geographic location, emission sources and weather. The size of particles and the duration of exposure are key determinants of potential adverse health effects. Particles larger than 10 μm are mainly deposited in the nose or throat, whereas particles smaller than 10 μm pose the greatest risk because they can be drawn deeper into the lung. The health risks associated with PM of less than 10 and 2.5 microns in diameter (PM10 and PM2.5, respectively) are especially well documented. The strongest evidence for effects on health is associated with fine particles (PM2.5) (WHO, 2018; PHE, 2019).

Although air pollution can be harmful to everyone, some people are more affected because they live in a polluted area, are exposed to higher levels of air pollution in their daily lives, or are more susceptible to health problems caused by air pollution. The most vulnerable face all of these disadvantages. Groups more affected by air pollution include older people, children, individuals with pre-existing cardiovascular or respiratory disease, pregnant women, communities in areas of higher air pollution and low-income communities (PHE, 2018).

Metrics and numeric limits

Guideline values for particulate matter: Fine particulate matter (PM2.5): 10 μg/m3 annual mean and 25 μg/m3 24-hour mean; coarse particulate matter (PM10): 20 μg/m3 annual mean and 50 μg/m3 24-hour mean (WHO, 2006).

In addition to guideline values, the WHO air quality guidelines provide interim targets for concentrations of PM10 and PM2.5 aimed at promoting a gradual shift from high to lower concentrations. If these interim targets were to be achieved, significant reductions in risks for acute and chronic health effects from air pollution can be expected. Achieving the guideline values, however, should be the ultimate objective.

The effects of PM on health occur at levels of exposure currently being experienced by many people both in urban and rural areas and in developed and developing countries – although exposures in many fast-developing cities today are often far higher than in developed cities of comparable size.

The WHO air quality guidelines report that reducing annual average fine particulate matter (PM2.5) concentrations from levels of 35 μg/m3, common in many developing cities, to the WHO guideline level of 10 μg/m3, could reduce air pollution-related deaths by around 15%. However, even in the European Union, where PM concentrations in many cities do comply with guideline levels, it is estimated that average life expectancy is 8.6 months lower than it would otherwise be, due to PM exposures from human sources.

There are serious risks to health not only from exposure to PM, but also from exposure to O3, NO2 and SO2. As with PM, concentrations are often highest largely in the urban areas of low- and middle-income countries. Ozone is a major factor in asthma morbidity and mortality, while NO2 and SO2 also play a role in asthma, bronchial symptoms, lung inflammation and reduced lung function.

Guideline value for ozone: O3 100 μg/m3 8-hour mean (WHO, 2006).

The recommended limit in the 2005 Air Quality Guidelines was reduced from 120 μg/m3 in previous editions of the WHO air quality guidelines based on recent conclusive associations between daily mortality and lower O3 concentrations.

Ozone at ground level – not to be confused with the ozone layer in the upper atmosphere – is one of the major constituents of photochemical smog. It is formed by the reaction with sunlight (photochemical reaction) of pollutants such as nitrogen oxides (NOx) from vehicle and industry emissions and volatile organic compounds (VOCs) emitted by vehicles, solvents and industry. As a result, the highest levels of O3 pollution occur during periods of sunny weather.

Excessive O3 in the air can have a marked effect on human health. It can cause breathing problems, trigger asthma, reduce lung function and cause lung diseases.

Guideline values for nitrogen dioxide: 40 μg/m3 annual mean and 200 μg/m3 1-hour mean (WHO, 2006).

The current WHO guideline value of 40 μg/m3 (annual mean) was set to protect the public from the health effects of gaseous NO2.

As an air pollutant, NO2 has several related impacts. Under short-term exposure to concentrations exceeding 200 μg/m3, it is a toxic gas which causes significant inflammation of the airways.

NO2 is the main source of nitrate aerosols, which form an important fraction of PM2.5 and, in the presence of ultraviolet light, of O3. The major sources of anthropogenic emissions of NO2 are combustion processes (heating, power generation, engines in vehicles and ships).

Epidemiological studies have shown that symptoms of bronchitis in asthmatic children increase in association with long-term exposure to NO2. Reduced lung function growth is also linked to NO2 at concentrations currently measured (or observed) in cities of Europe and North America.

Guideline value for sulphur dioxide: 20 μg/m3 24-hour mean and 500 μg/m3 10-minute mean (WHO, 2006).

An SO2 concentration of 500 μg/m3 should not be exceeded over average periods of 10 minutes’ duration. Studies indicate that a proportion of people with asthma experience changes in pulmonary function and respiratory symptoms after periods of exposure to SO2 as short as 10 minutes. Health effects are now known to be associated with much lower levels of SO2 than previously believed. A greater degree of protection is needed. Although the causality of the effects of low concentrations of SO2 is still uncertain, reducing SO2 concentrations is likely to decrease exposure to co-pollutants.

SO2 is a colourless gas with a sharp odour. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulphur. The main anthropogenic source of SO2 is the burning of sulphur-containing fossil fuels for domestic heating, power generation and motor vehicles.

SO2 can affect the respiratory system and lung function and can cause irritation of the eyes. Inflammation of the respiratory tract causes coughing, mucus secretion, aggravation of asthma and chronic bronchitis and makes people more prone to infections of the respiratory tract. Hospital admissions for cardiac disease and mortality, increase on days with higher SO2 levels. When SO2 combines with water, it forms sulphuric acid; this is the main component of acid rain which is a cause of deforestation.

Please note that the WHO Air quality guidelines are currently under revision.

It is also important to note that at present, there is no clear evidence of a safe level of exposure to air pollutants below which there is no risk of adverse health effects. Therefore, further reductions of concentrations of air pollutants below air quality standards are likely to bring additional health benefits. Actions that improve air pollution can also offer wider public health and wellbeing co-benefits.

Examples of drivers, outcomes and risk management

Most sources of outdoor air pollution are well beyond the control of individuals and require concerted action by local, national and regional level policy-makers working in sectors such as transport, energy, waste management, urban planning, and agriculture (WHO, 2018).

There are many examples of successful policies in transport, urban planning, power generation and industry that reduce air pollution (WHO, 2018):

• Industry: clean technologies that reduce industrial smokestack emissions; improved management of urban and agricultural waste, including capture of methane gas emitted from waste sites as an alternative to incineration (for use as biogas).
• Energy: ensuring access to affordable clean household energy solutions for cooking, heating and lighting.
• Transport: shifting to clean modes of power generation; prioritising rapid urban transit, walking and cycling networks in cities as well as rail interurban freight and passenger travel; shifting to cleaner heavy-duty diesel vehicles and low-emission vehicles and fuels, including fuels with reduced sulphur content.
• Urban planning: improving the energy efficiency of buildings and making cities more green and compact, and thus energy efficient.
• Power generation: increased use of low-emission fuels and renewable combustion-free power sources (like solar, wind or hydropower); co-generation of heat and power; and distributed energy generation (e.g., mini-grids and rooftop solar power generation).
• Municipal and agricultural waste management: strategies for waste reduction, waste separation, recycling and reuse or waste reprocessing; as well as improved methods of biological waste management such as anaerobic waste digestion to produce biogas, are feasible, low cost alternatives to the open incineration of solid waste. Where incineration is unavoidable, then combustion technologies with strict emission controls are critical.