Ambient air pollution

Essentials

  • Particles produced by combustion are the most important air pollutants with adverse health effects. Such particles arise both outdoors (traffic, residential wood combustion, long-distance transport) and indoors (smoking, other combustion).
  • Ozone, nitrogen dioxide and carbon monoxide are the gaseous ambient air pollutants that are the most important for health.
  • Exposure to ambient air pollution occurs mostly indoors.
  • In countries with low concentrations of pollutants, the associations between pollutants and adverse health effects are weak on an individual level and detectable primarily in large population surveys.
  • Particles of various sizes enter the respiratory system in various ways. Particles sized less than 10 µm (particulate matter, PM10) enter the lungs and particles sized less than 2.5 µm (fine particulate matter, also known as fine particles or PM2.5) go all the way to alveoli.
  • Health effects of fine particulate matter are mainly due to exposure over several years and to the increased risk of cardiovascular disease and mortality.

Sources and adverse health effects of ambient air pollutants

  • Particulate matter
    • Inhalable particulate matter is divided by size into fine particles (with a diameter ≤ 2.5 µm) and coarse (inhalable) particles (with a diameter of 2.5–10 µm). Fine particulate matter can further be divided into ultrafine particles (with a diameter < 0.1 µm) and accumulation mode particles (0.1–1 µm).
    • All incomplete combustion produces particulate matter either directly (e.g. residential wood combustion produces soot and car engines produce ultrafine particles) or indirectly (e.g. nitrogen and sulphur emissions from energy plants form accumulation mode particles).
    • Coarse particles form in traffic, for instance, as tyres grind asphalt and raise gritting sand and soil particles from the ground.
    • Fine particulate matter from outdoors easily enters indoor spaces. As we spend most of our time indoors, most of the exposure to fine particulate matter from outdoors occurs indoors.
    • Coarse particles and ultrafine particles do not readily enter indoor spaces.
    • Exposure to fine particulate matter for several years shortens the life expectancy, increases morbidity in cardiovascular diseases and lung cancer in particular, increases the risk of asthma and slows down the growth of lungs in children.
    • Short-term exposure to fine particulate matter increases circulatory and respiratory symptoms, hospital visits and mortality particularly among people with underlying disease.
    • Exposure to fine particulate matter during pregnancy is harmful and associated with preterm birth and low birth weight.
    • There is no safe lower limit for levels of fine particulate matter.
    • The WHO estimates that indoor and outdoor air pollution causes almost 7 million deaths globally each year https://pmc.ncbi.nlm.nih.gov/articles/PMC8140409 https://www.who.int/news-room/fact-sheets/detail/noncommunicable-diseases/.
    • In the EU, 239,000 premature deaths were attributable to PM2.5 during 2022 . The number has decreased by 45% since 2005 https://www.eea.europa.eu/en/analysis/indicators/health-impacts-of-exposur....
  • Nitrogen dioxide
    • The main source of nitrogen oxides is energy production, but traffic is the most important source of exposure.
    • In urban areas, increased nitrogen dioxide concentrations coincide with high concentrations of particulate matter. Recent studies suggest that nitrogen dioxide also has independent health effects and is not just an indicator of exhaust emissions.
    • In epidemiological studies, short-term exposure to nitrogen dioxide has been associated with aggravation of respiratory diseases in particular, and increased mortality.
    • Long-term exposure will probably also increase mortality.
  • Ozone
    • Ozone forms outdoors as solar UV radiation makes precursor gases react https://www.nature.com/articles/s41591-024-02976-x. Depending on the country, a considerable share may enter through long-distance transport.
    • As there are practically no indoor sources and ozone from outdoors does not readily enter indoor air, the level of ozone exposure depends on the time spent outdoors.
    • In controlled tests, ozone has been shown to affect pulmonary function and cause pulmonary hyperreactivity, respiratory tract inflammation and irritation symptoms in healthy subjects.
    • In epidemiological studies, daily variation in ozone levels has been associated with periods of hospital treatment and mortality from not only respiratory but also heart diseases.
    • Long-term exposure to ozone may also increase mortality.
  • Carbon monoxide
    • Traffic is the most significant outdoor source of carbon monoxide exposure, but carbon monoxide is also produced in small-scale burning in the home.
    • Outdoor carbon monoxide levels have decreased considerably since emissions from road traffic have decreased. In residential areas with a predominance of detached houses where plenty of wood is burned, carbon monoxide levels may be high for short times, particularly.
    • There is suggestive evidence that high carbon monoxide levels in urban areas have an independent effect, increasing mortality in myocardial infarction in particular on the following day.
    • Carbon monoxide poisoning, see (Carbon monoxide poisoning).
  • Other pollutants
    • Other ambient air pollutants include many volatile organic compounds (VOCs), such as benzene and polycyclic aromatic hydrocarbons, or PAH compounds, heavy metals and reduced sulphur compounds with a strong odour. Higher concentrations in ambient air are detected locally mostly in the surroundings of industrial plants or due to abundant residential wood combustion (PAH compounds).

Risk groups and protection

  • The effects of air pollutants range from very mild, transient effects to permanent disease or death.
  • The additional risk to individuals is often low but as practically everyone is exposed, the effect on public health may be significant.
  • The effects of particulate matter in ambient air on cardiovascular morbidity were not detected until the 1990s. As a considerable share of people die of these diseases, even a slightly increased risk will have significant effects.
  • Risk groups
    • People with sensitive lungs such as those with asthma react more easily to many irritants.
    • The developing respiratory tracts of children are sensitive to permanent damage.
  • On society level, morbidity can best be tackled by extensive measures to reduce emissions from traffic and residential wood combustion in particular, and pollutants coming from abroad.
  • On an individual level, exposure can be reduced by avoiding polluted living environments and decreasing physical exercise on days with high pollutant levels, by considering filtration of the air supply in homes, and by preventing emissions from residential wood combustion.

References

1. WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Geneva: World Health Organization, 2021. https://www.who.int/publications/i/item/9789240034228
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