Airborne particulate matter (PM) is a mixture of tiny solid particles and liquid droplets suspended in the air. These particles vary widely in size, shape, and chemical composition, and may contain inorganic ions, metallic compounds, elemental carbon, organic compounds, and compounds from the Earth’s crust.1
Breathing in unhealthy levels of PM can increase the risk of health problems like heart disease, asthma, cancer, and low birth weight. The health risk depends on particle size; smaller particles can penetrate more deeply into the lungs. The greatest health concern is fine particulate matter, defined as particles that are 2.5 microns or less in diameter (PM2.5).
Airborne PM 2.5 comes from a variety of natural and anthropogenic sources. Natural sources include wildfires, volcanic eruptions, sea spray that contains fine particles of salt, and biological sources such as pollen and fungal spores. Anthropogenic sources include the combustion of fuels from vehicles, especially diesel engines; emissions from factories and power plants that burn fossil fuels; residential heating that relies on burning wood, dings, coal, and other solid fuels in wood stoves and fireplaces; and the open burning of waste materials.
So-called secondary sources of PM2.5 derive from chemical reactions in the atmosphere. PM2.5 forms when gases like sulfur dioxide (SO₂), nitrogen oxides (NOₓ), and volatile organic compounds (VOCs) react in the atmosphere. For example, ammonia (NH3) is formed and released during the decomposition of manure and organic matter, mostly from animal farming and the associated manure storage and field application, with an additional contribution from (synthetic) nitrogen fertilizer use.2 NH3 in the atmosphere undergoes chemical reactions that produce ammonium salts, a form of PM2.5.
PM2.5 is unmistakably associated with many health impacts, including acute respiratory infections, heart disease, chronic obstructive pulmonary disease, and lung cancer.3 In 2017 about 3.83 million deaths in the world were attributable to annual ambient PM2.5 exposure.4 China and India account for more than one-half of all deaths.
The sources of PM2.5 show distinct geographic patterns. Windblown dust is the largest contributor to PM2.5 in arid regions such as North Africa, the Middle East, sub-Saharan Africa, and Central Asia.5 Coal was the dominant combustible fuel type contributing to the PM2.5 disease burden in 20 countries, including China, Eswatini, South Africa, and countries throughout Central and Eastern Europe. In the United States, agriculture dominates emissions in the upper midwest, windblown dust in the southwest, fossil fuel combustion in the east north central region, and fires across much of the west.
Air pollution regulations in affluent countries in Europe and North America resulted in a significant decline in PM2.5 levels over the last three decades.6 In Germany, the United States, and Canada, exposure to PM2.5 declined by 54%, 43%, and 37% respectively from 1990 to 2020.7 The Chinese government declared a “war on pollution” in 2013 due to a health emergency related to PM2.5 and other air pollutants.8 Exposure to PM2.5 dropped sharply over the next decade, although concentrations are still two to five times higher compared to many high-income nations. India made progress in PM2.5 exposure since the 2010s, although exposure in many regions remains far above health guidelines and remains a leading cause of morbidity and mortality.
1 California Air Resources Board, “Inhalable Particulate Matter and Health (PM2.5 and PM10),” accessed November 30, 2024, Link
2 Pozzer, A., Tsimpidi, A. P., Karydis, V. A., de Meij, A., and Lelieveld, J.: Impact of agricultural emission reductions on fine-particulate matter and public health, Atmos. Chem. Phys., 17, 12813–12826, https://doi.org/10.5194/acp-17-12813-2017, 2017.
3 Global Burden of Disease 2019, “Risk Factor Collaborators. Global burden of 87 risk factors in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019,” Lancet 396, 1223–1249 (2020), Link
4 McDuffie, E.E., Martin, R.V., Spadaro, J.V. et al. Source sector and fuel contributions to ambient PM2.5 and attributable mortality across multiple spatial scales. Nat Commun 12, 3594 (2021). https://doi.org/10.1038/s41467-021-23853-y
5 McDuffie et al., op. cit.
6 Li, C., van Donkelaar, A., Hammer, M.S. et al. Reversal of trends in global fine particulate matter air pollution. Nat Commun 14, 5349 (2023). https://doi.org/10.1038/s41467-023-41086-z
7 Organisation for Economic Co-operation and Development, “Exposure to air pollution,” accessed November 1, 2024, https://data-explorer.oecd.org/
8 Greenstone, M, G He, S Li, and E Yongchen Zou (2021), “China’s war on pollution: Evidence from the first 5 years”, Review of Environmental Economics and Policy, 15(2): 281-299, https://www.journals.uchicago.edu/doi/abs/10.1086/715550