Air pollution

The World Health Organization defines air pollution as contamination of the indoor or outdoor environment by chemical, physical or biological agents that modify the atmosphere’s natural characteristics. The term covers both gases and particulate matter (PM), a mixture of airborne solid particles and liquid droplets. Common pollutants of public-health concern include PM, carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide. Some pollution is visible as smoke or haze, but harmful concentrations are not always apparent to the eye.

Sources vary by place and by whether the air is indoors or outdoors. Household cooking and heating, vehicles, power generation, industrial facilities, agriculture, waste burning and forest fires can all contribute. Some pollutants are released directly; others form through chemical reactions in the atmosphere. For example, fine particles may be emitted from fires or dust, or form from gases such as sulfur dioxide and nitrogen oxides. Ground-level ozone is another secondary pollutant: sunlight drives reactions between nitrogen oxides and volatile organic compounds emitted by vehicles, power plants and other sources. This ozone is harmful to breathe and is a major ingredient of smog. It is different from ozone high in the stratosphere, which helps shield Earth from ultraviolet radiation.

Air pollution can affect people through both short- and long-term exposure. Fine particles can be inhaled deep into the lungs, and some can enter the bloodstream; evidence links exposure to cardiovascular and respiratory disease, including heart disease, stroke, asthma and lung cancer. Risk depends on the pollutant, concentration, duration of exposure and a person’s circumstances. Effects also extend beyond people. Ground-level ozone can reduce plant growth and crop yields, while nitrogen deposition can add excess nutrients to land and water, contributing to eutrophication. Sulfur and nitrogen compounds deposited from the air can change soil and water chemistry and contribute to ecosystem acidification. Some pollutants and their precursors can travel far from emission sources, so impacts need not be confined to source areas.

Air-quality monitoring measures pollutant concentrations and helps public authorities assess exposure, identify sources and evaluate whether standards are being met. The WHO publishes evidence-based global air-quality guidelines; national and regional governments use their own laws and standards to guide action. Because important sources lie across sectors, effective controls often combine cleaner household energy, lower-emission transport, industrial pollution controls, cleaner power generation, energy-efficient buildings and improved waste and agricultural practices. Air pollution and climate change are distinct problems, but they share some sources—especially fossil-fuel combustion—so policies that reduce emissions can bring benefits for both health and climate. The most relevant measures depend on local sources, geography and atmospheric conditions.