Pollution is the introduction of harmful materials into the environment. These harmful materials are called pollutants. Pollutants can be natural, such as volcanic ash. They can also be created by human activity, such as trash or runoff produced by factories. Pollutants damage the quality of air, water, and land.

Air pollution also increases the risk of respiratory infections, heart disease, stroke, and lung cancer, and more severely affects people who are already ill. People's health risks from air pollution vary widely depending on age, location, underlying health, and other factors.