Primary air pollutants are emitted directly into the atmosphere from sources such as vehicles, factories, and power plants. Particulate matter, which consists of small solid or liquid particles, is a primary air pollutant.

Sulfur dioxide is primarily emitted from industrial activities, such as the burning of fossil fuels for electricity generation and the smelting of metals.

Secondary air pollutants are formed in the atmosphere through chemical reactions between primary air pollutants and other atmospheric constituents. Ozone is a secondary air pollutant formed by the reaction of nitrogen oxides and volatile organic compounds in the presence of sunlight.

Exposure to particulate matter can cause a range of health problems, including respiratory problems such as asthma, bronchitis, and lung cancer.

Nitrogen oxides are primarily emitted from transportation sources, such as the burning of fossil fuels in vehicles.

Exposure to ozone can cause a range of health problems, including respiratory problems such as asthma, bronchitis, and lung damage.

Volatile organic compounds are emitted from a variety of sources, including natural sources such as trees and vegetation, as well as human activities such as the use of solvents and paints.

Exposure to carbon monoxide can cause a range of health problems, including neurological disorders such as headaches, dizziness, and impaired coordination.

Sulfur dioxide is primarily emitted from industrial activities, such as the burning of fossil fuels for electricity generation and the smelting of metals.

Exposure to nitrogen dioxide can cause a range of health problems, including respiratory problems such as asthma, bronchitis, and lung damage.

Particulate matter is emitted from a variety of sources, including transportation sources such as vehicles and aircraft, as well as industrial activities and natural sources such as dust and wildfires.

Exposure to lead can cause a range of health problems, including neurological disorders such as learning disabilities and behavioral problems.

Mercury is primarily emitted from industrial activities, such as the burning of fossil fuels and the mining and processing of metals.

Exposure to benzene can cause a range of health problems, including cancer, particularly leukemia.

Carbon monoxide is primarily emitted from transportation sources, such as the burning of fossil fuels in vehicles.