Carbon Monoxide (CO) is an odorless, colorless gas. After being inhaled, CO molecules can enter the bloodstream where they inhibit the delivery of oxygen throughout the body. Low concentrations can cause dizziness, headaches, and fatigue. High concentrations can be fatal. CO is produced by the incomplete burning of carbon-based fuels, including gasoline, oil, and wood. It is also produced from incomplete combustion of natural and synthetic products, such as cigarette smoke. It can build up in high concentrations in enclosed areas such as garages, poorly ventilated tunnels, and even along roadsides in heavy traffic.

Chlorofluorocarbons (CFCs) are chemicals used in industry, refrigeration and air conditioning systems, and consumer products. When released into the air, CFCs rise into the stratosphere. In the stratosphere, CFCs react with other chemicals and reduce the stratospheric ozone layer, which protects Earth's surface from the sun. Reducing CFC emissions and eliminating the production and use of ozone-destroying chemicals is very important to protecting Earth's stratosphere.

Hazardous Air Pollutants (HAPS) are chemicals that cause serious health and environmental effects. Health effects include cancer, birth defects, nervous system problems, and death due to massive accidental releases, such as occurred at a pesticide plant in Bhopal, India, in the mid-1980's. HAPs are released by sources such as chemical plants, dry cleaners, printing plants, and motor vehicles (e.g., cars, trucks, buses, and planes).

Lead is a highly toxic metal that produces a range of adverse health effects, particularly in young children. Lead can cause nervous system damage and learning behavior problems. Lead can also harm wildlife. Lead has been phased out of gasoline, which has reduced air contamination by lead considerably. However, lead can still be inhaled or ingested from other sources. These sources of lead include paint (both house and cars), smelters, manufacture of lead batteries, fishing lures, certain parts of bullets, some ceramic ware, water pipes, and fixtures.

Nitrogen Oxides (NO_x) are major contributors to ozone (smog) and acid rain. NO_x reacts with volatile organic compounds (VOCs) to form smog. In high doses, smog can harm humans by causing breathing difficulty for asthmatics, coughs in children, and general illness of the respiratory system. NO_x in combination with VOCs and in the presence of heat and sunlight form ground-level ozone, which can cause lung damage, chest pain, coughing, nausea, throat irritation, and congestion. Acid rain can harm vegetation and can run into lakes and rivers, which changes the chemistry of the water, making it potentially uninhabitable for all but acid-tolerant bacteria. NO_x are produced from burning fuels, including gasoline and coal.

Ozone (O₃) is a gas that consists of three oxygen atoms. Ozone forms naturally and is beneficial in the stratosphere (a layer of atmosphere high above Earth) where it filters harmful ultraviolet (UV) rays. However, ozone that is close to the ground (called ground-level ozone) can irritate the respiratory tract, cause chest pain and persistent cough, affect the ability to take a deep breath, and an increase susceptibility to lung infection. Ozone can also damage trees and plants and reduce visibility. Motor vehicles and industries are major sources of ground-level ozone. Ground-level ozone comes from the oxidation (breakdown) of VOCs found in solvents. It is also a product of reactions between chemicals produced by burning coal, gasoline, and other fuels and chemicals found in paints and hair sprays. Oxidation occurs readily during hot weather.

Particulate Matter (PM) includes fine solids suspended in the air in the form of smoke, dust, and vapors, which can remain suspended for extended periods. In addition to reducing visibility and soiling clothing, microscopic particles from the air can be breathed in and lodged in lung tissue, causing increased respiratory disease and lung damage. Particulates are also the main source of haze, which reduces visibility. Particulates are produced by many sources, including cars, trucks, and buses burning diesel fuels and other fossil fuels; the preparation and application of fertilizers and pesticides; road construction; industrial processes, such as steel making; mining; agricultural burning; and the operation of fireplaces and woodstoves. PM₁₀ is particulate matter smaller than 10 microns.

Sulfur Dioxide (SO₂) is an odorless gas at low concentrations, but at high concentrations can have a very strong smell. SO₂ is produced by burning coal, most notably in power plants. Some industrial processes, such as paper production and metal smelting, produce SO₂. Like NO_2, SO₂ is a major contributor to smog and acid rain. It can harm vegetation and metals and can cause lung problems, including breathing problems and permanent lung damage.