Particulate matter (PM) consists of soot and dust particles smaller than the diameter of a human hair. These have two classifications:

• PM10, or coarse size particles, have a diameter of less than 10 microns.
• PM2.5, or fine size particles, have a diameter of 2.5 microns or less. This is less than 1/8^th the diameter of a human hair. 

The size of these particles enables them to penetrate deeply into our lungs and become captured by lung tissue. The finer the particles, the deeper the penetration and therefore the more danger posed. Even a small increase in PM2.5 can significantly increase mortality. In fact, The American Lung Association believes that PM2.5 represents the most serious threat to our health. 

Exposure to PM pollution has been associated with respiratory and cardiac problems, infections, asthma attacks, lung cancer and decreased life expectancy—with 500,000 annual premature deaths estimated by the World Health Organization. Those most susceptible to this pollution include children, athletes, senior citizens, and people with pre-existing respiratory problems.

Sources of PM include diesel exhaust, soil dust, tire wear, and soot. Exhaust from diesel vehicles produces 79% of the particulate emissions from mobile sources; PM from this source is especially dangerous because of the hundreds of different chemicals that are adsorbed to the particle.

Ozone forms when hydrocarbons combine with nitrogen oxides and chemically react in sunlight. Hydrocarbons and nitrogen oxides are produced primarily by motor vehicles and various industrial practices. Ozone is a highly reactive oxidizing agent that breaks down organic materials, and it is the primary component of smog, which is formed when a natural phenomenon called an "inversion layer" traps these gases and prevents them from dissipating into the atmosphere. As populations grow, ozone and smog are becoming problems for large cities throughout the country.

Symptoms of ozone exposure include coughing, shortness of breath, wheezing, fatigue, throat dryness, chest pain, headaches and nausea. It has been linked to inflammation of lung tissue, reduced lung capacity, development of asthma, increased lung cancer mortality rates, and accelerated lung aging. Ozone also reduces the respiratory system's ability to fight infection and remove foreign particles such as particulate matter. Lung damage from long-term exposure to ozone can be permanent, while short-term exposure appears to be reversible. Those who are most susceptible to ozone pollution include children, athletes, senior citizens, and people with pre-existing respiratory problems.

Nitrogen monoxide (NO) and nitrogen dioxide (NO₂) are the two forms of nitrogen oxide (NO_x) found in the atmosphere. Nitrogen oxides contribute to the formation of ozone, production of particulate matter pollution, and acid deposition. The presence of nitrogen oxides gives smog its brown appearance. It is produced by factories, motor vehicles and power plants that burn fossil fuels. Diesel engines produce a disproportionately large amount of NO_x when compared to gasoline engines because of their high-temperature combustion process.

NO₂ has been shown to irritate lung tissue, cause bronchitis and pneumonia, and reduce resistance to respiratory infections; its presence in the atmosphere can have synergistic effects with other forms of air pollution. The health effects of ozone are magnified in the presence of NO₂. Frequent or long-term exposure to high levels of nitrogen oxides can increase the incidence of acute respiratory illness in children.

Hydrocarbons are a class of reactive organic gases (ROG), which are formed solely of hydrogen and carbon. They are formed in the incomplete burning of any organic matter such as oil, wood or rubber; the primary sources are combustion engine exhaust, oil refineries, and oil-fueled power plants. Another source is evaporation from petroleum fuels, solvents, dry cleaning solutions, and paint.

The health risks of hydrocarbons primarily stem from their contribution to ozone and the resulting smog problem. High levels of hydrocarbons in the atmosphere can interfere with oxygen intake by reducing the amount of available oxygen through displacement. Carcinogenic forms of hydrocarbons are considered hazardous air pollutants, or air toxics.

Carbon monoxide (CO) is a colorless, odorless gas produced by burning organic matter such as oil, natural gas, fuel, wood or charcoal. Motor vehicles produce 67% of the man-made CO that is released into the atmosphere. It displaces oxygen in red blood cells, reducing the amount of oxygen available to cells for respiration. Exposure to CO can result in fatigue, angina, reduced visual perception, reduced dexterity, and death. The elderly, young children, and people with pre-existing respiratory conditions are particularly sensitive to CO pollution. It is extremely deadly in an enclosed space, such as a garage or bedroom.