Most air toxics originate from human-made sources, including mobile sources (e.g., cars, trucks, buses) and stationary sources (e.g., refineries, petrochemical industries , power plants, steel plants), as well as indoor sources (e.g., some building materials and cleaning solvents). Some air toxics are also released from natural sources such as volcanic eruptions and forest fires.

People exposed to toxic air pollutants at sufficient concentrations and durations may have an increased chance of getting cancer or experiencing other serious health effects. These health effects can include damage to the immune system, as well as neurological, reproductive (e.g., reduced fertility), developmental, respiratory and other health problems. In addition to exposure from breathing air toxics, some toxic air pollutants such as mercury can deposit onto soils or surface waters, where they are taken up by plants and ingested by animals and are eventually magnified up through the food chain. Like humans, animals may experience health problems if exposed to sufficient quantities of air toxics over time.

People are exposed to toxic air pollutants in many ways that can pose health risks, such as breathing contaminated air , drinking water contaminated by toxic air pollutants, ingesting contaminated soil. Young children are especially vulnerable because they often ingest soil from their hands or from objects they place in their mouths. Touching (making skin contact with) contaminated soil, dust, or water . Once toxic air pollutants enter the body, some persistent toxic air pollutants accumulate in body tissues. Predators typically accumulate even greater pollutant concentrations than their contaminated prey. As a result, people and other animals at the top of the food chain who eat contaminated fish or meat are exposed to concentrations that are much higher than the concentrations in the water, air, or soil. People exposed to toxic air pollutants at sufficient concentrations and durations may have an increased chance of getting cancer or experiencing other serious health effects. These health effects can include damage to the immune system, as well as neurological, reproductive (e.g., reduced fertility), developmental, respiratory and other health problems.

Air toxics can pose special threats in urban areas because of the large number of people and the variety of toxics sources, such as cars, trucks, large factories, gasoline stations and dry cleaners.  Individually, some of  these sources may not emit large amounts of toxic pollutants.  However, all of these pollution sources combinedly can potentially pose significant health threats. EPA is also concerned about the impact of toxic emissions on minority and low income communities which are often located close to industrial and commercial urbanized areas.    EPA intends to collect and evaluate additional information to help identify and prioritize actions to decrease emissions that affect these and other residential communities.

The seven pollutants [alkylated lead compounds, polycyclic organic matter (POM) , hexachlorobenzene, mercury, polychlorinated biphenyls, 2,3,7,8-tetrachlorodibenzofurans (TCDF) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)] were among the pollutants of concern identified by the International Joint Commission of the United States and Canada, and the Great Lakes Commission, as well as EPA's Great Waters Program because of their persistence and tendency to bioaccumulate in the environment. These pollutants are also associated with adverse health effects such as nervous system damage and reproductive effects.