Toxic air pollutants can generally be described as those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or have adverse environmental effects. Hazardous Air Pollutants

EPA has identified some 188 priority hazardous air pollutants (HAP)   and is working with state, local, and tribal governments to reduce releases of these to the environment.

Impacts of Toxic (Hazardous) Air Pollutants
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.