Toxic air pollutants, also known as Hazardous Air Pollutants (HAPs) or Air Toxics, are chemicals that are known or suspected to cause adverse health effects. Air Toxics can present a serious threat to both public health and the environment. The U.S. EPA has defined a list of 188 chemical compounds that they regulate as Air Toxics. The U.S. EPA has also defined a list of 33 air pollutants (a subset of 32 air toxics on the Clean Air Acts list of 188 air toxics plus diesel particulate matter, diesel PM) used in National Air Toxics Assessment activities.

• Prolonged exposure to air toxics at significant concentrations can result in immunological, neurological, respiratory, or reproductive problems, as well as cancer.
• Air toxics deposited in soil or water bodies can affect ecological systems. Environmental effects include toxicity to plants and animals, reproductive and developmental problems in wildlife, and accumulation of pollutants in the food chain.
• Magnification of a pollutant up through the food chain is called bioaccumulation. Bioaccumulation is the build-up of a substance, like a toxic chemical, in the tissues of a living organism. This occurs when the substance is not removed from the tissues of an organism by bodily removal mechanisms such as synthesis and detoxification by the liver followed by excretion through the bladder or intestines. Predators generally accumulate a higher concentration of a toxin than the contaminated prey they consume. Therefore, bioaccumulation can cause animals (including humans) that have eaten contaminated food to have high or dangerous concentrations of toxic pollutants in their bodies. An example of a toxic air pollutant which undergoes bioaccumulation is mercury (PDF 2.28 MB)