In the atmosphere, toxic air pollutants are present in small amounts that can have adverse effects on the health of human populations and the environment. Toxic air pollutants include polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides, polychlorinated biphenyls (PCBs), and heavy metals, such as, mercury, copper, lead, zinc, arsenic, and cadmium. These pollutants are carried by particulate matter and can be mixed with precipitation. The Integrated Atmosphere Deposition Network (IADN) studies the transportation of specific toxic substances located in the atmosphere.
Source: Atmospheric Chemistry Glossary