Hazardous Air Pollutants (HAPs) are 187 toxic substances listed under Section 112 of the Clean Air Act that are known or suspected to cause cancer, birth defects, neurological damage, or other serious health effects. They include mercury (a neurotoxin), arsenic, lead, benzene, and other chemicals. Unlike common pollutants regulated under the NAAQS program, HAPs are controlled through technology-based standards that require pollution sources to install Maximum Achievable Control Technology (MACT). Coal-fired power plants emit multiple HAPs, with mercury being the most significant due to its persistence in the environment and ability to bioaccumulate in fish.