Which description best describes hazardous air pollutants (HAPs)?

• A. Compounds such as particulate matter that decrease visibility
• B. Compounds such as hydrocarbons and NOx that are photochemically oxidized
• C. Compounds such as asbestos and benzene that pose a serious risk to human health
• D. Compounds such as ozone that contribute to respiratory problems

Answer: (C)

Hazardous air pollutants are those substances known to cause serious health effects, including cancer, due to their toxic nature. The defining idea is the level of health risk posed, not how they influence visibility, ozone formation, or general respiratory irritation. Asbestos and benzene are classic examples because they have well-established links to severe health outcomes—asbestos exposure is strongly tied to mesothelioma and other lung diseases, while benzene exposure increases the risk of leukemia. This focus on proven toxicity and cancer risk is why these substances best describe hazardous air pollutants. The other descriptions refer to issues like visibility impairment, formation of secondary pollutants, or ozone’s respiratory effects, which are not the criteria used to define HAPs.