Which pollutants are classified as the six criteria air pollutants under the Clean Air Act?

The classification of the six criteria air pollutants under the Clean Air Act includes lead (Pb), sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), and ground-level ozone (tropospheric O3). These pollutants were identified due to their widespread presence in the atmosphere and significant impact on public health and the environment.

Lead is a heavy metal that can cause neurological issues, especially in children. Sulfur dioxide is primarily produced from fossil fuel combustion and contributes to acid rain. Nitrogen oxides are significant contributors to smog and respiratory issues. Carbon monoxide, a colorless and odorless gas, can interfere with oxygen transport in the body. Particulate matter consists of tiny particles that can penetrate the respiratory system and cause various health problems. Ground-level ozone is not emitted directly but forms from chemical reactions between volatile organic compounds and nitrogen oxides in sunlight, leading to respiratory difficulties and environmental damage.

The other options list pollutants either not recognized as criteria pollutants or include substances that do not have the same level of impact or regulation as those defined under the Clean Air Act. This specificity in the correct answer ensures that only the pollutants that require monitoring and control to protect public health and environmental quality are acknowledged.