Final answer:
The Clean Air Act is the U.S. law that sets emission standards and ambient air quality standards, involving the regulation of six 'criteria' pollutants. The EPA enforces primary standards for health and secondary standards for welfare, with industries facing command-and-control regulation to comply.
Step-by-step explanation:
The overarching law in the United States that sets standards for both emissions and ambient air quality is the Clean Air Act. Amended last in 1990, the Act authorized the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for harmful pollutants. The Act specifies two types of standards: primary standards for public health protection, including sensitive populations like children and asthmatics, and secondary standards for public welfare, such as protection from visibility reduction and environmental damage.
The EPA is responsible for setting NAAQS for six 'criteria' air pollutants, which include carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. These are considered the most significant in terms of health risks and environmental impact. The standards set limits on the concentration of these pollutants in the air, expressed in parts per million (ppm), parts per billion (ppb), and micrograms per cubic meter of air (µg/m³). The command-and-control regulations require industries to bear the social costs of pollution by mandating anti-pollution equipment installation, aiming to control emissions directly from the sources.