In setting national ambient air quality standards (NAAQS), the 1970 CAA set two levels of standards for criteria pollutants:

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The 1970 CAA established two levels of standards for criteria pollutants: primary standards, which set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly, and secondary standards, which set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings. These are the two types of NAAQS that are still used today.

In Setting National Ambient Air Quality Standards (NAAQS), the 1970