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The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (NAAQS) for wide-spread pollutants from numerous and diverse sources considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of at-risk populations such as people with pre-existing heart or lung disease (such as asthmatics), children, and older adults. Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings. The Clean Air Act requires periodic review of the science upon which the standards are based and the standards themselves. For more information on EPA's process for reviewing the NAAQS, see NAAQS Review Process.

EPA has set NAAQS for six principal pollutants, which are called "criteria" pollutants. They are listed below. For a summary table of the current NAAQS, see the NAAQS Table.

For further information about the current or prior review of any of the NAAQS or information related to the implementation of control programs designed to attain the standards, please use the links listed below:

Carbon Monoxide (CO)   Air Quality Standards   Implementation
Lead (Pb)   Air Quality Standards   Implementation
Nitrogen Dioxide (NO2)   Primary Air Quality Standards
Secondary Air Quality Standards
  Implementation
Ozone (O3)   Air Quality Standards   Implementation
Particulate Matter (PM)   Air Quality Standards   Implementation
Sulfur Dioxide (SO2)   Primary Air Quality Standards
Secondary Air Quality Standards
  Implementation

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