Reviewing National Ambient Air Quality Standards (NAAQS)

Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2) Secondary Air Quality Standards

The Clean Air Act requires EPA to set national air quality standards for six criteria pollutants (nitrogen oxides, sulfur oxides, particulate matter, ozone, carbon monoxide, and lead) based on primary (health-based) and secondary (welfare-based) considerations. The law also requires EPA to periodically review the scientific criteria upon which the standards are based  (learn more about this process). 

While EPA generally reviews criteria and standards for each of the six criteria pollutants individually, for this review of the secondary standards for NO2 and SO2, EPA has decided to examine nitrogen oxide (NOx) and sulfur oxide (SOx) compounds together.  EPA recognizes that NOx, SOx are inextricably linked from both an atmospheric chemistry and environmental effects perspectives.

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