National Ambient Air Quality Standards (NAAQS) — National standards set by the Environmental Protection Agency under authority of the Clean Air Act; NAAQS define the maximum allowable concentrations of specified air pollutants in outdoor (ambient) air. NAAQS have been set for carbon monoxide, particulate matter, sulfur oxides, nitrogen dioxide, lead, and ozone. "Primary" NAAQS protect human health, with a margin of safety; "secondary" NAAQS protect human welfare, which includes effects on soils, water, crops, vegetation, materials, etc.