EPA has set National Ambient Air Quality Standards (NAAQS) for six principal criteria air pollutants, which include oxides of nitrogen, oxides of sulfur, particulate matter, carbon monoxide, ozone and lead. Numerous sources emit these "criteria pollutants", which are considered harmful to public health and the environment.

Since 2008, EPA’s Integrated Science Assessments (ISAs) have formed the scientific foundation for the review of the NAAQS standards by providing the primary (human health-based) and secondary (welfare-based, e.g. ecology, visibility, materials) criteria assessments:

• Oxides of Nitrogen - Health Criteria
• Oxides of Sulfur - Health Criteria
• Particulate Matter
• Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter - Ecological Criteria
• Carbon Monoxide
• Ozone
• Lead