In response to strong public pressure for cleaner air, the Clean Air Act of 1970 marked a significant step toward effectively regulating air pollution emissions in the U.S.  The centerpiece of the 1970 amendments was the National Ambient Air Quality Standard (NAAQS) program. The NAAQS set standards for six pollutants: sulfur dioxide, nitrogen dioxide, particulate matter (PM-10), carbon monoxide, ozone, and lead.  These substances were regulated under the CAA because they were identified as dangers to public health and welfare.  New Source Performance Standards (NSPS) were enacted, and significant new sources of pollution regulated under these standards (however, existing plants were grandfathered in and were not subject to these standards).  The act called for lead to be phased out of gasoline by the mid-1980s—mitigating a significant public health risk.  Allowances for criteria pollutants from vehicles were to be significantly reduced by the late 1970s.

The Act established primary and secondary criteria for these six pollutants, and the term “criteria pollutant” originated from the CAA of 1970.  Primary criteria were designed to protect public health from air pollution, and secondary criteria were developed to protect public welfare (e.g. by preventing crop and property damage).

State Implementation Plans (SIPs) from the AQA of 1967 were rolled into these CAA amendments and substantial procedural requirements were enacted to ensure that SIPs were submitted to the newly founded EPA.  Additionally, this is the first version of the CAA that allowed for citizens to file lawsuits—an important enforcement mechanism