National Ambient Air Quality Standards Have Improved the Air

The first principle of the Clean Air Act (CAA) is defining what is, and what is not, clean air. This is accomplished numerically with the National Ambient Air Quality Standards (NAAQS). Compliance with the NAAQS is determined by monitoring air quality at stations all over the U.S and comparing the measured values to the standards. As seen in Figure 1, monitored concentrations of every pollutant measured (ozone, carbon monoxide, lead, nitrogen dioxide, and sulfur dioxide) have decreased since 1980. Particulate matter has been monitored nationally for a shorter period (since 1990 for PM10 and since 1999 for Pm².5) but also shows a downward trend. This data was normalized so that all pollutants would fit on the same scale. Carbon dioxide and other greenhouse gases (GHGs) are not shown since there are no NAAQS for GHG and therefore no GHG non-attainment areas.

Cleaner air makes sense when you realize that the total tons of pollution emitted from factories, power plants, and other industrial sources has been cut by almost two-thirds, as shown in Figure 2. In addition, from 1990 to 2005, emissions of air toxics declined by approximately 42 percent. These reductions are the result of implementing stationary and mobile source regulations.

At the same time however, the total number of cities with "bad" air has not shown the same consistent downward trend. This is due to the EPA redefining "unhealthy" air. From 2003-2005, the number of nonattainment areas doubled. This does not mean that air quality became twice as bad. As required, EPA reexamines the NAAQS every five years and this dramatic uptick in the number of nonattainment areas is due to two actions by EPA. First, EPA shortened the ozone NAAQS averaging time from an 8-hour basis to a 1-hour basis. NAAQS with shorter averaging times are harder to meet. Secondly, EPA added particulate matter less than 2.5 microns in diameter (Pm².5) as a NAAQS pollutant.

EPA recently added 1-hour averaging periods to the NO2 and SO₂ NAAQS. When the nonattainment areas are designated for these new NAAQS in 2014-2015, another surge in nonattainment areas can be expected, without a causal increase in pollutant emissions or degradation of air quality.
The NAAQS aren't the only aspects of the CAA that works. The Acid Rain reduction program, emissions inventories, and Title V operating permits have also met their objectives of reducing pollution and better defining a source's emissions and responsibilities. Not so successful are Prevention of Significant Deterioration (PSD), Mercury and Air Toxics Standards (MATS), and Cross-State Air Pollution Rule (CSAPR). These programs have been riddled with errors, fraught with confusion, and haunted by legal delays. (Pollution Engineering, by Robyn Andracsek, June 2012)