The Clean Air Act, last amended in 1990, is the comprehensive federal law that regulates air emissions from stationary and mobile sources.  These pollution limits are called National Ambient Air Quality Standards (NAAQS). The Clean Air Act requires the EPA to set NAAQS for pollutants considered harmful to public health and the environment. The standards track six common "criteria" pollutants: particulate matter (PM), ground-level ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NO2) and lead (Pb).

When an area's monitored air pollution exceeds the NAAQS a certain number of times, EPA designates it as a "nonattainment" area. This designation is determined by a formula that uses the number of times the standard is exceeded each year. The formula includes three years of data to ensure that an area isn't designated as nonattainment because of one unusual year and the law also includes a periodic review process.

A "nonattainment" classification means that air quality in a particular region does not meet (or "attain") the federal air quality standard. This information is maintained by the EPA in the form of a map that is updated regularly (Figure 1):  

Figure 1 - In the colored areas it makes sense to use Dynamic Air Cleaners to clean both the indoor air and outdoor air, reduce the volume of outside air, and use the savings in energy and maintenance costs to pay for the air cleaning systems.

 

As you can see, the areas where outdoor air does meet the federal standard are essentially limited to areas where you don't have population centers. This leaves us with a situation whereby building codes require ventilation air to meet their standard, but the ventilation air is bad, according to the EPA.