"Ambient air" is the outside air that we all breathe. This term is specifically defined by EPA as "that portion of the atmosphere, external to buildings, to which the general public has access."
In the early 1970s, the EPA listed six major air pollutants that affected the quality of ambient air and established concentration limits for these pollutants. These limits are known as the National Ambient Air Quality Standards (NAAQS). Primary limits or standards were established to protect human health and secondary standards were established to protect human welfare and the quality of life. Through the years, the NAAQS have been revised and amended to account for evolving scientific understanding of air pollution and its impacts. Currently, the six criteria pollutants are:
- Ozone (O3)
- Particulate Matter (PM 2.5 and PM 10)
- Carbon Monoxide (CO)
- Sulfur Dioxide (SO2)
- Nitrogen Dioxide (NO2)
- Lead (Pb)
These six pollutants can cause serious human health problems (including premature mortality) and damage the environment and property. Common sources of these pollutants are coal-fired power plants, industrial manufacturing sources, and on-road and off-road vehicles. These standards can be viewed on-line at the National Ambient Air Quality Standards (NAAQS) page.
The Clean Air Act (CAA) requires the states to set up air monitoring networks to measure these six pollutants. The methods used to sample these pollutants are referred to as either "reference" or "equivalent" methods and must be approved by EPA before being used. The required methods for sampling these six pollutants and ambient limits themselves are codified in the Code of Federal Regulations at 40 CFR Part 50.
Background information about the annual Air Monitoring Network Design Plan and a general overview of how the state's air monitoring networks are operated can be found here.