77% of the nationwide CO emissions are from transportation sources. The largest emissions contribution comes from highway motor vehicles. Thus, the focus of CO monitoring has been on traffic oriented sites in urban areas where the main source of CO is motor vehicle exhaust. Other major CO sources are wood-burning stoves, incinerators and industrial sources.
The National Ambient Air Quality Standard for carbon monoxide is 9 ppm 8-hour nonoverlapping average not to be exceeded more than once per year. The rounding convention in the standard specifies that values of 9.5 ppm, or greater, are counted as exceeding the level of the standard. An area meets the carbon monoxide NAAQS if no more than one 8-hour value per year exceeds the threshold. (High values that occur within 8 hours of the first one are exempted. This is known as using "nonoverlapping averages.") To be in attainment, an area must meet the NAAQS for two consecutive years and carry out air quality monitoring during the entire time. Air quality carbon monoxide value is estimated using EPA guidance for calculating design values (Laxton Memorandum, June 18, 1990).
Title 40, Part 50 of the Code of the Federal Regulations lists the ambient air quality standard for carbon monoxide.
Sections 107(d)(4)(A) and 186 of the Clean Air Act lists the requirements for designations and classifications of carbon monoxide areas.