Carbon Monoxide, or CO

1971 CO NAAQS (1-hour standard: 35 ppm, 8-hour standard: 9 ppm)

The CO NAAQS were established in 1971. Although the CO NAAQS have gone through multiple periodic EPA reviews as required by the Clean Air Act, the NAAQS have not been changed since that time. Therefore, no boundary designation processes for CO have occurred in many years.

Additional information is available on EPA's Carbon Monoxide webpage.

1971 Standard

One-hour standard: 35 ppm; Eight-hour standard: 9 ppm

The one-hour and eight-hour values for this standard did not change in 2011. A violation of this standard occurs if either the one-hour average or eight-hour average standards are exceeded more than once in a year.

St. Louis Area




MACC Adoption: March 2014

80 FR 59614

1971 Limited Maintenance Plan for the St. Louis Nonclassifiable Maintenance Area for the 8-Hour Carbon Monoxide NAAQS

Nitrogen Dioxide, or NO2

Additional information is available on EPA's Nitrogen Dioxide webpage.

2010 Standard

Annual Standard: 0.053 ppm; One-hour standard: 100 ppb

In 2010, EPA promulgated a revision to this NAAQS by establishing a new one-hour standard and retaining the annual standard. The one-hour standard is based on a three-year design value, which is calculated by taking the 98th percentile of the daily high one-hour average concentrations recorded each year, for three years, and averaging the three years together. The annual standard is based on the annual arithmetic mean and is not to be exceeded.

Infrastructure SIP

Date EPA Approval Document

MACC Adoption:
April 2013


State of Missouri Plan for Implementation, Maintenance and Enforcement of National Ambient Air Quality Standards - Plan for the 2010 Nitrogen Dioxide 1-hour Standard

Boundary Recommendation

Date Action

December 2010

2010 NO2 NAAQS Boundary Recommendation Letter to EPA

August 2011

State Response to EPA 120-Day Letter for the 2010 NO2Boundary Designations

Related Links

Summary of Monitored Values of Carbon Monoxide
Preliminary 1-hour CO parts per million, CST
Missouri Carbon Monoxide (CO) Air Monitoring Sites

Summary of Monitored Nitrogen Dioxide Values
Preliminary 1-hour NO2 parts per billion, CST
Missouri Nitrogen Dioxide (NO2) Air Monitoring Sites

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