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Carbon Monoxide (CO)

Carbon monoxide is a colorless, ordorless gas emitted by motor vehicles as a result of incomplete combustion of fossil fuels. This occurs when carbon or substances that contain carbon are not burned completely, such as gasoline, wood or coal.

Emissions primarily occur from motor vehicle exhausts -- by automobiles, buses and trucks -- and some industrial processes. Carbon monoxide is found in high concentrations along the roadside, especially where there is heavy traffic. Other areas might include parking garages and poorly ventilated tunnels.

A poisonous gas that replaces oxygen in the blood, carbon monoxide can affect the cardiovascular and nervous systems. It enters the blood via the lungs and permanently binds to hemoglobin (the iron-containing protein in red blood cells), preventing hemoglobin from carrying oxygen. Lower concentrations of CO have been shown to affect people with heart disease such as angina, can cause dizziness, headaches and fatigue, and at high concentrations, even death.

The U.S. Environmental Protection Agency sets National Ambient Air Quality Standards to protect public health. The limits for carbon monoxide are set at:

  • 1-hour average: 35 parts per million
  • 8-hour average: 9 parts per million

Carbon Monoxide Trends

(click on graphs below to view the pdf version of the graph)

Short-term trends:

Carbon monoxide (CO) is a localized pollutant that disperses rapidly. In Pima County, on-road motor vehicle exhaust produces 50 percent of the carbon monoxide in the air.

CO concentrations tend to be highest in winter, when temperatures are cool, wind speeds are low and a temperature inversion is present. This occurs when a stable atmospheric layer restricts the mixing of pollutants.

The Tucson Air Planning Area exceeded the 8-hour Environmental Protection Agency health standard once in 1988 at two monitoring sites. A violation is defined as more than two exceedances at a site per year. The last violation was recorded in 1984, where two exceedances occurred at one site. No violation has been recorded since 1984.

Since the region violated the EPA health standard frequently in the 1970s, the Tucson Air Planning Area was classified as a nonattainment area for carbon monoxide and a state implementation plan was required to comply with the Clean Air Act.

In 2000, Arizona requested the EPA reclassify the Tucson region in attainment with the national health-based standard for carbon monoxide and, in the same year, the EPA approved a maintenance plan to ensure ongoing compliance. The CO Maintenance Plan was recently updated and approved by the EPA in 2009, with an effective date of January 2010.

The plan maintains existing controls and contingency provisions and replaces the 2000 plan. This revision ensures maintenance of the CO standard through 2020.

Long-term trends:

Since monitoring began in Pima County in 1973, carbon monoxide levels have decreased. The primary reasons for this are due to technological advances with the implementation of the following:   

  • Federal Motor Vehicle Control Program (tailpipe emission standards for new cars)
  • Annual state Inspection and Maintenance Program
  • Oxygenated fuels have been used each winter from September 30 to March 31, since the winter of 1990-1991

Other effective controls in the Tucson area that help reduce levels of this pollutant and promote the use of alternate modes are:

Alternate modes such as carpooling, taking the bus, walking, bicycling, and the use of telecommuting all serve to reduce the number of motor vehicles on the road. 

For more tips on ways to keep our air healthy, visit and the Clean Water Starts With Me Facebook page. 

High carbon monoxide concentrations tend to occur when wind speeds are low and strong inversions are present. The levels tend to rise following traffic peaks, with the highest levels occurring soon after rush hour.

Carbon Monoxide Limited Maintenance Plan Revision

The U.S. Enviromental Planning Agency approval of revisions to the Arizona State Implementation Plan : The 2008 Revision to the Carbon Monoxide Limited Maintenance Plan (CO LMP) for the Tucson Air Planning Area including a statutory provision for the vehicle emissions inspection program extension (Dec. 21, 2009).


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