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Carbon Monoxide (CO)

Carbon monoxide (CO) is a colorless, odorless gas emitted by motor vehicles and other sources. CO is formed when fuels such as gasoline, wood or coal, are not completely burned. 

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.

CO can affect the cardiovascular and nervous systems and cause death in high concentrations. 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 environment for seven air pollutants.

Current CO Standards:

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

Carbon Monoxide Trends

Short-term trends:

Carbon monoxide (CO) is a localized pollutant that disperses rapidly. In Pima County, on-road motor vehicle exhaust produces about 60% of the carbon monoxide.

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.

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. 

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.


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