In the Phoenix area, exceedances of the national health-based air quality standard for carbon monoxide occur exclusively during the months of November through January when lower temperatures increase CO emissions from automobiles and weather conditions trap these emissions close to the ground.
Carbon monoxide enters the bloodstream and reduces oxygen delivery to the body's organs and tissues.
The health threat from CO is most serious for those who suffer from cardiovascular disease. At higher levels of exposures, healthy individuals are also affected. Visual impairment, reduced work capacity, reduced manual dexterity, poor learning ability, and difficulty in performing complex tasks are all associated with exposure to elevated CO levels. Under the Clean Air Act, the State of Arizona was required to develop a plan to reduce carbon monoxide in the Phoenix area to healthy levels. This plan was due to EPA by February 28, 1998.