Carbon monoxide (CO) is a colorless, odorless gas that is formed by incomplete burning of fuels such as gasoline, oil, natural gas, and wood.  Approximately 58 percent of CO comes from motor vehicle exhaust and 22 percent comes from non-road engines and vehicles (such as boats, construction equipment, etc).  Other sources include industrial processes, residential wood burning, and forest fires. 

CO can reduce oxygen delivery to the body’s organs and tissues, straining vital organs such as the brain and heart.  In high concentrations, CO inhalation can cause death.  In 2003, EPA determined that Maricopa County had met the CO standards; however, air quality programs in the region must continue to be implemented to ensure that CO levels continue to meet the standard.