Carbon monoxide (CO) is a colorless, odorless gas that is a product of the incomplete combustion of carbon-based fuels — including gasoline, diesel fuel, crude oil and wood — and other natural and synthetic products. Main contributors of carbon monoxide emissions include vehicle exhaust from cars, trucks, and buses; gas-powered furnaces; and portable generators.


The greatest sources of carbon monoxide in outdoor air are cars, trucks, and other vehicles or machinery that burn fossil fuels.

Health Affects of Carbon Monoxide

Breathing air with a high concentration of CO reduces the amount of oxygen that can be transported in the blood stream to critical organs like the heart and brain. At very high levels, which are possible indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness and death. Very high levels of CO are not likely to occur outdoors. However, elevated CO levels outdoors (in ambient air) can particularly affect people with some types of heart disease. These people already have a reduced ability to get oxygenated blood to their hearts and are especially vulnerable to the effects of CO when exercising or under increased stress. In these situations, short-term exposure to elevated CO may result in reduced oxygen to the heart accompanied by chest pain.

Air Quality Standards and Monitoring

The primary National Ambient Air Quality Standards (NAAQS), based on health effects, for CO are 35 parts per million (ppm) averaged over one hour and 9 ppm averaged over eight hours (not to be exceeded more than once per year).

Missouri monitors CO concentrations in the air at locations across the state. Two of these sites are near roadways with heavy traffic in St. Louis and Kansas City. Click on the Monitoring tab to learn more about these air monitoring sites. Click on the Data tab to learn more about the data collected from this sites.

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


Carbon Monoxide

Carbon monoxide has no color, taste or odor. It comes from the incomplete combustion of fuels with carbon, such as oil, coal, wood, gasoline and natural gas. Vehicle emissions account for the largest source. Breathing air with high concentrations of CO can result in multiple health effects.

Since 1971, EPA has maintained two standards for carbon monoxide. The eight-hour standard stands at nine parts per million, and the one-hour standard stands at 35 parts per million. If design values are at or below the standard, then an area is in compliance. Design values correspond to an annual average of eight-hour concentrations and one-hour concentrations. 

Departmental information about carbon monoxide (CO)

  • 1971 standard and related document


Carbon Monoxide Monitoring Network