2. RELEVANCE TO PUBLIC HEALTH 2.1 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO CARBON MONOXIDE IN THE UNITED STATES Carbon monoxide is a colorless, odorless, non-irritating, and tasteless

Carbon monoxide is a colorless, odorless, non-irritating, and tasteless gas that is ubiquitous in the atmosphere. It arises from both natural and anthropogenic sources. It is produced as a primary pollutant during the incomplete combustion of fossil fuels and biomass. Carbon monoxide is also produced indirectly from the photochemical oxidation of methane and other volatile organic compounds (VOCs) in the atmosphere. Vegetation can emit carbon monoxide directly into the atmosphere as a metabolic byproduct, and the photooxidation of organic matter in surface waters (lakes, streams, rivers, oceans) and surface soils also results in the formation of carbon monoxide. Volcanic activity is an additional natural source of carbon monoxide in the atmosphere. The vast majority of anthropogenic carbon monoxide emissions arise from gasoline-powered automobile usage, although the total amount of carbon monoxide emitted to the environment from this source has declined significantly over the past several decades due to the use of catalytic converters and other emission control devices that are standard equipment on modern passenger vehicles.