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Carbon monoxide, the most abundant air pollutant found in the community atmosphere, generally exceeds the mass of all other air pollutants combined (excluding CO2). Total annual tonnages of CO from man‐made sources exceeded 132 × 106 metric tons (146 × 106 short tons) in the United States and 359 × 106 metric tons (396 × 106 short tons) globally in 1970. The largest single technological source of CO is the internal combustion engine. The atmospheric CO abundance in the biosphere in urban areas, where man resides, is not uniformly distributed and is dominated by anthropogenic sources. Community concentrations are quite variable and unequally distributed, being related to proximity to anthropogenic sources, patterns of human activity, and meteorological factors. These concentrations range from about 1 to >70 ppm and have brief peak levels in dense traffic as high as 140 ppm. The removal of CO from the atmosphere, as well as the sources and dispersion, controls the local concentration. Temporal characteristics of community CO concentrations are described. Although localized buildup of CO in cities may still represent a serious health hazard, man's CO production considered on a global basis is relatively minor in comparison with production from the larger natural sources. These include the atmospheric oxidation of methane, the ocean, decay of chlorophyll in plants, and terpene oxidation from plant sources. Their total source strength is estimated as 3–25 times the man‐made sources in the atmosphere as a whole. Background concentrations, a product of both natural and anthropogenic sources, normally range from 0.04 to 0.20 ppm, occasional concentrations being as high as 0.80 ppm at ground level in remote clean air areas, the higher concentrations being found in the northern hemisphere.

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