the gases produced during the combustion of fuel in internal-combustion engines. Toxic components such as CO and NO₂ are among the constituents of exhaust gases. The CO content in the exhaust gases of carburetor engines varies from 0.5 to 13 percent. Most of the CO is the result of incomplete combustion caused by poor carburetion or other engine malfunction. Especially harmful components such as nitrogen oxides and certain carcinogens are contained, although in small amounts, in diesel exhausts.

The toxicity level of engine exhaust gases can be reduced by installing catalytic afterburners in the exhaust system, by improving carburetors, and by a newly developed torch ignition system that ensures more complete combustion of the fuel. In the USSR the maximum permissible content of CO in the exhaust gases of carburetor engines and of fumes from diesel engines is regulated by special standards.