Carbon monoxide, CO

The substance
Carbon monoxide is a colourless, odourless, poisonous gas. It is formed when fuels containing carbon are burnt in conditions where oxygen is limited. It is slightly lighter than air. Carbon monoxide can form explosive mixtures with air.

Carbon monoxide was formerly widely encountered by the public as a constituent of 'town' gas - producing by roasting coal - which was used for domestic heating and even lighting since the 19th century. Carbon monoxide is the main reason for the very high toxicity of town gas. Town gas was replaced in the early 1970s by natural gas, which is mostly methane, and is very much less toxic. Poorly maintained natural gas burners may produce dangerous amounts of carbon monoxide, especially if not properly ventilated. Carbon monoxide is used for metal refining (notably nickel, using the Mond process) and as an intermediate in the production of certain chemicals.

Major emissions
Carbon monoxide is produced when fuels containing carbon are burnt in conditions where oxygen is limited. Petrol engines are the main source of carbon monoxide. However, emissions are much lower from modern vehicles fitted with catalytic converters. Carbon monoxide concentrations in urban areas are closely related to motor traffic density and to weather conditions. Concentrations can vary greatly during the day reflecting traffic levels and speed. Vehicles produce most carbon monoxide when idling or decelerating. Other minor sources are power stations and waste incinerators. In the home, faulty gas boilers may release carbon monoxide and tobacco smoking releases carbon monoxide directly into the smoker's inhaled air. Carbon monoxide is also formed in large quantities in all reducing metallurgic processes such as e.g. blast furnaces. Natural processes also produce small amounts of carbon monoxide.

Impacts on human health and environment
Excessive exposure to carbon monoxide may affect the blood, brain, heart and the unborn child. Carbon monoxide reacts with other pollutants to produce ground-level ozone, which can harm human health, damage buildings and plants.

Emission to air reporting threshold: 500000 kg/year

Data source: European Pollutant Emission Register