Carbon monoxide

Carbon monoxide (CO) is a highly toxic, colourless, odourless and flammable gas. It is toxic at levels of only 0.1 per cent (1000 ppm). Carbon monoxide's toxicity arises from its ability to bind to transition metals such as iron in haemoglobin, the oxygen-carrier protein in blood. By binding strongly to the Fe-centre of haemoglobin, CO blocks the uptake of oxygen and thereby its transport around the body. 

A product of volcanic activity, CO is present in the atmosphere at ca  0.1 ppm. But today CO is recognised as a polluting gas produced by the incomplete combustion of carbon-based fuels, eg in the internal combustion engine with CO present in car exhaust fumes. In urban areas such as Mexico City CO levels can rise to 100-200 ppm.  

Carbon monoxide has, however, many important industrial uses. For example, the gas is used extensively for the extraction of nickel in the Mond process, and in the reduction of iron oxide in the blast furnace, where the CO is produced in situ from the reaction between carbon dioxide and red-hot coke.