Carbon monoxide is a colourless, odourless poisonous gas produced by incomplete, or inefficient, combustion of fuel including 'cold' or badly tuned engines. It is estimated that road transport is responsible for almost 90 per cent of all carbon monoxide emissions in the UK. Badly ventilated domestic fuel appliances (gas, oil or solid fuel) can cause high levels indoors, as can smoking.

The gas affects the transport of oxygen around the body by the blood. At very high levels, this can lead to a significant reduction in the supply of oxygen to the heart, particularly in people suffering from heart disease.

As traffic is a major source of carbon monoxide, ambient concentrations will generally be highest close to busy roads. Monitoring data suggests that annual average carbon monoxide levels have been decreasing over the last few years. This is probably due to improved vehicle engine efficiency and the introduction of catalytic converters. The effect of technological improvements has been cancelled out to some degree by an increase in traffic levels. Concentrations will generally be highest close to busy roads.