Carbon monoxide CO is a gas and is found in air. High levels of carbon monoxide are poisonous to humans and, unfortunately, it cannot be detected by humans as it has no taste or smell and cannot be seen.
The natural concentration of CO gas in air is around 0.2 parts per million (ppm), and that amount is not harmful to humans. Natural sources of carbon monoxide include volcanoes and bushfires.
The main sources of additional carbon monoxide CO are motor vehicle exhaust and some industrial activities, such as making steel.
Tobacco smoke is one of the main indoor sources of carbon monoxide CO.
