Carbon monoxide, represented by the chemical formula CO, is a colorless and odorless gas. It contains one atom each of carbon and oxygen connected by a triple bond. Carbon monoxide is formed by incomplete combustion of carbon-containing compounds (typically fossil fuels, such as coal, kerosene, natural gas, etc. and in automobile engines). Naturally it is produced by a photochemical reaction in the troposphere.
Carbon monoxide is mostly known for its harmful effects on human health. It combines with hemoglobin in our blood, creating a compound known as carboxyhemoglobin which reduces the oxygen carrying capacity of our blood. Each year a large number of deaths are reported due to carbon monoxide poisoning. Examples include being stuck in a running car, sleeping in a (non-ventilated) room with a coal or wood-based heater, etc.