Colorless, odorless, combustible, and lethal gas produced by incomplete combustion of fossil fuels (coal, natural gas, petroleum), biomass, and carbon containing products (such as wood). CO is a hematopoietic agent because hemoglobin in blood can absorb it 200 times more than oxygen. Therefore, specially in closed spaces, CO works as an asphyxiant by drastically depleting the blood's ability to carry oxygen.