An odorless, colorless, and highly poisonous gas. More: If breathed in, carbon monoxide blocks oxygen in the blood from being delivered to the rest of the body. Prolonged exposure can lead to brain damage and even death. It is released by the incomplete combustion of fuels such as natural gas, charcoal, gasoline, and tobacco. Sometimes referred to as the "silent killer", carbon monoxide can for instance be released in homes by heating systems that do not work properly, claiming many lives every year.