Carbon monoxide poisoning: A potentially deadly condition caused by breathing carbon monoxide gas, which prevents oxygenation of the blood. Common causes of carbon monoxide poisoning include malfunctioning furnaces and the use of kerosene heaters or similar devices in unventilated indoor spaces. Carbon monoxide is also emitted by automobile and other engines, so these should not be run in unventilated spaces, such as closed garages. Inexpensive alarms are available that can detect dangerous buildups of carbon monoxide. The treatment for carbon monoxide poisoning is immediate reoxygenation of the blood in a hospital.