A colorless, odorless, poisonous gas released during the incomplete combustion of carbon-containing fuels.

Above definition is Copyright © The Home Ventilating Institute. Some content originally appeared in John Bower's book, Understanding Ventilation, published in 1995 by The Healthy House Institute.

From HHI:

Carbon monoxide is formed when fossil fuels such as gasoline, diesel, natural gas, heating oil, kerosene or propane don’t burn completely. Once inhaled, CO hijacks the hemoglobin cells that normally carry oxygen to all parts of the body.

Early symptoms of carbon monoxide poisoning are similar to those of influenza, including nausea, headaches, confusion, fatigue, dizziness and breathing difficulties. Left untreated, CO poisoning can cause death.

Most people are familiar with the warning against leaving an automobile, lawn mower or generator running in a closed garage. However, carbon monoxide is a potential hazard with fireplaces and any fuel-burning appliance. Following are a few safety tips:

• Never use a portable heating device fueled by kerosene, propane, liquefied natural gas or other fossil fuels in any enclosed space. These appliances were designed as space heaters for open sheds or open garages, NOT as heating appliances inside a dwelling.

• Never use a charcoal or gas grill inside a home or other enclosed space.

• If you prefer wood fires in your fireplace, have the chimney or exhaust stack inspected and cleaned regularly by a qualified chimney sweep.

• Natural-gas appliances such as furnaces and hot-water heaters should be cleaned and inspected regularly by a heating professional. Insufficient venting of exhaust gases and cracked furnace heat exchangers are frequent culprits in CO production. Contractors use sensitive instruments capable of detecting the presence of carbon monoxide long before it becomes a hazard throughout the home.

• Install at least one carbon monoxide alarm in your home as a “fail-safe” measure, in case something goes wrong with one of the appliances mentioned above.

More info about carbon monoxide.