Carbon monoxide is an odorless, colorless poisonous gas that is produced by the incomplete burning of any fossil fuel -- "anything from coal to wood to gas to gasoline to kerosene … any fuel except electricity," said Dr. Ron Roth, an emergency room physician at UPMC Presbyterian.

Fumes can build up around improperly installed or vented appliances; cars left running in closed, attached garages; and by outdoor charcoal grills brought indoors because of inclement weather.

"It can kill rather quickly," said Dr. Kenneth Katz, chief of medical toxicology at UPMC Presbyterian. "It doesn't allow your blood to deliver oxygen to tissue. It's essentially chemical asphyxia."

All people and animals are at risk for CO poisoning, but certain groups, like unborn babies, infants and people with chronic heart disease, anemia and respiratory problems, are more susceptible, according to the Centers for Disease Control and Prevention.

More than 500 Americans die from accidental CO poisoning each year, the CDC says, and more than 2,000 commit suicide by intentionally exposing themselves.

Initial symptoms mimic many common illnesses: headache, fatigue, shortness of breath, nausea and dizziness.

Some tips to avoid exposure:

• \u2002Install CO detectors on every level of your home. "It does diffuse through the house if it's leaking in the basement," Dr. Roth said. "If you have an alarm in the basement you can catch it earlier."

• \u2002Have furnaces and vents (which should point upward) checked for leaks before winter each year.

• \u2002Flues on working fireplaces also should be inspected regularly.

• \u2002Make sure all appliances are installed properly.

• \u2002Never use gas-powered kitchen appliances to heat your home.

• \u2002Never use an outdoors-type appliance like a charcoal grill or kerosene light in your home or any other enclosed space, including tents.

One also can get carbon monoxide poisoning from sitting too close to the exhaust of a boat motor; in a car with a leak in its exhaust system if the windows are closed; and by sitting within an unvented plastic cap on the back of a pickup truck.

In short, said Dr. Roth, who also teaches emergency medicine at the University of Pittsburgh medical school, "a lot of it is common sense."