Victims of carbon monoxide poisoning often mistake their symptoms for the flu and fail to get help, a sometimes deadly mistake, say doctors who see more cases in the winter months.
That's when malfunctioning furnaces and improperly ventilated fireplaces come on and some people use appliances such as stoves for heat.
"It's a problem across the country," said Dr. Clifford S. Mitchell, from the Maryland Department of Health and Mental Hygiene's Infectious Disease and Environmental Health Administration.
"While these deaths can be prevented, tragically, often people in tougher economic circumstances, or people in natural disasters who lose power and heat, focus on the short-term problems, so they can be vulnerable to a silent killer."
The gas is odorless and colorless, yet still lethal at low concentrations, he said. Victims will feel dizzy or have a mild headache and may fail to recognize the cause.
Carbon monoxide exposure causes an estimated 15,000 trips to emergency rooms across the country every year and 500 deaths, according to the U.S. Centers for Disease Control and Prevention.
In March, Baltimore joined other cities including Denver, New York and Boston in mandating carbon monoxide detectors in its dwellings. Baltimore's law requires the detectors outside all sleeping areas in properties with appliances that burn gas, petroleum products, wood and other fuels.
Maryland law requires hard-wired carbon monoxide alarms outside sleeping areas in houses, apartments, hotels and dormitories constructed after Jan. 1, 2008.
There are several steps property owners can take to prevent carbon monoxide poisoning, according to a paper prepared by Dr. Robert E. Rosenthal, head of hyperbaric medicine at the University of Maryland Shock Trauma Center.
They include: having a professional inspect the central heating system annually and repairing leaks, never idling the car inside a garage, keeping gas appliances properly adjusted, never using generators or gasoline-powered tools inside, installing an exhaust fan vented outside over gas stoves, opening flues when the fireplace is used, choosing properly sized wood stoves that meet federal emission standards and ensuring doors on all wood stoves fit tightly.
Shock Trauma sees the most serious carbon monoxide emergencies because it has a hyperbaric chamber and can treat other life-threatening problems at the same time, said Dr. Cynthia Cotto, one of the hyperbaric medicine physicians.
Carbon monoxide replaces oxygen in the blood sent around the body, and in severe cases the chamber is needed to administer oxygen at higher pressure, Cotto said. It's like getting two or three times as much oxygen, and can be a more effective antidote.
Recovery for adults and children depends on the amount of damage already done and other injuries, such as respiratory systems burned in related fires, and cardiac distress.
"I tell people that a $25-$30 carbon monoxide detector is a lot cheaper than a trip to the ER," she said.