Every winter it happens: A fuel-burning appliance such as a gas stove, clothes dryer or water heater malfunctions. Or a house is so tight and snug that a fireplace or kerosene heater depletes the oxygen supply. The house fills with carbon monoxide, and residents become ill or die.
According to Underwriters Laboratories, the product-testing organization based in Northbrook, Ill., more than 500 people in the United States are killed each year in carbon-monoxide-related incidents. Of that number, more than 200 are killed by carbon monoxide emitted from a consumer product such as a stove or water heater. Another 10,000 seek medical attention after accidental carbon monoxide poisoning.
"You can't hear, taste, see or smell it," said Paul Patty, Underwriters Laboratories' resident expert on carbon monoxide. "It's nicknamed the "silent killer' because it sneaks up on its victims and can take lives without warning."
It can also cause brain damage. Unborn babies, infants and children are particularly susceptible.
Carbon monoxide (chemical symbol, CO) is a byproduct of incomplete combustion, Patty said. Sources of CO include malfunctioning appliances _ furnaces, stoves, ovens and water heaters _ that burn fossil fuels such as natural gas or propane.
Other sources include vehicle exhaust, blocked chimney flues and gas or charcoal grills that are used in the home or in an unventilated tent, camper or garage.
Patty said the initial signs of carbon monoxide poisoning are flulike symptoms, including nausea, fatigue, headaches, dizziness, confusion and difficulty breathing. "Because CO poisoning often causes a victim's blood pressure to rise, the victim's skin may take on a pink or red cast," he said. "Young children and household pets are typically the first affected."
Fortunately, relatively inexpensive devices can detect harmful levels of carbon monoxide even before the symptoms of CO poisoning are felt.
Ken Giles, a spokesman for the Consumer Product Safety Commission in Washington, said that although early versions of carbon monoxide alarms were somewhat unreliable _ some would detect atmospheric carbon monoxide and trigger a false alarm _ alarms on the market today recognize carbon monoxide from a fuel-burning appliance in the home and then trigger an alarm before CO levels become dangerous.
"A carbon monoxide alarm today is triggered by a combination of the amount of carbon monoxide and the duration of its presence," Giles said. For example, he said, a carbon monoxide alarm should sound when it detects 30 parts per million of CO for 30 days; 70 parts per million for 180 minutes; 150 parts per million for 50 minutes; and 400 parts per million for 15 minutes.
Heather Peets, vice president of operations for Senco Sensors, a manufacturer of carbon monoxide alarms, based in Vancouver, British Columbia, said three technologies are used in CO detectors: biomimetic, metal oxide semiconductor and electrochemical.
Biomimetic detectors, Peets said, use chemicals that darken when carbon monoxide is present, thereby blocking an internal light source from reaching a receiver and sounding the alarm. Though biomimetic technology is still used, such detectors are sensitive to temperature and humidity and are not as precise as newer technologies.
With a metal oxide semiconductor detector, Peets said, carbon monoxide is detected through the periodic heating of a chemical in the unit to more than 500 degrees. Though such detectors are accurate, they are typically "hard-wired" to the home's electrical system. The most advanced technology today is electrochemical, which uses platinum electrodes imbedded in an electrolyte solution to form a sensor. Such units are precise and battery-operated and can provide instantaneous monitoring of carbon monoxide at even very low levels.
Senco's Model One _ which was highly rated by Consumer Reports in a 2001 study _ has three distinct danger-level alarms: low, high and crisis. The portable battery-powered unit has a digital display and a test function that displays an instant carbon monoxide reading in any room or near any appliance.
"A typical detector takes a reading once every minute," Peets said. "But we have a test mode that allows the unit to take two readings per second down to 10 parts per million." The Model One, she said, retails for about $50 (www.sencosensors.com).
Joe Hlavacek, product manager for Aprilaire, a Madison, Wis., manufacturer of air-quality products, said one big problem with carbon monoxide detectors is that none lasts indefinitely. "The sensor degrades," he said, adding that in most cases, carbon monoxide detectors should be replaced at least once every five years.
His company's detector constantly monitors itself, he said, and will sound an alarm when it is no longer functioning efficiently.
"The Aprilaire carbon monoxide alarm actually informs you when it's working, when it needs a new battery and _ most important _ when it needs to be replaced," Hlavacek said.
The Aprilaire unit, sold and installed by local dealers and contractors, costs $45 to $75 installed. (Dealers are listed on the company's Web site, www.aprilaire.com.)
Another Aprilaire product _ CO Detectagas _ can be used by homeowners who want to make sure their existing carbon monoxide detectors are functioning properly.
Lauren Hackett, a spokeswoman for Consumers Union _ the company based in Yonkers, N.Y., that publishes Consumer Reports _ said that though most carbon monoxide detectors have a test button, pushing that button generally means that you are checking the electrical circuits, not that the sensor is responding to the presence of CO.
It is possible to check the functioning of the sensor, however, by using Aprilaire's CO Detectagas kit.
To test an alarm, she said, the detector is sealed inside the plastic bag provided with the kit, a hole is punched in the bag and a five-second burst of canned carbon monoxide is injected into the bag, which is then sealed. "The alarm should respond within 15 minutes," Hackett said, adding that the test uses an amount of CO that is small enough not to be a cause for concern. (Hackett said Consumers Union bought the kit online for $18 plus shipping at www.gogeisel.com.)
She said that all homes should have at least one carbon monoxide detector, but ideally alarms should be installed on each level.
Carbon monoxide tips
+ Never burn charcoal indoors, even in the fireplace. It can give off lethal amounts of carbon monoxide. Don't use a charcoal grill to cook inside the house or in a closed garage.
+ Never close your damper with hot ashes in the fireplace. This can cause the ashes to heat up again and force toxic carbon monoxide into the house.
+ If you use a kerosene heater or other fuel-burning heater, keep a window open to ensure a constant supply of fresh air. Never go to bed with a space heater in operation.
+ Don't operate gasoline-powered engines in confined areas such as garages. Don't leave a car, mower or other vehicle running in an attached garage, even if the door is open. Carbon-monoxide fumes can seep into your home via heating ducts or other openings.
+ Don't block or seal shut exhaust flues or ducts for appliances such as ranges, clothes dryers and water heaters.
+ Don't use ovens or household appliances (stoves, cooktops) that run on fossil fuel for heating purposes.
+ Keep chimneys clear of bird and animal nests, leaves and residue to ensure proper venting.
+ Low-level symptoms of carbon-monoxide poisoning are often misdiagnosed as the flu. Headaches, nausea, fatigue and dizziness are symptoms of carbon-monoxide poisoning. Persistent symptoms may mean you have a slow CO leak in your home.
Source: Kidde, manufacturer of home safety products