A by-product of combustion, this invisible, colorless, odorless, and tasteless toxic gas gets into the bloodstream and weakens the body's defenses.

Carbon monoxide is produced when natural fuels are burned at high temperatures with incomplete combustion in a gas or oil furnace, gas water heater, fireplace, space heater, wood stove, or any appliance that operates on a flammable fuel like natural gas, fuel oil, wood, coal, or kerosene, as well as gasoline, propane, and diesel. This also includes gas ranges and ovens, gas clothes dryers, and charcoal grills.

Inadequately ventilated fireplaces, cracked furnace heat exchangers, blocked chimneys, flue pipes that leak, and even low air pressure inside your home that sucks flue gases back down the chimney can create potentially lethal carbon monoxide situations. All chimneys should have screens to keep out birds and small animals, greatly eliminating the chance of blockage.

Remember too, fumes can be sucked into a home from the outside from a malfunctioning pool heater, through air conditioning systems, and most often from an attached garage where a vehicle has been left running.

While there are classic signs of carbon monoxide's initial symptoms - headaches, nausea, diarrhea, drowsiness, and fatigue - they are too often dismissed as a flu-like illness. Some victims go to bed thinking they will feel better, but they are merely staying in the same dangerous environment in which they became ill.

Other symptoms can be chest pains, weakness, vomiting, confusion, disorientation, and sleepiness.

With any of these reactions to suspected carbon monoxide poisoning, get fresh air immediately. Get away from the contaminated environment and seek medical attention. Ask for a blood test that will show if carbon monoxide is the cause of your symptoms.

However, just because you don't have a physical reaction to carbon monoxide, it doesn't mean everything is okay. Some people can have a carbon monixide build-up without experiencing any pain or sickness.

According to The Journal of the American Medical Association (JAMA), 1,500 people die each year from accidental carbon monoxide poisoning, and another 10,000 seek medical attention as a result of inhaling this toxic gas.

Every situation of carbon monoxide poisoning is different. In some cases a low-level accumulation of the gas is gradual; with others a high concentration accelerates the problem. Either way it can be deadly. When carbon monoxide builds up in a home, it becomes, in effect, a gas chamber.

Carbon monoxide is the same weight as air, and mixes with oxygen we breathe. As a result, it does not dissipate immediately. And, because it has a half life, it can take twice as long to leave your body as it did to accumulate.

When we breathe in air, oxygen passes through the lungs and then enters the bloodstream, where it bonds to a protein molecule called the hemoglobin. The oxygen is then carried to the body's muscles and organs.

But when carbon monoxide is inhaled, it latches onto the hemoglobin, and pushes the blood's normal partner oxygen out of the way. Carbon monoxide is 240 times stronger than oxygen and easily displaces it in the bloodstream, depriving tissues of their life-sustaining needs. As the carbon monoxide gradually overwhelms the oxygen in one's blood, the victim slips into a slow, relaxed suffocation from the inside. Death comes quietly from the fumes of this silent time bomb.

At increasing stages of carbon monoxide exposure there are advancing symptoms. At a 10 percent level you may not notice anything. However, as the percentage of carbon monoxide increases in the blood, those affected advance through progressively more serious stages beginning with a headache, developing into drowsiness and nausea, then confusion, fainting, and finally slipping into a coma.

Everyone is different, and how quickly the CO level in your blood rises varies. Unborn babies, infants, the elderly, and those suffering from cardiac and/or respiratory problems are considered more vulnerable to this deadly toxin.

Sadly, unborn babies are at the greatest risk. A single exposure of expectant mothers to high levels of carbon monoxide can result in the death of the baby, while the mother survives.

This is because there is a much higher metabolic rate in developing fetuses (as well as young infants), and their affinity for carbon monoxide is hundreds of times greater than that for grown adults.

Carbon monoxide poisoning also has lingering effects. Survivors of severe incidents can have permanent damage, including mental impairment, speech disorders, and/or problems with vision and haring. Long-term effects can also include memory loss, neurological damage, and personality changes.

There are many common sense steps to avoid carbon monoxide exposure, but the most obvious one is to have an annual inspection of your central heating system, as well as gas water heaters, gas dryers, ovens, ranges, and cooktop vents.

Modern construction has made newer homes more airtight. Gone are the leaky windows, door gaps, and drafts that let outside air into the house. Beter insulation also keeps the house sealed. Couple this with the growing number of exhaust divices that vent not only furnaces and water heaters, but kitchens, bathrooms, and clothes dryers, and you help create negative air pressure inside the home - a perfect situation to draw carbon monoxide gas right back into your energy-effecient structure, and to trap the poluted air inside.

The U.S. Consumer Product Safety Commission in Washington, D.C., calls carbon monoxide poisoning a senseless killer because the installation of just one carbon monoxide detector near a sleeping area in every home could save many lives.

For more information about carbon monoxide poisoning and detectors:

The Consumer Product Safety Commission offers a brochure on carbon monoxide. Send a post card with your name and address to A Senseless Killer - Carbon Monoxide, Consumer Produce Safety Commission, Washington, D. C. 20207. For a recorded message you can call (800) 638-2772 and follow the prompts. To order the brochure electronically, visit their website at www.cpsc.gov.

The American Lung Association, through local chapters or the national office at 1740 Broadway, New York, New York 10019; ph. (212) 315-8700.

American Sensors Electronics, healthy home hot line (800) 387-4219. American Sensors has produced a brochure called, Is Your Family Safe? available through their toll-free number, or you can write them at Suite 15 East, 100 Tempo Avenue, Toronto, Ontario, Canada M2H 3S5; www.nadi.com.

Underwriters Laboratories offers a free booklet, Questions and Answers about Carbon Monoxide and CO Detectors. Write: UL Literature Department, CO Brochure, 333 Pfingsten Road, Northbrook, Illinois 60062; ph. (847) 272-8800; www.ul.com.

The Gas Appliance Manufacturers Association (GAMA) offers free pamphlets on gas space heaters and carbon monoxide. Phone (800) 603-9293.

Symptoms of carbon monoxide poisoning

The heart and the brain require the most oxygen, and are the organs at greatest risk of injury from lack of oxygen associated with carbon monoxide poisoning. Most signs are nonspecific and are often confused with those of other medical problems.

Percentage of hemoglobin carrying carbon monoxide instead of oxygen:

10% Usually no sign in healthy individuals; reduced exercise tolerance in those with lung disease; lower threshold for angina in those with coronary heart disease.

15% Slight headache, shortness of breath on mild exertion, angina in coronary patients, dilation of surface blood vessels.

20% Throbbing headache, nausea or vomiting, fatigue, irritability, and difficulty concentrating.

30% Severe headache, dizziness, fatigue and weakness, faintness on exertion, impaired thinking.

40% Rapid breathing, rapid heartbeat, fainting, confusion.

50% Respiratory failure, collapse, intermittent convulsions or seizures, coma.

60% Severe respiratory failure, extremely low blood pressure, frequently fatal coma.

70% Rapidly fatal coma.