Canadians battling blizzards and plunging temperatures may feel they have escaped from harm once they're safely snuggled up in their warm, well-insulated houses, condominiums and apartment buildings. However, each year, hundreds of Canadians living in homes heated with fuel-burning furnaces discover an invisible danger lurking within their walls -- a danger that may turn into a silent killer.

The Technical Standards and Safety Authority (TSSA), Ontario's fuels safety organization, reports a number of carbon monoxide poisonings over the past few weeks that have resulted in serious injury and, in extreme cases, death. A recent incident hospitalized three residents when carbon monoxide (CO) was sucked into a newly occupied Mississauga highrise condominium through an air intake installed too close to an exhaust valve.

TSSA is among the safety, government and firefighting organizations intent on reminding Canadians to remain vigilant against this odourless, tasteless, invisible gas, which at high levels becomes a silent killer, often striking while people sleep.

Carbon monoxide is produced when fossil fuels such as natural gas, oil, wood, charcoal, gasoline, propane and kerosene do not combust completely because they do not receive enough air while they are consumed in furnaces, space heaters, water heaters, clothes dryers, ovens and other fuel-burning appliances. CO production is at a maximum during the startup of a cold gasoline-powered engine. Idling your car or gas mower in the garage can cause dangerous levels of CO to enter the home through connecting walls or doorways. Damaged or blocked venting as well as inadequate airflow can allow carbon monoxide to build up inside a house, apartment building, cottage, camper or even a tent. Homes that are too well sealed can also collect deadly CO.

Carbon monoxide, which accumulates in the blood reducing its ability to carry oxygen, can cause serious health problems at even low levels, including suffocation, brain damage or death. Early warning signs of poisoning may be confused with flu symptoms: tiredness, headaches, dizziness, nausea, vomiting and shortness of breath.

To protect your family, make sure carbon monoxide never enters your home:

1. Have a qualified service technician inspect and clean your fuel-burning appliances and furnace once a year.

2. Install at least one carbon monoxide detector in your home, carefully following manufacturer's instructions and complying with local CO detector bylaws. If you have only one, install it outside your bedroom. A second detector could be added near the furnace. Ideally, each level with a sleeping area should have a detector. To avoid sensor damage or false alarms, do not install in unheated, humid areas, near vents or within two metres of heating and cooking appliances. Put a label on the detector that will remind you when its reliable lifetime ends, i.e. "2008" if it will last three years. Test your detector regularly and replace batteries at least twice a year (perhaps, to coincide with Daylight Savings Time changes).

3. Cracks, corrosion or blockage, like birds' nests, in vent pipes, exhaust fans and chimney flues may interrupt ventilation and cause carbon monoxide build up. Arrange for annual inspections and cleaning by qualified professionals.

4. Do not use natural gas, propane or charcoal grills, space heaters, pressure-washers or outdoor fuel-burning apparatus inside your home or garage, even if the doors are open.

Most CO detectors sound an alarm when CO reaches a high level. Since long term, low-level exposure is of special concern for pregnant women, young children, the elderly and those with a history of heart or respiratory problems, detectors that can display both high and low levels may prove safer in the long run. These models may also be useful if outdoor CO produced by nearby heavy traffic elevates indoor CO levels, but does not trigger an alarm. Potentially-harmful CO levels may be monitored using digital display and memory features.

Not all carbon monoxide alarms sold in Canada are certified to Canadian safety standards so look for the CSA or ULC symbols. Canada Mortgage and Housing Corporation (CMHC) describes the three basic types of CO sensors as follows:

1. Metal Oxide Semi-conductor - The original technology for CO detection. Must connect to house power. Battery backup is available for up to 20 hours.

2. Biomimetic - Less expensive technology. Gel-coated discs darken in the presence of CO and colour change sounds an alarm. Can be battery operated.

3. Electrochemical - Chemical reaction with CO creates an electrical current, setting off an alarm. Highly sensitive and accurate readings at all CO levels. Most units come with a continuous digital readout and a memory feature for checking past CO levels.

When a CO alarm sounds, get out immediately. From outside the building, call the Fire Department at 911. Do not return to the home until it has been professionally tested, the problem identified and the source removed.

Carbon monoxide remains the primary cause of accidental poisoning deaths so don't let your guard down once you step across your threshold.

For more on CO safety visit www.safetyinfo.ca.