It's what people don't know that can kill them.

We know that carbon monoxide is a killer.

We don't know how many people die from accidental CO poisoning. Estimates range from a couple hundred a year in the United States (Consumer Product Safety Commission) to a couple thousand (American Medical Association), but nobody knows for sure.

We know the symptoms of CO poisoning include headache, dizziness, nausea, weakness, sleepiness and disorientation in various combinations.

We don't know how often CO poisoning gets misdiagnosed as flu, food poisoning or scores of other ailments with similar symptoms. The CPSC claims that more than 10,000 people a year get treated in U.S. hospitals for CO sickness, but it's likely the statistics for both fatalities and hospital visits would escalate quite a bit if the symptoms were more clear-cut.

We know that around two-thirds of Americans use gas, oil, wood, kerosene or coal as their major heat source. And at least the vast majority of the American public doesn't know that combustion of these fuels inevitably produces CO.

We know that CO danger is greatest at night when residents are asleep and cannot detect the symptoms.

We don't know why only 27 percent of U.S. homes have CO alarms, and why more people don't install CO detectors like they do smoke detectors - on every level of the home and especially in sleeping areas - to protect themselves from CO buildup at night. We also don't know why more heating contractors don't sell CO detectors, or even give them away as premiums with expensive jobs.

We know that in 1994 Chicago became the first major city in the country to require CO detectors in all residences heated by fossil fuel-burning appliances. This was in reaction to a horrifying 1991 incident in which a family of 10 was wiped out from CO poisoning due to a faulty furnace installation.

We don't know why more municipalities haven't made CO detectors mandatory.

We know the American public and news media freak out over isolated health problems caused by exposure to toxic mold.

We don't know why they don't get more agitated about CO hazards that result in far more illnesses and deaths.

Actually, we know why. It's because such stories don't lend themselves to exciting visuals involving flames or heaps of twisted metal that once were recognizable as automobiles. In the world of TV news, if it doesn't bleed, it doesn't lead - unless it involves celebrities, such as tennis star Vitas Gerulaitas, who died of CO poisoning in 1994.

We know that CO can result from incomplete combustion of flammable fuels and improper venting.

We don't know why so many half-baked do-it-yourselfers try to mess with installing fuel-burning appliances that can create CO hazards.

We know that CO danger is greatest in winter when heating equipment gets its most strenuous workout, and when doors and windows are shut tight.

We don't know why all homeowners don't get an annual fall heating system inspection and tune-up by a professional contractor.

We know that a competent heating technician will routinely inspect flue piping connections and vents for cracks, gaps, rust, corrosion or debris. Likewise, the technician will check the combustion chamber and heat exchanger for cracks, holes, metal fatigue or corrosion, as well as the filters and filtering system for dirt and blockages. The competent technician also will clear debris off the burner and test safety switches.

We don't know why heating contractors don't detail all this as a way of showing the value they offer as part of annual clean-and-check inspections.

We know that CO levels over 200 parts per million lasting for 90 minutes can cause headaches in healthy adults, and 500 ppm exposure could result in brain damage or death.

We don't know why so many heating contractors don't carry calibrated CO detection equipment capable of detecting dangerous CO buildup.

We know that the heating contractors of our industry understand as well as anyone the dangers of CO, and how to protect against it.

We don't know why this isn't common knowledge with the American public.

Actually, we know why. It's because the people of our industry don't do enough to draw attention to the life-saving value they offer.

We don't know why exactly, except heroes tend to have an aversion to tooting their own horn.

We know it's time for people in the heating business to start asserting themselves as CO abatement experts.