CO is toxic to humans because it is attracted to hemoglobin, the main component of red blood cells. Normally, hemoglobin carries oxygen throughout our bodies, releasing it to tissues as needed. When CO gas is present, it replaces the oxygen, and in heavy concentrations, can kill in minutes. In lower concentrations the symptoms mimic the flu or other viruses which are common in cold weather months.

Carbon monoxide is an invisible, odorless, colorless gas created when fossil fuels (such as gasoline, natural gas, heating oil etc) burn incompletely. Typically the exhaust contains high concentrations of CO, particularly at mixture settings richer than peak EGT. The most common way for this CO to find its way into the cabin or home is through the venting system.

Normally, oxygen inhaled into your lungs combines with the hemoglobin in the red cells of your blood to form "oxy hemoglobin."The oxygen is then transported throughout your body by your arteries and capillaries, where it disassociates from the hemoglobin and oxygenates the cells of your tissues and organs (including your brain). The deoxygenated hemoglobin then returns through your veins to your lungs, where it is combines with more oxygen and the cycle repeats.

When carbon monoxide is inhaled, the CO combines with your hemoglobin to form "Carboxyhemoglobin" (COHb). The COHb bond is over 200 times stronger than oxygen's bond with your hemoglobin. Thus, the CO effectively puts your hemoglobin "out of commission" and deprives your body of the oxygen it needs to survive. The strong COHb bond explains why even very tiny concentrations of carbon monoxide can poison you slowly over a period of several hours, and why it may take a long, long time for your body to eliminate CO buildups from your bloodstream.

How long? According to an authoritative medical text (Rosen's Emergency Medicine, 3rd Ed., 1992), COHb has a "half-life" of more than 5 hours for a patient breathing fresh air. If you're taken to the emergency room and they put you on oxygen therapy, the half-life drops to 1.5 to 2.5 hours (depending on whether the medics put you on a ventilator or just use a face mask). In extreme cases of CO poisoning, you may be rushed to a large medical center and put into a hyperbaric chamber with pure oxygen at three times normal atmospheric pressure, which reduces the half-life to under a half-hour.

According to the FAA Civil Aeromedical Institute, cigarette smoking will normally produce a COHb saturation of 3% to 10%. Smokers are consequently far more vulnerable to CO poisoning in flight, since they're already in a partially-poisoned state when they first get into the aircraft. Because of COHb's long half-life, smokers would do well to abstain from smoking for 8 to 12 hours prior to flight. (Unfortunately, the more common scenario is that the last cigarette is stubbed out on the tarmac moments before flight, and the next one is lighted seconds after the aircraft comes to a stop at the destination.)

As the CO level in your blood increases, the amount of oxygen transported to your body's cells decreases. It is this oxygen deprivation that makes CO so deadly. Sensitive parts of your body like your nervous system, brain, heart and lungs suffer the most from this lack of oxygen. Symptoms of mild CO poisoning include headache, fatigue, dizziness, vision problems (particularly double vision), nausea, and increased pulse and respiration. Unfortunately, these symptoms are often attributed to flu, indigestion, or the common cold. At higher levels of COHb saturation, you may suffer difficulty in breathing, loss of consciousness, collapse, convulsions, coma, and even death.

Just how sick you'll get from CO exposure varies greatly from person to person, depending on age, overall health, the concentration of CO (measured in parts per million), and the duration of exposure. High concentrations can cause incapacitation within minutes, but low concentrations can still be extremely dangerous if you're exposed for a period of hours. As CO continues to be inhaled, the percentage of COHb gets higher and higher, and you get sicker and sicker. Your eyes are particularly vulnerable to the effects of CO poisoning, and permanent damage can easily occur.

Whereas hypoxia tends to make you turn blue (the medical term is "cyanotic"), CO poisoning has the opposite effect — it makes you turn red. Carboxyhemoglobin is red in color, just as oxyhemoglobin is. (That's why a pulse oximeter is unable to detect CO poisoning.) But, since CO does not disassociate readily from hemoglobin the way O2 does, your venous blood remains red rather than turning the normal bluish color. This morbid little fact is useful mostly to coroners and morticians, however, because by the time CO poisoning has progressed far enough to turn you noticeably red, you're at least comatose if not dead.

The accompanying tables give you some idea of how various levels of CO concentration in the air and COHb saturation of the blood affect an average person. As you can see, a CO concentration of one tenth of one percent (1,000 parts per million) is enough to render you unconscious in an hour. OSHA has established the maximum permissible CO level for continuous 8-hour-per-day exposure in the workplace at 35 parts per million.

• Stale, stuffy air that never seems to clear.
• Excessive humidity that condensates on windows.
• A hot draft venting from the chimney into the home, or no draft at all in the chimney.
• Soot which accumulates around the outside of a fireplace, chimney, or furnace.
• A smell of exhaust fumes in the air.

• Persistent, severe headaches
• Dizziness or blurred vision
• Nausea and/or vomiting
• Chronic fatigue and drowsiness
• Rapid pulsed or fluttering of the heart, tightness of the chest
• Dizziness, fainting, unconsciousness, or dimmed vision
• The absence of any of these symptoms once you leave your house
• Confusion, anxiety, irritability, disorientation, loss of muscle control

An Ounce of Prevention...Here is list of things you can do to prevent the invisible killer from entering your home!

• Purchase and install Carbon Monoxide Detectors outside of sleeping areas and near all fuel-burning appliances.
• Check all your duct work which is vented to the outside (chimneys, water heaters etc) on an annual basis for any signs of blockage (bird nests, large twigs etc)
• If you have a fireplace or wood-burning stove, have the chimney cleaned annually.
• Have your heating system inspected annually, (before the cold weather sets in) to check for proper and safe operation of all parts.
• If your home is extremely airtight, you may want to contact your local utility company for information regarding adequate ventilation, back-drafting concerns, or to measure the carbon monoxide level in your home.
• Inspect your vehicle's exhaust system for leaks. Most muffler shops will do this for free.
• Don't forget your water heater or any of the fuel-burning appliances. Make sure that the appliance is properly vented and the ductwork does not leak any exhaust gases into the room.
• Use a Carbon Monoxide Detector. It's an economical and accurate way to warn you if dangerous levels of carbon monoxide are accumulating in your home or RV.
• Avoid using charcoal grills inside a home, tent, camper, or in an unventilated garage.

In 1992, Underwriter's Laboratory issued its UL2034 Standard for low-cost residential CO detectors. A number of manufacturers, including American Sensor, BRK Brands (First Alert), and Nighthawk Systems, quickly introduced uL approved CO detectors priced in the $50 range. A few companies ran a massive campaign of "scare tactic" TV ads and quickly became the leading supplier of residential CO detectors. The industry really took off when the City of Chicago mandated the installation of CO detectors.

The worst false-alarm offenders were the market-leading units that made use of the "biomimetic" (color-change) sensor technology. The sensor module simply passed a light beam through the "biomimetic" spot, and alarmed if the light was sufficiently attenuated (presumably because the spot had turned dark in color). Not only did this mean that the units had a limited sensor life and cross-sensitivity to gases and vapors other than CO), but the detector was plagued by false alarms due to the fact that other things could attenuate the light beam (smoke, contamination, even insects that crawled inside the sensor module).

In response to the false-alarm crisis, Underwriter's Laboratory revised its UL2034 Standard in June of 1995, but the false alarm problems didn't get any better. Meantime, in late 1995 and early 1996, the gas utility industry and the Consumer Product Safety Commission (CPSC) started getting concerned about the very opposite problem: CO detectors that would not go off when they should! While First Alert had obtained an exclusive license on the biomimetic sensor technology for residential CO detectors, virtually all other detectors sold prior to 1996 (including #2 and #3 market leaders American Sensor and Nighthawk) made use of a metal oxide semiconductor (MOS) sensor, which was the only other low-cost sensor technology available at the time. CPSC tests revealed that some of the MOS based units would fail to alarm even at life-threatening CO concentrations of 1,000 PPM or more! Many of these units were recalled.

In short, your choice in 1996 was between two sensor technologies, one (biomimetic) plagued by false positives and the other (MOS) plagued by false negatives. Since then, the industry has gone through considerable improvements. Pittway Corporation wound up divesting itself of First Alert, which subsequently went public, then nearly bankrupt, and finally was acquired by Sunbeam in 1998. American Sensor wound up going bankrupt, while the assets of Nighthawk Systems were acquired by fire extinguisher giant Kidde Safety who subsequently redesigned their CO detector products to use a more reliable sensor.

In 1998, Underwriters Laboratory revised its uL 2034 specification, but implementation was delayed until January 1, 2000. For a CO detector to be uL approved for residential use after that date,
uL requires that it must:

• alarm at:
  • 70 PPM between 60 & 240 minute of continuous exposure
  • 150 PPM between 10 & 50 minutes of continuous exposure
  • 400 PPM between 4 & 15 minutes of continuous exposure
• ignore CO levels less than 30 parts per million (PPM) for at least 30 days and ignore levels of 70 PPM or less for at least an hour. This requirement was imposed by uL at the request of gas utilities and firefighters to minimize the number of unnecessary emergency calls from homeowners.
• have a silence button. Pushing it will silence the alarm for four to six minutes. If the concentrations stay above 70 PPM it will alarm again in six minutes.
• not alarm in the presence of Methane (below 500 PPM), Butane (below 300 PPM), Heptane (below 500 PPM), Carbon Dioxide CO2(below 5,000 PPM) , Isopropyl Alcohol (below 200 PPM) and Ethyl Acetate (below 200 PPM)