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Carbon Monoxide (CO Gas) is The Invisible Killer

How it Kills. Carbon monoxide (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) burn incompletely. In a piston-powered aircraft, engine 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 home is through the home heating system.

Normally, oxygen inhaled into your lungs combines with the hemoglobin in the red cells of your blood to form "oxyhemoglobin." 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. 

CO Concentration
(parts per million)
35 No obvious symptoms after 8 hours of exposure.
200 Mild headache after 2 to 3 hours.
400 Headache and nausea after 1 to 2 hours.
800 Headache, nausea and dizziness after 45 minutes; collapse after 2 hours.
1000 Unconsciousness after 1 hour.
1600 Unconsciousness after 30 minutes.
Table 1. Effects of various CO
concentrations at sea level.
(At altitude, the effects of CO
poisoning and altitude hypoxia
are cumulative.)
Saturation (%)
0 - 10 None. (Smoking yields 3% to 10% COHb.)
10 - 20 Tension in forehead, dilation of blood vessels.
20 - 30 Headache and pulsating temples.
30 - 40 Severe headache, weariness, dizziness, vision problems, nausea, vomiting, prostration.
40 - 50 Same as above, plus increased breathing and pulse rates, asphyxiation.
50 - 60 Same as above, plus coma, convulsions, Cheyne-Stokes respiration.
60 - 70 Coma, convulsions, weak respiration and pulse. Death is possible.
70 - 80 Slowing and stopping of breathing. Death within hours.
80 - 90 Death in less than 1 hour.
90 - 100 Death within minutes.
Table 2. Effects of various COHb saturations.

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 five hours for a patient breathing fresh air. In other words, if you crash-land in a hay field with COHb saturation of 40%, your COHb level can be expected to drop to about 20% after five or six hours, to 10% after another five or six hours, and so forth. 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 docs 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.

Early Warning Signs 

In Your Home - Due to its nature, carbon monoxide cannot be detected by the human senses. But sometimes the gas will leave clues to its presence. These are things to look for which might indicate (but not always) that you have excessive concentrations of CO:

  • 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.

In Your Body

The physical symptoms of CO poisoning are often misdiagnosed as the flu or virus. Some of the symptoms may include:

  • 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

Infants, small children and persons with cardiovascular problems are more susceptible to the ill effects of carbon monoxide. Even low concentrations of the gas can cause problems for individuals in these categories. Since the physical symptoms of CO poisoning are similar to the flu or virus, it is important to remember that if you do experience some of these symptoms, you may indeed have the flu. But if the conditions persist and nothing seems to help, or your entire family seems to be experiencing the same illnesses, you should seek medical advice immediately and mention your fear of carbon monoxide poisoning.

An Ounce of Prevention . . .

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

  • Purchase & install a Carbon Monoxide Detector.
  • 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. Make sure that 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.

 This year, many people will needlessly die from Carbon Monoxide poisoning.

Passive CO Detectors

Passive CO Detector Badges

Each year, thousands of people die or are seriously injured from exposure to carbon monoxide gas. Low levels of exposure can cause symptoms which mimic the flu or other viruses. High levels of exposure can kill in minutes. Fortunately, there is a simple and inexpensive method for detecting this silent and invisible killer. The passive Carbon Monoxide Detector has an easy-to-see, visual indicator which warns of the presence of CO gas. The circle on the indicator will turn gray or black, depending on the concentration of CO. It will return to normal when exposed to fresh air again.

The detector has a convenient adhesive strip on the back, and mounts almost anywhere. Shelf life in an unopened pack is three years; once opened it should be replaced approximately every 60 days.

The detector is made of non-toxic materials, making it safe for use around children or pets.

It can detect as little as 100 ppm (parts per million) carbon monoxide gas. The detector will be inactivated and damaged by the presence of halogens (iodine, chlorine, bromine), and ammoniac or nitrous gases. Persons with cats may not be able to use the detector due to the presence of ammonia in the air from the cat litter box. Also, the detector is not an appropriate device for detecting CO gas in connection with diesel engines.

Active CO Detectors

In the early 1990s, a number of companies started selling low-cost electronic carbon monoxide detectors for consumer use. These seemed to offered great promise, but their history has been something of a roller coaster ride.

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 in residences beginning October 1, 1994.

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 electrochemical sensor technology.

In 1998, Underwriters Laboratory finally 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 not indicate CO levels less than 30 parts per million (PPM), nor alarm at levels below 100 PPM. This requirement was imposed by UL at the request of gas utilities and firefighters to minimize the number of unnecessary emergency calls from homeowners.


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