Carbon Monoxide Detectors

The dramatic crash of Piper Dakota N8263Y madeall the evening TV newscasts on Friday, January 17, 1997. The experiencedinstrument-rated pilot and his 71-year-old mother had departed FarmingdaleAirport on New York's Long Island at 11:15 a.m. on a VFR flight to Saranac Lake,N.Y., about two hours' flying time to the north. But less than a half-hour intothe flight, something went terribly wrong: the pilot-in-command passed out cold.Thirty-six minutes into the flight, the passenger (who was herself a low-timeprivate pilot) radioed Boston Center and told the controller that the pilot wasunresponsive and vomiting, and they were in trouble. After determining that thepassenger was pilot-rated, the controller spent the next 20 minutes trying totalk her down to a landing at Bridgeport, Conn. An Air National Guard helicopterjoined up with the aircraft and participated in the talk-down attempt, butwithout success.

Forty-five minutes into the flight, the woman reported that she, too,was getting tired and nauseated, and was unable to awaken the pilot. Shortlythereafter, the airplane turned north and started climbing, and the womanstopped responding to radio calls. The aircraft gradually climbed into the5,000-foot cloud bases and continued climbing to 8,800 feet. The helicopter lostsight of the emergency aircraft but attempted to follow it with the help of ATC.About two hours into the flight, the aircraft descended out of the clouds andthe helicopter established visual contact, reporting that the cabin appeared tobe full of smoke and nobody was visible through the windows. Not longafterwards, the helicopter pilot reported that the Dakota had started descendingrapidly and crashed into the woods near Lake Winnipesaukee, N.H. Both occupantswere fatally injured in the crash.

Toxicological tests in the FAA lab in Oklahoma City revealed that the pilot'sblood had a carboxyhemoglobin (CO) saturation of 43%, and the passenger'smeasured 69%. Those concentrations are sufficient to produce convulsions andcoma. NTSB metallurgists determined that the muffler contained a large crack andan irregular hole, both of which appeared to have been leaking exhaust gas forsome time.

A week after the Dakota crash, the AOPAAir Safety Foundation issued a press release cautioning pilots about thedangers of carbon monoxide poisoning, and recommending that pilots ofsingle-engine aircraft install a CO detector. However, ASF executive directorBruce Landsberg noted that CO accidents are "extremely rare," adding that "asearch of the Air Safety Foundation accident database revealed only twoaccidents caused by carbon monoxide between 1985 and 1994."

Much as I hate to contradict my good friend Bruce, my own quick search of theNTSB accident database suggests that CO-related accidents and incidents occurfar more frequently than the AOPA Air Safety Foundation's statement would haveyou believe. But don't take my word for it. Take a look at just some of theaccidents and incidents my brief search turned up:

Still think in-flight CO poisoning occurs too rarely to worry about? I didn'tthink so!

The fact is that deaths from unintentional carbon monoxide poisoning havedropped sharply in recent years, thanks mainly to lower CO emissions fromautomobiles with catalytic converters (60% of CO deaths are motorvehicle-related) and safer heating and cooking appliances. In contrast,CO-related airplane accidents seem to be on the increase as the fleet growsolder and the maintenance infrastructure deteriorates. The recent mufflerproblem on new Independence-built Cessna singles demonstrates that even newairplanes aren't immune.

Carbonmonoxide is an invisible, odorless, colorless gas created when fossil fuels(such as gasoline) burn incompletely. In a piston-powered aircraft, engineexhaust contains high concentrations of CO, particularly at mixture settingsricher than peak EGT. The most common way for this CO to find its way into thecabin is through the cabin heat system. The vast majority of single-engineaircraft obtain their cabin heat by ducting ventilation air over the surface ofthe muffler. Therefore, when cabin heat is being used, any cracks or holes inthe muffler can allow CO-rich exhaust gas to contaminate the cabin air. Otherpossible causes include inadequate sealing of the firewall, wheel wells, orother air leak that allows exhaust to leak into the cabin.

Normally, oxygen inhaled into your lungs combines with the hemoglobin in thered cells of your blood to form "oxyhemoglobin." The oxygen is then transportedthroughout your body by your arteries and capillaries, where it disassociatesfrom the hemoglobin and oxygenates the cells of your tissues and organs(including your brain). The deoxygenated hemoglobin then returns through yourveins to your lungs, where it is combines with more oxygen and the cyclerepeats.

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

How long? According to an authoritative medical text (Rosen's EmergencyMedicine, 3rd Ed., 1992), COHb has a "half-life" of more than fivehours for a patient breathing fresh air. In other words, if you crash-landin a hay field with COHb saturation of 40%, your COHb level can be expected todrop to about 20% after five or six hours, to 10% after another five or sixhours, and so forth. If you're taken to the emergency room and they put you onoxygen therapy, the half-life drops to 1.5 to 2.5 hours (depending on whetherthe docs put you on a ventilator or just use a face mask). In extreme cases ofCO poisoning, you may be rushed to a large medical center and put into ahyperbaric 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 willnormally produce a COHb saturation of 3% to 10%. Smokers are consequently farmore vulnerable to CO poisoning in flight, since they're already in apartially-poisoned state when they first get into the aircraft. Because ofCOHb's long half-life, smokers would do well to abstain from smoking for 8 to 12hours prior to flight. (Unfortunately, the more common scenario is that the lastcigarette is stubbed out on the tarmac moments before flight, and the next oneis 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 toyour body's cells decreases. It is this oxygen deprivation that makes CO sodeadly. Sensitive parts of your body like your nervous system, brain, heart andlungs suffer the most from this lack of oxygen. Symptoms of mild CO poisoninginclude headache, fatigue, dizziness, vision problems (particularly doublevision), nausea, and increased pulse and respiration. Unfortunately, thesesymptoms are often attributed to flu, indigestion, or the common cold. At higherlevels of COHb saturation, you may suffer difficulty in breathing, loss ofconsciousness, collapse, convulsions, coma, and even death.

Just how sick you'll get from CO exposure varies greatly from person toperson, depending on age, overall health, the concentration of CO (measured inparts per million), and the duration of exposure. High concentrations can causeincapacitation within minutes, but low concentrations can still be extremelydangerous if you're exposed for a period of hours. As CO continues to beinhaled, the percentage of COHb gets higher and higher, and you get sicker andsicker. 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. Carboxyhemoglobinis red in color, just as oxyhemoglobin is. (That's why a pulse oximeter isunable to detect CO poisoning.) But, since CO does not disassociate readily fromhemoglobin the way O2 does, your venous blood remains red rather than turningthe normal bluish color. This morbid little fact is useful mostly to coronersand morticians, however, because by the time CO poisoning has progressed farenough 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 COconcentration in the air and COHb saturation of the blood affect an averageperson. As you can see, a CO concentration of one tenth of one percent (1,000parts per million) is enough to render you unconscious in an hour. OSHA hasestablished the maximum permissible CO level for continuous 8-hour-per-dayexposure in the workplace at 35 parts per million. Personally, I would not careto fly in an airplane that exceeded that level.

Giventhe insidious nature of carbon monoxide poisoning and the apparent increase inthe CO-related accident rate, it seems astonishing that so few pilots install COdetectors in their airplanes (particularly piston singles, which are by far themost vulnerable). Furthermore, among those pilots who do use CO detectors,almost all seem to be using those adhesive-backed cardboard chemical spotdetectors that are commonly sold for about $4.00 apiece under tradenames like"DeadStop" and "HeadsUp" by pilot shops and mail-order outfits.

While I suppose these chemical spot detectors are better than nothing, theyleave a great deal to be desired. For one thing, they have a very short usefullife, claimed to be 30 to 60 days (and experts tell me that anything more than30 days is wildly optimistic). Unfortunately, most pilots who use thesedetectors are very bad about replacing them once a month religiously. C'mon,fess up, you know I'm right!

Oh, by the way, if you did replace them once a month, they'd cost you$50 a year!

Furthermore, these chemical spots are extremely vulnerable tocontamination from all sorts of aromatic cleaners, solvents, and other chemicalsthat are routinely used in aircraft maintenance. Read the fine print on thesethings, and you'll learn that the detectors will be inactivated and damaged bythe presence of ammonia, chlorine, iodine, bromine, and nitrous gases. Itdoesn't take much, either. One brand of spot detector actually warns that theammonia produced by the presence of a cat litter box in the home may render thedetector unusable! What's worse, there's not necessarily any warning that thedetector has been contaminated. The bottom line is that you might easily beflying around with an inoperative detector (because it's too old orcontaminated) and not know it. In some ways, that's worse than not having adetector at all.

Finally, the chemical spot detectors are incapable of detecting low levels ofCO. If you're lucky, they'll just barely start turning color at 100 PPM, but soslowly and subtly that you'll never notice it. For all practical purposes,you'll get no warning until concentrations rise to the 200 to 400 PPM range (andthat assumes a fresh, uncontaminated detector). Even at these levels, it cantake so long for the color change to take place that you could easily becomeimpaired before you notice it. As I said, these things are arguably better thannothing, but not by much.

An improved version of the chemical spot detector - the Quantum Eye -is manufactured by the Quantum Group Inc. in San Diego, Calif. This unit sellsfor about $10 and claims to have a useful life of 18 months (although my expertstell me that 12 months is more realistic). It has an expiration date printedright on its face to help ensure that it won't be used beyond its time. It alsohas a color reference wheel printed on its face, making it easier to noticesubtle color changes.

The Quantum Eye utilizes a "biomimetic" sensor element, which is essentiallyan artificially engineered chemical whose affinity for CO is as similar aspossible to that of hemoglobin. The idea is that CO binds to this sensormaterial at approximately the same rate that it binds to hemoglobin, and thusthe biomimetic detector will change color even in the presence of fairly lowlevels of CO if the exposure time is long enough. That's a big improvement overthe four-dollar chemical spot detectors that are basically insensitive to lowlevels of CO.

The Quantum Eye is not without its problems, however. Just as with thecheaper chemical spot detectors, the Quantum Eye is quite vulnerable to exposureto a wide range of aromatic chemicals commonly used around airplanes, such ascleaners and solvents containing alcohol, ammonia or chlorine. Such contaminantshave a cumulative effect that progressively degrades detector performance overtime. Unfortunately, there's really no good way to determine the degree ofcontamination or degradation. The only real solution is to replace the detectorregularly, and to try to avoid exposure to aromatics.

Another problem with this type of detector is that it can't distinguishbetween a short exposure to a high concentration of CO and a long exposure to alow concentration - both produce the same color change. To put this intoaviation terms, the detector may be able to warn you that you're in trouble(assuming you keep it in your visual scan and notice the color change), but itcan't tell you what the concentration of CO is and therefore how much time ofuseful consciousness you have left. This might be okay in the home environment -where you can call 911 and then run out the door - but it leaves a lot to bedesired in an airplane where running outside is not exactly a viable option.

If you're thinking of buying a chemical spot detector, the Quantum Eye is theonly one worth considering, in my opinion. But as you'll see, there are farbetter alternatives available.

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

In 1992, Underwriter'sLaboratory 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. First Alert(then a division of Pittway Corporation) ran a massive campaign of "scaretactic" TV ads featuring the basso profundo voice of actor William Conrad(Cannon, Jake and the Fatman), and quickly became the leadingsupplier of residential CO detectors. The industry really took off when the Cityof Chicago mandated the installation of CO detectors in residences beginningOctober 1, 1994.

Then a funny thing happened. By December 20, 1994, the Chicago firedepartment had logged some 8,500 calls of CO detector alarms, and found that 86%of them turned out to be false alarms! Then, on December 21, 1994,Chicago experienced a temperature inversion and consequent smog problem, and allhell broke loose: more than 1,800 calls were made to "911" within 24 hours,almost all of which turned out to be false alarms. Other cities experiencedsimilar problems. Los Angeles recorded some 3,300 nuisance alarms in onemonth.

By far the worst false-alarm offenders were the market-leading FirstAlert units. These made use of the same "biomimetic" (color-change) sensortechnology used by the Quantum Eye spot detector. The sensor module used byFirst Alert simply passed a light beam through the biomimetic spot, and alarmedif the light was sufficiently attenuated (presumably because the spot had turneddark in color). Not only did this mean that the First Alert detector shared allthe problems of the Quantum Eye (such as limited sensor life andcross-sensitivity to gases and vapors other than CO), but the detector wasplagued by false alarms due to the fact that other things could attenuate thelight beam (smoke, contamination, even insects that crawled inside the sensormodule).

In response to the false-alarm crisis, Underwriter's Laboratory revised its UL2034 Standard in Juneof 1995, but the false alarm problems didn't get any better. Meantime, in late1995 and early 1996, the gas utility industry and the Consumer Product SafetyCommission (CPSC) started getting concerned about the very opposite problem: COdetectors that would not go off when they should! While First Alert had obtainedan exclusive license on the biomimetic sensor technology for residential COdetectors, virtually all other detectors sold prior to 1996 (including #2 and #3market leaders American Sensor and Nighthawk) made use of ametal-oxide-semiconductor (MOS) sensor, which was the only other low-cost sensortechnology available at the time. CPSC tests revealed that some of the MOS-basedunits would fail to alarm even at life-threatening CO concentrations of 1,000PPM 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 falsenegatives.

Since then, the industry has gone through considerable upheaval. PittwayCorporation wound up divesting itself of First Alert, which subsequently wentpublic, then nearly bankrupt, and finally was acquired by Sunbeam in 1998.American Sensor wound up going bankrupt, while the assets of Nighthawk Systemswere acquired by fire extinguisher giant Kidde Safety who subsequently redesigned the Nighthawkproducts to use a more reliable electrochemical sensor technology.

In 1998, Underwriters Laboratory finally revised its UL 2034 specification,but did so in a fashion that made all UL-approved residential CO detectors farless attractive for aircraft use. UL published its revised spec in 1998, butimplementation was delayed until January 1, 2000. For a CO detector to beUL-approved for residential use after that date, UL requires that it must notindicate CO levels less than 30 parts per million (PPM), nor alarm at levelsbelow 100 PPM. This requirement was imposed by UL at the request of gasutilities and firefighters to minimize the number of unnecessary emergency callsfrom homeowners. I'm sure this has made the firefighters and gas company folksvery happy. But it sure didn't please me.

That's because I believe that aviation safety is best served by a sensitivelow-level detector, not one that's intentionally "blinded" to concentrationsbelow 30 PPM. For in-flight use, we're not simply worried about high CO levelsthat can make you ill - we care even more about low levels of CO that canproduce subtle cognitive impairment. Furthermore, when flying at altitude in anunpressurized aircraft, we're already somewhat impaired by altitude hypoxia, soit doesn't take much CO to increase impairment to a dangerous level. That'sbecause the effects of altitude and CO are additive. The bottom line is that ULdidn't do pilots any favors with its 1998 revision to UL-2034.

Most residential CO detectors sold today are designed to operate from115-volt AC power, which makes them poor candidates for in-flight use. However,I found several battery-operated CO detectors with digital readouts that canbe used for aviation applications: the Kidde Nighthawk 900-0089, the AIM Model935, the Senco Sensors Model One (BS7860 version), and the CO Experts Model2002. All are powered by alkaline batteries, and incorporate an electrochemical sensor, an 85dB horn, and a largeLCD digital display. (First Alert also offers a battery-operated model, but itlacks a digital readout, so I didn't bother to test it.)

Unfortunately, since I originally wrote this article, the units from AIM andSenco Sensors (both Canadian manufacturers) became unavailable. AIM changedhands and ceased production of their CO detectors, and Senco decided no longerto permit their BS7860 detector to be sold in North America (apparently onadvice of their lawyers). This leaves only the Kidde Nighthawk 900-0089 and theCO Experts Model 2002 in the running for use in the cockpit.