A Simple Little Molecule

I really appreciate all of you who have been following along on the series of articles about sensory illusions and motion sickness over the last few months.  I especially appreciate the emails I’ve gotten with comments and feedback from all over – even a fellow pilot crossing the Drake Passage telling me I was right – the weather there is terrible.

So, if you’ve been paying attention lately this little pop quiz should be real easy:  You’re flying along and feel a headache coming on and then start to feel nauseous, dizzy and a little fatigued.  What’s your problem?  Of course, it’s all the classic symptoms of motion sickness.  But what if the air is perfectly clear and there’s no turbulence?  Or for that matter, you’re not even flying or driving and just at home sitting on the couch watching TV?  Still think it motion sickness?  It’s not, what you might have is carbon monoxide (CO) poisoning

The symptoms are pretty much the same but the difference between feeling ill from motion sickness or ill from CO poisoning is the difference between life and death.  That’s because in addition to the basic sick feeling, CO intoxication also causes weakness, shortness of breath, confusion, blurred vision, rapid and irregular heartbeat, cognitive dysfunction, and ultimate loss of consciousness and death.  Obviously none of this is conducive to safe piloting.  The onset of symptoms of CO poisoning are very similar to motion sickness and also hypoxia.  Some patients have even described CO intoxication as feeling like the flu is coming on. Compounding all of these dangers for pilots is another issue, and that is the fact that self-diagnosis is almost impossible.  CO intoxication, just like high altitude hypoxia (low oxygen levels in the blood), leads pilots and passengers to develop a sense of euphoria, a false sense of security that shuts down your capacity to make proper judgments of your predicament.  A possible early tip off that you’re breathing CO, if you can pick up on it before disaster strikes, is that everyone on your airplane or in the house with you starts to feel the same way at about the same time.

Carbon monoxide (CO) is the simplest of all possible molecules, just one carbon atom and one oxygen atom combined and there it is.  It’s a colorless, odorless, and nonirritating gas but that’s where the nice stuff ends -  it’s highly toxic to humans and every other organism that breathes air. CO is primarily produced by incomplete combustion of organic material; that engine under your cowling or under the hood of your car is the most common source. There’re also lots of home sources of CO like gas- or charcoal-burning stoves and power generators. Natural sources also exist, including fires that can be a source of severe toxicity for unfortunate victims trapped in burning buildings and why you see fire fighters always carrying oxygen tanks. CO toxicity is defined as a level of 15% or more in the blood.  It’s in all types of smoke and that includes cigarettes (and everything else some people smoke). Chronic smokers can run CO levels of almost 10% to start so they have no leeway to inhaling any more CO without become quickly incapacitated. CO toxicity is estimated to be responsible for about 50,000 emergency department visits each year and 13,500 calls to poison control centers in the United States. The annual mortality from CO varies but runs about 1,500 to 2,000 deaths.

Carbon monoxide is toxic because it blocks the most essential function in the body - oxygen intake and transport to the tissues.  When CO is inhaled it immediately gets into our blood stream and sticks to the iron molecules in hemoglobin where oxygen is supposed to be carried.  Then it forms something called carboxyhemoglobin (HgbCO), which kicks out oxygen from hemoglobin and literally chokes off cell metabolism, a kind of internal strangulation.  An added problem is that CO has an affinity for the oxygen binding sites of hemoglobin that’s 200 to 250 times stronger than oxygen itself (two oxygen molecules, abbreviated as O2).  Once CO molecules adhere to hemoglobin it’s nearly impossible to push them off and make room for the O2 that should be there.  Since symptoms start when CO levels reach about 15% it means O2 percent is pushed down to around 85% (O2 saturation should be about 98%) and that low an O2 level (hypoxia) is what causes incapacitation.  Less 02 in hemoglobin means decreased oxygen delivery to the tissues and when the body tissues are deprived of oxygen, the cells can generate only a fraction of the energy needed for essential chemical reactions.  When deprived of O2, cells can only produce energy by turning to anaerobic (without air) metabolism, and the byproduct of anaerobic metabolism is acid.  Lactic acid is the specific acid that builds up in your body and leads to a total shift in your homeostasis (internal environment), which brings on all the symptoms.  That’s called “lactic acidosis” and basically it’s the same effect as suffocating.

Dumping acid into the cells lowers cellular pH (remember that from high school chemistry?) and none of our cell-based chemical reactions work in an acid environment - so the wheels literally come off all of our metabolic processes.  Everyone who has exercised (hopefully that’s everyone) is familiar with one of the symptoms of anaerobic metabolism – it’s that achy, crampy feeling in your arms and legs during a hard workout.  That’s because rapid or prolonged muscle exertion outpaces O2 delivery and forces your muscle cells to convert to anaerobic metabolism and produce that lactic acid.  Brain and nerve cells don’t have the capacity to convert to anaerobic metabolism, which explains the early neurological symptoms and clouding of judgment and problem-solving skills.  Anaerobic metabolism is an incredibly inefficient way to generate energy; almost 15 times less energy is produced than from aerobic (with air) metabolism.  Ultimately, the lack of oxygen and the acid byproducts lead to coma, cellular breakdown, and eventually death if not quickly treated. There are only a few things you can do in the air if you think you’re being exposed to CO.  If you even suspect CO exposure in the cockpit, open the air vents and breath supplemental oxygen through a mask.  If you turned on the heater, shut it off right away. Land as soon as possible, and if you or one of your passengers is still sick call for emergency medical attention.  At home the process is similar; open the windows, shut off any possible source of CO like stoves, heaters, and generators, and just go outside quickly. 

For sure, one of the really simple things you can do in your airplane and your home is to avoid the problem in the first place by using CO sensors that you can get really cheap online or at any pilot shop or home supply store. The NTSB has already made a recommendation to require CO detectors in general aviation cockpits after finding the pilot of a Cessna 172 that crashed and killed the occupants died of CO poisoning.  The report, released in December 2021 is linked here: Aviation Investigation Report (AIR-22-01).  Both the FAA and AOPA have also recently called attention to this deadly risk. Patients who are brought to the hospital with high carboxyhemoglobin levels need immediate treatment like breathing 100% and even hyperbaric oxygen chambers, but it has to happen quickly.  CO poisoning isn’t all that common but the results are fatal.  Fly smart, use a CO detector, and always be suspicious of CO the minute you start to feel any of the aforementioned symptoms.

Kenneth Stahl, MD, FACS
Kenneth Stahl, MD, FACS is an expert in principles of aviation safety and has adapted those lessons to healthcare and industry for maximizing patient safety and minimizing human error. He also writes and teaches pilot and patient safety principles and error avoidance. He is triple board-certified in cardiac surgery, trauma surgery/surgical critical care and general surgery. Dr. Stahl holds an active ATP certification and a 25-year member of the AOPA with thousands of hours as pilot in command in multiple airframes. He serves on the AOPA Board of Aviation Medical Advisors and is a published author with numerous peer reviewed journal and medical textbook contributions. Dr. Stahl practices surgery and is active in writing and industry consulting. He can be reached at [email protected].

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