Carbon monoxide (CO) is a colorless, tasteless, and odorless molecule made up of one carbon atom and one oxygen atom, and held together by three bonds. It isn’t the composition that’s so interesting. What’s interesting is the fact that this little combination of atoms can be a very sneaky problem.
Carbon monoxide is formed during the incomplete combustion of fuels, including extremely tiny amounts of it that are naturally formed in the body as the product of cellular metabolism. It may not be terribly likely that you’re burning a charcoal grill in the cockpit of the airplane for warmth, but this doesn’t mean you’ll never have to deal with carbon monoxide. Large amounts of it are present in the exhaust gases of the aircraft, which are normally vented to the atmosphere by an intact exhaust system. However, introduce an imperfection in that system, the same one that provides cabin heat, and you have the most common way that carbon monoxide poisoning occurs in pilots and passengers.
Carbon monoxide is dangerous in two very distinct ways. The primary threat comes from carbon monoxide’s technique of disrupting the way that hemoglobin transports oxygen to tissues throughout your body, ultimately inducing a hypoxic state. Hemoglobin (Hb) is a critical red blood cell component. It is an iron-based protein with four distinct binding sites—a tetramer—whose job is to transport four oxygen molecules at a time from the lungs to the rest of the body’s tissues, and to bring carbon dioxide back to the lungs for removal.
The Train Car. Think of hemoglobin as a huge fleet of train cars, each with precisely four seats, being whisked along as blood moves through the body. In the lungs, four oxygen molecules get in the car, and then as the car passes by a tissue that needs it, some of the oxygen jumps out, and some carbon dioxide jumps in to take those seats, on its way back to the lungs.
Hemoglobin likes oxygen, but it loves CO—about 230 to 250 times as much—so there’s really no competition. While in the lungs, if there is a CO molecule present that was inhaled, the hemoglobin will grab on to it and throw it in the train car, along with three oxygen molecules. Hemoglobin has such an affinity for CO, that once it has successfully latched onto a molecule of it, it tightens its grip and the three remaining oxygen molecules also become trapped—locked into place in the train car. The car, still free to move about, travels through the blood to the tissues that are wanting the oxygen, but the oxygen can’t get free. They are firmly locked in place, and continue to travel right past where they are needed most. So in this situation, one single CO molecule has effectively taken down an entire defenseless hemoglobin protein.
What does this mean? CO attaching to only a handful of hemoglobin molecules produces little effect in the body, but as the amount of CO begins to increase (and it doesn’t take much), and therefore as more and more of the hemoglobin cars are disrupted, tissues slowly begin to be starved of oxygen. There may be plenty of oxygen available, there may be plenty of blood to move it around, and there may be plenty of movement of that blood—but the oxygen just can’t get off the train cars to get to the tissue. Sound familiar? That’s because it’s very similar to another aeromedical issue: hypoxia. Carbon monoxide poisoning manifests first by creating a state of hypemic (or anemic) hypoxia.
The only difference in this case is that instead of a decreased amount of blood (or hemoglobin) to carry oxygen around, you have a decreased ability for blood to carry oxygen.
Tissue Toxicity. The other insidious aspect of CO poisoning occurs when the carbon monoxide does finally get free. Instead of breaking free in the lungs where it might get ventilated out, it tends to do so in muscle tissue, brain and other nervous system tissue, and other susceptible tissues, where it continues to do damage and prevent normal oxygen uptake. For example, carbon monoxide that releases from hemoglobin, but becomes bound in muscle tissue, further decreases the muscle’s ability to use oxygen.
Even with an increase in the blood oxygen levels, muscle tissue still will have a difficult time utilizing the oxygen that it does get, producing rapid muscle fatigue, difficulty moving, and, since the diaphragm is a muscle, a decreased ability to breathe. Remember also that the heart is a muscle, and so this also produces a reduction in cardiac output, and therefore decreased blood flow (and oxygenation) to other tissues, which further worsens the hypoxic problem. Additionally, dissolved carbon monoxide interferes with the body’s individual cells’ ability to process oxygen, and so they go into a state of oxygen starvation, attempt in vain to compensate, and eventually this results in cell death.
The Risk. Mild exposure to carbon monoxide is likely to induce symptoms such as lightheadedness, dizziness, headache, and mild confusion. With higher exposures you could experience fatigue, muscle cramps, nausea, vomiting, and severe confusion and disorientation. Still higher concentrations lead to unconsciousness, seizures, cardiac and brain damage, and death. Any exposure is dangerous, and a large exposure is likely deadly.
While nobody is immune to the effects of carbon monoxide poisoning, some people have higher risk than others. Pilots and passengers who smoke are more susceptible. People who are anemic, those with poor cardiac function, those with blood clots, people who have lost blood because of injury or recent surgery, and even those who have recently donated blood have an increased susceptibility to the condition. When flying at higher altitudes, smaller doses of carbon monoxide will have larger effects.
Aviate, Ventilate, Navigate, Communicate. As previously mentioned, carbon monoxide itself is colorless and odorless. However, the exhaust gases which contain it have characteristic odors. If you smell exhaust gases in the cabin, there is a serious threat that you are inhaling carbon monoxide. This can happen anytime, but is particularly true when using cabin heat during the colder months. If you or your passengers begin showing any of the symptoms, you may have an emergency situation, and prompt action is necessitated.
The first step is to declare an emergency to yourself right away, and to ATC when workload permits. Remember Aviate, Navigate, Communicate? Let’s modify this slightly in this instance. You should Aviate, Ventilate, Navigate, and Communicate. Don’t forget to ventilate. Turn off the cabin heat, open vents, open a window; get fresh air flowing through the cabin. This will help to dilute the amount of carbon monoxide in the air, and in your lungs.
If available, use oxygen set on a high flow rate, which will further dilute the toxic gas. Descend to a lower altitude if possible. Oxygen availability in the atmosphere decreases with increasing altitude, so we need to try to reverse that as much as is possible.
Divert to a nearby airport and land as soon as possible. Seek medical attention right away, and tell the medical provider that you suspect that you have been exposed to carbon monoxide. The 5D Model on this page may help you remember.
Even if you feel better, it’s essential to get medical attention, because CO can remain trapped in your body for a long time, continuing to do damage, and you could still be suffering hypoxic and toxic effects without even being aware. After a serious exposure, it can take days before a sufficient amount of carbon monoxide can be removed from your body.
The best cure is prevention. Once you have been exposed to carbon monoxide in any significant quantity, the potential health effects can be quite serious, and of course the costs associated with medical treatment cannot be ignored either. A much safer and cost-effective measure is to make sure you don’t get exposed in the first place. The hazard will always be there, but the risk can be managed by reducing the likelihood of exposure, and by keeping any exposure you experience as small as possible.
Avoid smoking. Cigarette smoke contains carbon monoxide, and it is readily absorbed by your blood and can put you in a state of chronic low-level carbon monoxide poisoning. Ensure your airplane’s exhaust and cabin heating systems are inspected regularly. Leaks detected on the ground can be repaired before they have the chance to become emergencies.
Naturally, it’s not possible to guarantee that you’ll never be exposed, so in that case getting warned as early as possible, and with as little exposure as possible, is desired. There are a number of options available. Many companies offer a very low-cost chemical-based carbon monoxide detector that changes color when exposed to the gas. However, in order to be effective, they should be replaced regularly.
And relatively new to the market are electronic versions. They claim to be very sensitive, and have a 24-month battery. What about fingertip pulse oximeters? They are designed to warn of hypoxia, and can be fooled by carbon monoxide. Typical models use a light source that is designed to measure the amount of oxygenated versus deoxygenated hemoglobin. However, the train car hemoglobin locking effect mentioned earlier tends to fool them into thinking that there’s plenty of oxygenated blood in the body, even when that’s not the case, and so a normal reading does not guarantee safety.
Related Articles
