What happens when high-altitude hypoxia strikes mid-flight?

High-altitude hypoxiaCC: Calle Macarone at Unsplash

Modern aircraft must feature oxygen masks as standard. There’s a good reason for this and it comes in the form of high-altitude hypoxia. As a condition where the brain abandons its useful consciousness, high-altitude hypoxia has resulted in some of the world’s worst air crashes. Interestingly, it’s also influenced an alternative means of executing prisoners on death row. If you have a fear of flying, you may find learning about this topic is more reassuring that you first assumed.

First of all, what is high-altitude hypoxia?

Hypoxia is a state where your body’s organs aren’t receiving enough oxygen. In the medical world, it’s usually due to a physiological mishap. For example, those with COPD are more likely to suffer from hypoxia than the rest of us.

In the air, high-altitude hypoxia occurs when the partial pressure of the air drops. Our bodies aren’t designed to cope with a reduced partial air pressure, which is why mountaineers train hard to cope with their ascents.

As high-altitude hypoxia worsens, our brains begin to respond in a bizarre way. We start to lose what’s referred to as useful consciousness. Although we remain conscious, we may enter a state of euphoria. That state of euphoria can cause us to abandon all sense of how dangerous a situation is.

Why is the point of useful consciousness so important?

To understand why the point of useful consciousness is so important in high-altitude hypoxia, you need to know when it begins. Let’s say your onboard an aircraft and the cabin has decompressed. If you’re below 10,000 feet, the partial pressure of the air around you is high enough. If you then reach 10,000 feet, it will drop. At this point, you’re not in the danger zone. Your oxygen saturation levels may drop to around 90%, but you can remain usefully conscious.

Beyond 10,000 feet, your situation will start to change. However, it isn’t until 14,000 feet that your time of useful consciousness begins to drop. Fortunately, modern aircraft will deploy oxygen masks at 14,000 feet, giving the flight crew an opportunity to descend to a safe level.

If you’re at 20,000 feet and the cabin has depressurised, you have 30 minutes of useful consciousness left. That’s more than enough time to don your own oxygen mask and someone else’s! At 30,000 feet, you have between one and two minutes. At 40,000 feet, that drops to 20 seconds.

Just in case it isn’t clear, those figures should illustrate why high-altitude hypoxia should prompt you to put your own oxygen mask on first. If you don’t do so before your time of useful consciousness runs out, you may be so euphoric that you simply don’t see the need. Like medical hypoxia, high-altitude hypoxia can become fatal.

How does high-altitude hypoxia happen on an aircraft?

Fortunately, high-altitude hypoxia is rare. There are two main scenarios where it can happen: a rapid decompression or a slow decompression. One example of a rapid decompression was Qantas Flight 30, where a 747 suffered from a sudden explosion in the hold. As rapid decompressions are obvious events, the pilots were able to descend to a safe level while the oxygen masks supplied the passengers with the air they needed.

A sadder and more fatal case was Helios Airways Flight 522, where a slow decompression was the likely cause of the pilots flying into a mountain. Such events are so rare that they’re isolated.

Am I at risk of high-altitude hypoxia when I fly?

In a nutshell: no. Aircraft are routinely maintained and screened between flights. Pilots also receive warning lights in the flight deck if an event is occurring. But if you are instructed to put your oyxgen mask on first, it’s probably best to listen unless you’re 100% certain you’re flying at 20,000 feet or below.

About the Author

Laura McKeever
Laura has been a freelance medical writer for eight years. With a BSc in Medical Sciences and an MSc in Physician Assistant Studies, she complements her passion for medical news with real-life experiences. Laura’s most significant experience included writing for international pharmaceutical brands, including GSK.