If you're a pilot, you know that hypoxia can be deadly. But what is hypoxia, and why does it happen?

What Is Hypoxia?

Hypoxia occurs when your body is deprived of oxygen. When the tissues in your body don't get enough oxygen, they start performing poorly. And there's one tissue that's particularly at risk: your brain. When your brain is starved of oxygen, it impairs your ability to make critical decisions, inhibits your motor skills, and eventually, causes you to pass out.

How Does Hypoxia Occur?

There are 4 types of hypoxia (hypoxic, hypemic, stagnant, and histotoxic), but for today, we'll stick with hypoxic hypoxia. Hypoxic hypoxia happens when your entire body is starved of oxygen. And for pilots, that typically happens when you aren't getting enough O2 when flying at high altitudes.

When you inhale air into your lungs, atmospheric pressure forces oxygen through your lungs' membranes and into your bloodstream. But as you climb, atmospheric pressure decreases, and the amount of oxygen forced into your blood also decreases. The percentage of oxygen in the air doesn't change - it's still 21 percent. But at 18,000 feet, the atmospheric pressure is half that of sea-level.

When Are You At Risk?

Obviously, pilots flying at high altitudes are at risk of hypoxia, and there are typically three scenarios where pilots are subjected to it:

• Flying at high altitude with no oxygen system
• Slow, rapid or explosive decompression
• Failure to properly use a pressurization system

Flying at high altitude with no oxygen system
As you start climbing in an unpressurized airplane, the amount of oxygen that reaches your bloodstream starts decreasing. But the decrease isn't linear: as you climb, the atmospheric pressure drops of exponentially. Just look at the graph below, which charts your time of useful consciousness at different altitudes (the maximum time you have to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen).

You probably know the oxygen rules by heart. As a crew member, you must use supplemental oxygen when you're above 12,500 feet MSL cabin pressure altitude for more than 30 minutes, and anytime you're above 14,000 feet MSL. Above 15,000 feet MSL, you have to provide it to your passengers - and many aviation attorneys would suggest you make them use it.

But the FAA rules aren't always a failsafe. Depending on your age, health, and many other factors, hypoxia can start creeping up on you at altitudes as low as 10,000 feet.

Slow, rapid or explosive decompression
If you're flying a pressurized airplane, you need to be prepared for a decompression. There are three types of aircraft decompressions:

• Slow - more than 10 seconds
• Rapid - 1 to 10 seconds
• Explosive - less than 1 second

Decompressions can be incredibly dangerous. With slow decompressions, you may not even realize it's happening until you start getting hypoxic. And with rapid and explosive decompressions, your time of useful consciousness is cut in half or less, meaning your reaction time to put on supplement oxygen can be a matter of seconds.

Failure to properly use a pressurization system
Finally, the third situation where you're at risk of hypoxia is not using your aircraft's pressurization system properly. How does that happen? Typically, by either forgetting to turn it on, or by setting an incorrect cabin altitude.

How Can You Tell If You're Getting Hypoxic?

There a lots of symptoms of hypoxia, and unfortunately, everyone reacts differently. Here are some of the most common symptoms:

• Euphoria - a carefree feeling
• Cyanosis - blue fingernails and lips
• Visual impairment - losing your color vision
• Headache
• Impaired judgement
• Drowsiness

One of the most dangerous things about hypoxia is the fact that it's impairing your judgement before you even know it's there. Add on the fact that many people's first symptom is a feeling of euphoria - that carefree feeling that everything is ok - and you have a recipe for disaster.

What Should You Do If You Think There's A Problem?

So what should you do if you think you're getting hypoxic? If you have a supplemental oxygen system on board, put it on immediately, then, get below 10,000 feet as fast as possible.

After you've put on your oxygen, request an altitude below 10,000 feet from ATC. If they aren't able to give you that altitude, declare an emergency and start descending. Remember, you only have a precious few minutes, or in some cases, seconds, to make the decisions that will save your life.