Time of Useful Consciousness

Tonights Human Factors class was particularly interesting. Especially the discussion we had on high altitude and decompression.

When talking about decompression and the effects on the human body an item called “time of useful consciousness” (TUC) gets mentioned all the time. You wont pass out at this magic number but instead start feeling the effects of hypoxia and just end up getting so messed up that your pretty much useless. Below are some TUCs for different altitudes:

20,000′ : 5-12 Minutes

30,000′ : 45-75 Seconds

40,000′ : 10-30 Seconds

So lets say your aircraft has a rapid decompression at 40,000′. You have at least 10 seconds to realize what happened and put on your oxygen mask. What happens to your body at 40,000′ if you don’t put on your mask?

A condition known as negative aspiration occurs. At that high altitude the partial pressure of oxygen is much, much lower than the partial pressure of oxygen in your blood. We all remember from physics (or was it chemistry) that gases move from an area of high pressure to low pressure. So the oxygen in your body is literally sucked out of your blood as it moves to the lower pressure area in your lungs…

Hypoxia then kicks in, your pass out, and a few minutes later: die.

So the moral of the story is? Put your mask on no matter what. Don’t waste precious seconds trying to figure out the cause of the problem. Do that after your mask is on.

Now of course this is an extreme example.. but there are some situations where people didn’t put their masks on in a timely fashion. Of course, there are those that were able to survive the exact problem I described above.

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