What happens to air in your lungs at altitude?

by Stephanie Chasteen on October 10, 2009

Here’s a provocative question about the atmosphere, from one of those most curious citizens of the world — 6th graders.

“At sea level you take a breath and fill a sandwich bag with it easily.  On Mt Everest, not using bottled air, could you do the same thing? I guess the question is “How full are your lungs at 28,000 feet?” If you filled a sandwich bag on top of Mount Whitney, would it still be full when you brought it back down?”

When you fill your lungs at high elevation, the air has the same volume, but it’s less dense. That means that there are fewer air molecules in a breath of air at 28,000 feet. So your lungs feel just as “full” but there is actually less mass of air there. So, the answer to the question depends on what you mean by “full.” The air pressure on Mt. Whitney is ½ the pressure at sea level. So, says Paul Doherty (my old mentor), it’s like someone ripped out one lung. You’re only getting half the oxygen as you are at sea level.

The same is true of the air in the sandwich bag – it’s less dense. But the air outside the sandwich bag is also less dense than it was at sea level. The bag fills easily because the air outside the bag exerts less pressure. If you blow up a balloon underwater, using pressurized air, the same thing should be true.

But then when you bring that sandwich bag down from Mt. Whitney, since it’s only got about ½ the air molecules in it that it would have if you had filled it at sea level, it looks a little deflated. It will have half its original volume. This is why your water bottle crinkles in on itself when your airplane lands.

Underwater, it’s the same story. You fill a sandwich bag with air at 100 feet. The air in your lungs is compressed to ¼ of its original volume. So you fill the sandwich bag with this compressed, dense air. As you come up, the air expands to 4 times its original volume. Says Paul Doherty,

Bang, it explodes.

And, adds Paul:

On a free dive your lungs don’t explode on the way back up. They just expand to their original volume. However if you took a breath from a scuba tank at 100 feet and then held your breath on the way up , DON’T DO THIS! your lungs would do what the sandwich bag did…not good.

This has killed many divers before.

Picture of Mt. Whitney from user Ziemusu on Wikimedia commons.

{ 1 comment }

Rhett October 11, 2009 at 1:38 am

Actually, if you took a breath from a scuba tank just 5 feet underwater (a full deep breath) and then ascended you could cause a problem.

Of course, on the other hand, if you are 100 feet deep and take a full breath from a scuba tank, you can ascend the whole way while exhaling (because the air in your lungs is expanding).

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