Nitrogen FAQ

I know nitrogen is not metabolized by the body, and I've read that tech divers are facing issues with helium bends. Is the size of the molecules relevant? Helium molecules are smaller and lighter than oxygen molecules, while nitrogen molecules are larger. Some people are filling their car tires with nitrogen because it supposedly leaks out of the tire more slowly than air.
There are times when size matters and times when it is a red herring. This is one of those cases where it's a little bit of both. Nitrogen is a slightly larger molecule than oxygen (by about 2 percent), but that does not have anything to do with its reactivity in the body. A better analogy would be how wood and steel interact with a magnet. The steel reacts, while the wood does not.

A helium molecule, on the other hand, is one-third the size of a nitrogen molecule. Molecule size can make a difference in passing through some barriers, but it is almost certainly not the only factor in uptake and elimination or its nonnarcotic nature relative to nitrogen. Interestingly, while helium is considered a fast gas in terms of exchange, it can cause challenges in decompression, resulting in the need for more buffering of limits based on expectations of exchange speed. You may be hearing more about decompression related to helium in large part because of its increased use by recreational technical divers. Broadening the use sometimes brings out new issues not discovered in more limited traditional use.

As I like to say, physiology does not equal math. Physiology is much more dynamic. Seemingly logical ideas, such as size being a critical factor, can be attractive but do not answer the question. Asking questions, though, and having a lifelong interest in learning are critical in getting closer to the elusive truth.

By the way, it is not worth paying to fill your tires with nitrogen.
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