Deep Dive

Somehow SCUBA diving came up in a conversation a long, long time ago. I was in the Netherlands for work and the client was kind enough to spend an evening entertaining me during my visit there. He told me that apart from the obvious dangers of being underwater and surfacing too quickly from too great a depth, SCUBA diving had a more subtle danger: hypothermia. Since I don’t do diving of any kind (SCUBA, sky- , platform, spring-board or dumpster) this was new information.

But first, a diving joke:

Q: What’s the difference between SCUBA diving and skydiving?

A: If you’re SCUBA diving and you run out of air, there are some strategies and techniques you can use to help you survive. If you’re skydiving and you run out of air—you’re simply out of air.

Hypothermia made some sense, since being in very cold water can certainly be a serious hazard, but a lot of people dive in warm water, wearing wetsuits or drysuits. Heat loss through contact with the water is still happening, though. Consider that if you dive for work, say, doing underwater construction, you probably have to wear a heated suit and mask to keep from getting too cold. A further complication is that when you get out of the water and you sit in the boat with a wetsuit on, the evaporating water removes an enormous amount of heat from your body.

But the really difficult issue is that you’re breathing cool, compressed air or, if you’re diving deep, a compressed mixture of oxygen and another gas like nitrogen or helium. In any case, it’s already cold (since it’s in the water) and when it comes out of the SCUBA tank, it expands and as it does so its temperature drops even more. (This is described by Boyle’s Law, well-known by chemistry students everywhere.) You’re losing heat through your lungs with each breath.

So you’re in water that’s most certainly colder than your skin and the only “air” you have available is quite chilly: You’re getting colder from the outside-in and the inside-out.

Our bodies will begin to shiver involuntarily if we become cold, but this is a little tricky, too, because our skin is really only good at detecting relative changes in temperature. You can slowly get very cold without starting to shiver or even feeling very cold. And on the other hand, it can be very challenging to successfully re-warm a severe hypothermia victim: if they suddenly feel warm and cozy inside a rescue blanket, their body may actually stop shivering and subsequently freeze to death.

It’s strange to think that the obvious problem—breathing underwater—is in some ways rather straightforward to deal with, and that hypothermia is more serious. Some hazards are more dangerous simply because they can happen slowly, and some solutions are dangerous if they’re applied too hastily.