Friday, December 4, 2009

Hot Tubs and Heat Stroke

In Chapter 10 (about Feedback and Control) of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss hot tubs and heat stroke.
The body perspires in order to prevent increases in body temperature. At the same time blood flows through vessels near the surface of the skin, giving the flushed appearance of an overheated person. The cooling comes from the evaporation of the perspiration from the skin. If the perspiration cannot evaporate or is wiped off, the feedback loop is broken ad the cooling does not occur. If a subject in a hot tub overheats, the same blood flow pattern and perspiration occur, but now heat flows into the body from the hot water in the tub. The feedback has become positive instead of negative, and heat stroke and possibly death occurs.
Were we overly alarming about hot tubes? Not according to an article by Nicholas Bakalar in the November 23rd issue of the New York Times, which indicates hot tub accidents are a growing problem.
A hot tub might not seem an especially dangerous place, but over a period of 18 years, 1990–2007, more than 80,000 people were injured in hot tubs or whirlpools seriously enough to wind up in an emergency room. Almost 74 percent of the injuries occurred at home… About half the injuries were caused by slipping or falling, but heat overexposure was the problem in 10 percent of the accidents, and near-drowning in about 2.5 percent. Almost 7 percent of the injuries were serious enough to require hospitalization… The Consumer Products Safety Commission reported more than 800 deaths associated with hot tubs since 1990, nearly 90 percent of them in children under age 3.
This means that about 8000 people suffered from heat stroke accidents in hot tubs over 18 years, or over one per day. Perhaps a better understanding of biological thermodynamics and feedback loops has more than merely academic value.

To learn more, see “Death in a Hot Tub: The Physics of Heat Stroke,” by Albert Bartlett and Thomas Braun (American Journal of Physics, Volume 51, Pages 127–132, 1983).

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