Can spas or hot tubs cause infertility?
Anything that warms your testes can reduce sperm production — spas, hot tubs, jacuzzis, baths, tight underwear that holds your testes close to your body, and even a fever from an infection. The consequence of reduced sperm production is reduced fertility.
Normal body temperature (about 37°C) is too warm for optimal sperm production. That’s why we (and lots of other warm-blooded animals) have a scrotum. The scrotum usually holds the testes away from the abdomen, so they don’t get too warm. In humans, the temperature in the scrotum is about 1.5-2 degrees lower than your body temperature.
Exactly how much your sperm production is affected by soaking in the tub will depend on the water temperature, how long you’re in it, and how often you do it.
There are experiments dating back 80-90 years that show a reduction in sperm count for five to seven weeks after only 30 minutes of scrotal warming, but the size of the effect was different for different men. Twelve consecutive days of warming caused large reductions in sperm count from about five to seven weeks later, lasting for about five weeks.
Because of the importance of temperature for effective sperm production, our bodies have a variety of ways to help keep our testes cool. You’ll have noticed that your scrotum relaxes when you’re hot. That’s so your testes hang further from your body to stay slightly cooler.
There’s also a heat exchanger within your scrotum to keep the testes cool. Blood in the veins passes closely by the warmer blood flowing in the arteries, so that heat is lost from the arterial blood before it reaches the testes. This is the same kind of system arctic foxes use to prevent heat loss from their feet.
Whales and dolphins don’t have scrotums (their testes lie in their bellies) but they do have a counter-current heat exchange mechanism, with cool blood returning from the fluke and dorsal fin taking heat away from the arterial blood supply to the testes.
In cattle and sheep, the concentration of sweat glands is greater on the scrotum than anywhere else in the body.
There’s no fat, which provides insulation against heat loss, in our scrotum.
So, remember to avoid things like hot tubs if you’re trying to make a baby. But don’t rely on a hot bath or sitting in a spa for a while as a reliable method of contraception!
Answered by: Associate Professor Tim Moss
Associate Professor Tim Moss has PhD in physiology and more than 20-years’ experience as a biomedical research scientist. Tim stepped away from his successful academic career at the end of 2019, to apply his skills in turning complicated scientific and medical knowledge into information that all people can use to improve their health and well being.
Tim has written for crikey.com and Scientific American’s Observations blog, which is far more interesting than his authorship of over 150 academic publications. He has studied science communication at the Alan Alda Centre for Communicating Science in New York, and at the Department of Biological Engineering Communication Lab at MIT in Boston.