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Fitness instructors always tell you to breathe in through the nose and out through the mouth while exercising. Why?
鈥� Some rituals may produce benefits in the same way that a placebo can, but in principle the practice your questioner describes has some concrete advantages.
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Gyms are often dusty or may be humidified largely by the exhalations of exercisers, who may also be sneezing out germs. In this environment, inhaling through your nose may be healthier than using your mouth, as the nose can filter out dust and fine droplets.
What鈥檚 more, inhaling through your nose rather than your mouth makes it less likely that you will hyperventilate 鈥� and thus you are less likely to suffer the associated faintness and tingling, brought on by a shortage of carbon dioxide in the blood. Air inhaled through the nose also tends to be warmer and moister than air entering via the mouth, so does not dry or irritate the throat.
As for exhaled air, if it leaves via the nose it deposits heat in the nasal passages, retaining it within the body. Exhaling through the mouth is preferable as it increases evaporation, removing unwanted heat following exertion.
Jon Richfield, Somerset West, South Africa
Many of you have written in over the years asking if we could interview Jon Richfield. We can at last oblige; see 鈥淭he man who knew so much鈥� 鈥� Ed
鈥� At peak effort while exercising, you may well perform what is called a . This also happens during evacuation of the bowel and in childbirth, and involves closing the glottis and contracting the abdominal muscles. The increased pressure in the abdomen results in a reduced flow of blood back to the heart from the abdominal organs and legs. If this manoeuvre is sustained at sufficient pressure, fainting can occur. As it happens, breathing out through your mouth makes it much harder to do a Valsalva manoeuvre.
Additionally, breathing in through the nose moistens the incoming air and so avoids drying out the airways. On the other hand, you lose moisture faster if you breathe out through the mouth than when exhaling through the nose, and this could explain why people who tend to exhale with their mouth when they exercise also tend to need more fluids.
Don L Jewett, Former Director of Physical Therapy, University of California, San Francisco, US