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Why don't we breathe equally out of both nostrils?

A man with a beagle licking his nose. Humans have two nostrils, but most people have one that is dominant.
A beagle licks a man's nose. (Image credit: Boris Zhitkov via Getty Images)

If you get close to a mirror and breathe out through your nose, the mirror will fog up. Two marks of water vapor will pool on the surface, one for each nostril. But one mark will be larger than the other, because people breathe mostly out of one nostril at a time.

So why do we rarely breathe out of both nostrils at once?

At any given time, people do about 75% of their breathing from one nostril and 25% from the other, said Dr. Michael Benninger, a head-and-neck doctor at the Cleveland Clinic. The dominant nostril switches throughout the day. This is called the nasal cycle. 

Related: Why do we breathe so loudly when we sleep?

Although we don't usually notice it, during the nasal cycle one nostril becomes congested and thus contributes less to airflow, while the other becomes decongested. On average, the congestion pattern switches about every 2 hours, according to a small 2016 study published in the journal PLOS One. Right-handed people tended to spend more time favoring their left nostril, according to the study.

No one is sure why the nasal cycle occurs, Benninger said. But there is one popular theory: "Some people have speculated that it has to do with allowing moisture to build up on one side so that it doesn't get too dry," he said.

Most people aren't aware of the nasal cycle, Benninger said. However, it may become more noticeable during sleep. If a person lies on their right side, for example, gravity will cause that lower nostril — the right nostril — to become more congested. If the cycle has designated the right nostril to be naturally more congested at the time, there's no appreciable effect. But if the nasal cycle has made the left nostril more congested and the right nostril is congested because of side sleeping, breathing may be difficult, and the person may wake up.

Usually, people notice the cycle only if they have consistent blockage of one side of their nose, Benninger noted. One way this can happen is if a person has a deviated septum, in which the wall between the nostrils is displaced, causing it to push into one side. Some people also grow soft, painless growths — called polyps — in their nose, which can cause the same effect. Although less common, tumors can draw attention to it as well.

Although your nose may feel stuffy when you have a cold, that’s not due to the nasal cycle. In general, both nostrils are congested when you’re sick, so you’re going to have trouble breathing through your nose no matter where you are in the cycle, Benninger said.

There are ways to decongest both nostrils at once, which can temporarily lead someone to breathe more equally out of them until the nasal cycle resumes. Medications such as nasal decongestant sprays, as well as exercise, can have the same effect, Benninger said. Sex can also open up the nose and may be a natural substitute to decongestant medicine, according to a 2021 study in the Ear, Nose & Throat Journal.

Originally published on Live Science.

Tara Santora

Tara Santora is a contributing editor at Fatherly and a freelance science journalist who covers everything related to science, health and the environment, particularly in relation to marginalized communities. They have written for Popular Science, Scientific American, Business Insider and more. Born and raised in the Philadelphia suburbs, Tara graduated from Oberlin College with a bachelor's degree in biology and New York University with a master's in science journalism.