Just about everyone with an unobstructed nose will breathe through their nose when at rest or during casual activities, but most people will breathe through their mouth during exercise. The more intensely you exercise, the more likely that you will have to breathe through your mouth because you may not be able to get enough air through your nose to feel comfortable (Respiration Physiology, 1983;53(1):129–133).

Possible Advantages of Breathing Through Your Nose
Why would you even consider trying to control whether you breathe through your mouth or your nose? Compared to mouth breathing, nasal breathing:
• helps to filter out pollutants
• helps to filter germs
• adds moisture to the air you breathe
• heats the air you breathe
• may reduce asthma attacks in people who suffer from exercise-induced asthma.

Nasal Breathing Takes Practice
You can breathe far more air into your lungs through your mouth than you can breathe through your nose. You can exercise intensely when you breathe just through your nose, but you will need to practice (PNAS, May 19, 2015;112(20):6425-6430).
• How intensely you can exercise depends on how fast oxygen can pass from red blood cells into muscle cells.
• The cells lining your nose and sinuses release large amounts of a gas called nitric oxide while the cells lining your mouth and throat do not (Nat Med, 1995;1:370–373).
• Breathing through your nose releases far larger amounts of nitric oxide, which specifically widens the very small blood vessels next to muscles to bring the red blood cells closer to muscle cells, to increase markedly the rate that oxygen passes from red blood cells to muscle cells.

One study showed that with training, you can get enough air while breathing through your nose to exercise at up to 85 percent of your maximum capacity (Int J of Kinesiology and Sports Sci, Apr 30, 2018;6(2):22). Ten recreational runners practiced nasal breathing during exercise for six months and when they breathed through their noses while exercising at up to 85 percent of their maximal capacity, they had the same:
• time to exhaustion,
• maximal capacity to take in and use oxygen (VO2max), and
• peak lactate levels. (Lactate levels increase when you don’t get enough oxygen).
Nasal breathing brought in the same maximal amount of oxygen as mouth breathing, but nasal breathing markedly reduced:
• respiratory rate (breaths per minute), and
• ratio of oxygen intake to carbon dioxide output.

Nasal Breathing May Help with Exercise-Induced Asthma
People who have exercise-induced asthma may benefit from nasal breathing when they exercise. Within minutes after starting to exercise, they often suffer wheezing, coughing, shortness of breath, a tight chest, decreased endurance, or a sore throat. These symptoms are usually caused by breathing dry and cold air (Allergy, 2013;68:1343–1352). Practicing nasal breathing for several months can help to decrease asthma attacks (Clinical Allergy, 1981;11(5):433-9). However, nasal breathing has been shown to hinder performance of elite athletes who suffer from exercise-induced asthma (British J of Sprts Med, 2012;46:413-416).

My Recommendations
• Most people can learn to breathe comfortably through their noses during intense exercise if they want to (International J of Ex Sci, 2017;10(4):506-514), but nasal breathing is not recommended for competitive athletes since it is likely to reduce their maximum exercise intensity (Australian J of Sci and Med, 1995;(273):512-55).

• You don’t need to breathe through your nose when you exercise in very cold weather. Your nose warms the air much more than your mouth does, but exercise causes your body to produce such large amounts of heat that air taken through your mouth at 40 degrees below zero Fahrenheit during exercise will be warmed almost 100 degrees before it reaches your lungs. Breathing air that cold would burn your nose so much that you would quickly lose interest in exercising and seek shelter.

• Your nose clears pollutants far more efficiently than your mouth does, but people with healthy lungs can exercise safely using mouth-breathing on mildly polluted days. Your air tubes are lined with small hairs, called cilia, that sweep pollutants towards your mouth where you swallow them with your saliva and they pass from your body. However, breathing heavily polluted air when you exercise can damage your lungs, whether you use your mouth or your nose. Air quality experts tell us that if you can see ash or smell smoke, stay indoors with windows and doors closed.

• If you want to try nasal breathing, you may find that commercially-available nasal strips that fit over the bridge of your nose make it easier and more comfortable.

• The bottom line is that you can breathe through either your mouth or your nose during exercise. Do whatever feels most natural for you.