Foundation for Airway Health believes mouth breathing is one of the most visible signs of an airway disorder. Now a new study presents the benefits of nasal breathing during exercise. The Washington Post's Wellness Perspective explains this in Could Nasal Breathing Improve Athletic Performance? Says author Jae Berman: "The nose is built with a specific purpose: to support our respiratory system (the primary purpose of the mouth, on the other hand, is to start the digestive process). The nostrils, hair, and nasal passageways are designed to assist in filtering allergens and foreign bodies from entering the lungs. The nose also adds moisture and warmth to inhaled air for smoother entry to the lungs." Further, "Nasal breathing also activates the part of the nervous system that supports rest, recovery and digestion, rather than the part of the nervous system that is responsible for survival or stress states, such as flight or freeze. That means that, even if the body is in a stressful state of high-intensity exercise, nasal breathing can provide a sense of calm and allow us to function better."

Read through the article for more on the study:
International Journal of Kinesiology and Sports Science