The nose and lungs form a single, unified airway. This means that where inflammation is present, it can travel from the nose to the lungs and from the lungs to the nose. This is one reason people with asthma experience high levels of nasal congestion. The normal response to a stuffy nose is to begin breathing through the mouth. This, however, increases breathing volume, draws cold, unfiltered, dry air into the airways, and makes the symptoms of asthma worse. Habitual mouth breathing also affects sleep and causes or contributes to stress.
A substantial 2016 study in Allergy demonstrated that mouth breathing was associated with the worsening of asthma. Even participants who did not have asthma experienced poorer lower lung function and more sensitivity to house dust mites when they mouth-breathed. Researchers examined questionnaires completed by 9,804 people from Nagahama City in Japan and found that mouth breathing causes asthma to become more severe, possibly by increasing vulnerability to inhaled allergens. Scientists concluded, “The risk of mouth breathing should be well recognized in subjects with allergic rhinitis and in the general population.” A separate study found that habitual mouth breathing reduced lung function in people with mild asthma and exacerbated the symptoms of acute asthma.
Bartley, James, and Conroy Wong. “Nasal Pulmonary Interactions.” In Nasal Physiology and Pathophysiology of Nasal Disorders, pp. 559-566. Springer, Berlin, Heidelberg, 2013.
Izuhara, Y., Hisako Matsumoto, T. Nagasaki, Y. Kanemitsu, K. Murase, I. Ito, T. Oguma et al. “Mouth breathing, another risk factor for asthma: the Nagahama Study.” Allergy 71, no. 7 (2016): 1031-1036. https://onlinelibrary.wiley.com/doi/abs/10.1111/all.12885
Hallani, M., J. R. Wheatley, and T. C. Amis. “Initiating oral breathing in response to nasal loading: asthmatics versus healthy subjects.” European Respiratory Journal 31, no. 4 (2008): 800-806. https://pubmed.ncbi.nlm.nih.gov/18032447/
