How Does Sauna Bathing Improve Respiratory Tract Function?

Abstract

Sauna bathing is a custom deeply rooted in several cultures and has many beneficial effects on health.1Heinonen I Laukkanen JA Effects of heat and cold on health, with special reference to Finnish sauna bathing.Am J Physiol Regul Integr Comp Physiol. 2018; 314: R629-R638Crossref PubMed Scopus (24) Google Scholar With regard to the respiratory tract, sauna bathing may reduce the incidence of common colds2Ernst E Pecho E Wirz P Saradeth T Regular sauna bathing and the incidence of common colds.Ann Med. 1990; 22: 225-227Crossref PubMed Scopus (48) Google Scholar and relieve the symptoms of bronchial asthma.3Laitinen LA Lindqvist A Heino M Lungs and ventilation in sauna.Ann Clin Res. 1988; 20: 244-248PubMed Google Scholar The condition of the air in a Finnish sauna should fall within a range of temperature (80-100°C) and relative humidity (10-20%).4Hannuksela ML Ellahham S. Benefits and risks of sauna bathing.Am J Med. 2001; 110: 118-126Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar Because of the low relative humidity, the air is usually described as “dry.” Most of the health benefits are attributed to physiologic responses to heat.1Heinonen I Laukkanen JA Effects of heat and cold on health, with special reference to Finnish sauna bathing.Am J Physiol Regul Integr Comp Physiol. 2018; 314: R629-R638Crossref PubMed Scopus (24) Google Scholar, 2Ernst E Pecho E Wirz P Saradeth T Regular sauna bathing and the incidence of common colds.Ann Med. 1990; 22: 225-227Crossref PubMed Scopus (48) Google Scholar, 3Laitinen LA Lindqvist A Heino M Lungs and ventilation in sauna.Ann Clin Res. 1988; 20: 244-248PubMed Google Scholar, 4Hannuksela ML Ellahham S. Benefits and risks of sauna bathing.Am J Med. 2001; 110: 118-126Abstract Full Text Full Text PDF PubMed Scopus (186) Google Scholar However, Anderson et al5Anderson SD Daviskas E Schoeffel RE Unger SF Prevention of severe exercise-induced asthma with hot humid air.Lancet. 1979; 2: 629Abstract PubMed Scopus (28) Google Scholar reported that the inhalation of air at 37°C and 100% relative humidity prevents exercise-induced asthma; at first blush, this result seems inconsistent with other effects of sauna bathing because the air temperature is lower and the relative humidity is higher. Stressing the distinction between relative and absolute humidity resolves the conundrum. Although absolute humidity is usually measured in grams of water per cubic meter of air, for physiologic purposes, it is clearer to use partial pressures of water. The two quantities are proportional according to the gas law, and the partial pressure answers to the “driving force” of water from air into the tissue. The equilibrium vapor pressure at body temperature is 47 torr. A partial pressure of water greater than 47 in inspired air will hydrate the respiratory tract mucus; whereas, a partial pressure less than 47 tends to desiccate it. Taking the midpoints of the ranges for Finnish saunas (90°C; 15 % relative humidity), the equilibrium vapor pressure is 526 torr, and the partial pressure of water is 79 torr. Therefore, under usual conditions, sauna air will hydrate respiratory tract mucus, which can account for the beneficial effects. From the perspective of physiology, sauna air is not “dry.” It may be noted that stressing the distinction between relative and absolute humidity has recently clarified that fluctuation of absolute humidity is the primary driver of seasonal variation of influenza as a clinical syndrome,6Shaman J Kohn M Absolute humidity modulates influenza survival, transmission, and seasonality.Proc Natl Acad Sci U S A. 2009; 106: 3243-3248Crossref PubMed Scopus (599) Google Scholar both in temperate and tropical climates.7Deyle ER Maher MC Hernandez RD Basu S Sugihara G Global environmental drivers of influenza.Proc Natl Acad Sci U S A. 2016; 113: 13081-13086Crossref PubMed Scopus (126) Google Scholar Although relative humidity may be more important for meteorology, absolute humidity (partial pressure of water) is the better quantity for physiology.