Regular sauna baths boost the immune system and keep us healthy during the virus-infested winter season. Sauna’s healthy effect can be explained by two things, mucus and heat. A fair warning: the following is based on scientific research.
Some basic facts first. The blanket of mucus in the nasal cavity and paranasal sinuses form the first line of defence against viruses. Despite its nasty name, this watery mucus is highly useful—it catches virus particles and stops them from entering our organism. We then expel the foreign particles and pathogens by sneezing, coughing and blowing our nose, thereby cleaning the blanket of mucus. Every child is thought to use a hanky to cover her nose and mouth when sneezing or coughing to prevent the virus from spreading. And we are all learning how to wear face masks to stop the COVID-19 pandemic for the same reason.
Dry air inhibits mucus regeneration
Most virus particles are caught by the layer of mucus before they can do harm. However, sometimes the air we breathe dries up the watery mucus, making it easy for the viruses to enter the body. Dry air is perfect for different viruses, including the coronavirus which causes the COVID-19 pandemic, rhinovirus and the influenza virus. Human beings feel comfortable if the indoor humidity is between 25%–70%. But if the relative humidity drops below 20%, the blanket of mucus dries up and our defense against viruses is impaired. In consequence, viruses enter our body and we may fall ill.
Sauna to the rescue
The relative humidity in a typical sauna is only 10–20%. Yet few people take sauna baths without throwing water on the stove. This activity drastically increases air humidity (up to 80%) and creates perfect conditions for regenerating mucus and repairing tissue. After the layer of mucus is restored in the humid air we are again prepared to fight off viruses—virus particles will be trapped inside the new watery layer on our nasal cavity and paranasal sinuses.
High temperature vs. common cold
Even if some viruses slip through the first line of defence, sauna’s hot air will kill them off quickly. Many viruses do not tolerate hight temperatures. Human rhinoviruses which cause common cold is one such virus. According to different studies, rhinoviruses reproduce in temperatures lower than 33°C (91°F). It is probably for this reason that rhinoviruses cause upper respiratory infections as they cannot survive deep in the human body where temperatures are higher. Spending only 20 minutes at 45°C (113°F) activates our immune system and reduces rhinovirus’ capability to multiply more than 90%. In sum, sauna is less than an ideal place for rhinoviruses as well as many other viruses.
Sauna and COVID-19
Also the coronavirus does not like high temperatures, albeit for a different reason. Coronaviruses are surrounded by an envelope which protects the virus when traveling from one host cell to another. Such viruses are relatively resistant to cold and therefore are more likely to cause sickness in winter months. However, the protective lipid envelope is destroyed if the temperature is held between 55–65°C (131–149°F) for 15–30 minutes. Only one minute will do if the temperature is 80°C (176°F). Human beings can endure such heat, the virus can not.
In addition, coronavirus does not tolerate an alkaline environment. In the intense heat of sauna, our breeding becomes deeper and more frequent. This makes our blood pH levels rise. Different strands of the coronavirus cannot reproduce in such conditions, become inactive or disintegrate complete.
Is it really so simple?
The conclusion seems obvious—taking sauna baths regularly will protect us against viruses. Alas it is not that simple. We have to keep in mind that, even if the sauna thermometer reaches 120°C (248°F), the core temperature of our body rises no more than a few degrees. Yet the virus particles that have already entered into our organism are untouched by the high environmental temperatures and remain intact. Taking sauna baths is not a miracle cure against viruses if we are already sick. Sauna can be used as a preventive measure to boost our immune system, regenerate the mucus blanket in our nasal cavity, and kill off free virus particles in our upper respiratory tracts. But it will not cure us.
Even though in most cases taking sauna baths may be good for us, it is best to avoid sauna if you are already sick. Because our body is weak, it cannot deal with sudden and steep temperature changes. A sick and already weak organism simply cannot take it. We may easily weaken our organism further by exposing our sweaty body to colder temperatures in the antechamber or outside.
 Täheväli Stroh, Lea. Maja ja niiskus: Praktilisi nõuandeid niiskuskahjustuste ennetamiseks (Buiding and moisture: Practical guidlines to prevent moisture damage). Tallinn, Ajakirjade Kirjastus, 2005.
 Kudo et al. Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proceedings of the National Academy of Sciences of the United States of America (PNAS), May 28, 2019, 116 (22).
 Conti et al. Antiviral Effect of Hyperthermic Treatment in Rhinovirus Infection. Antimicrobial Agents and Chemotherapy, April 1999.
 Cohen, Mark. Turning up the heat on COVID-19: heat as a therapeutic intervention. F1000Research, 2020, 9:292.
 Cohen, Mark. 2020, p. 4.