Viruses such as SAR-CoV-2 are sensitive to heat and are destroyed by temperatures tolerable to humans. We all use fever to deal with infections and heat has been used throughout human history in the form of hot springs, saunas, hammams, steam-rooms, sweat-lodges, steam inhalations, hot mud and poultices to prevent and treat respiratory infections and enhance health and wellbeing (1)
Heat applied to the whole body can further boost the immune system’s second line of defence by mimicking fever and activating innate and acquired immune defences and building physiological resilience (1). Heat-based treatments also offer psychological benefits and enhanced mental wellness by focusing attention on positive action, enhancing relaxation and sleep, inducing 'forced-mindfulness', and invoking the power of positive thinking.
Viruses, such as rhinoviruses and coronaviruses, are most active in cool dry conditions, which are associated with increased occurrence of respiratory tract infections including infections with SARS-CoV (2) and SAR-CoV-2 (3). But whilst these viruses can stay active for long periods in cold conditions, their lipid envelopes are destroyed by temperatures tolerable to humans. The heat sensitivity of viruses is used routinely to deactivate viruses within vaccines, and temperatures of 55 to 65°C for 15 to 30 minutes are reported to deactivate a range of enveloped viruses, including coronaviruses (4, 5, 6).
The first line of defence against respiratory viruses is the nasal cavity and sinuses, which maintains a protective mucosal barrier that allows viruses to be trapped, identified by the immune system and swept away, as well as serving an important thermoregulatory role (1). The nasal cavity and sinuses in humans serve to cool inhaled air in summer and warm and humidify air in winter (7). So if a respiratory viruses get past the first line of defence, fever is produced as part of the acute phase response, which forms the immune system’s second line of defence.
Although heat therapies have been used for 100s of years as mentioned above, Heat-based therapies are not widely used in mainstream medicine, other than the local application of hot packs for symptomatic relief, or the use of novel technologies such as microwaves, radiofrequency energy, ultrasound, infrared radiators, ferromagnetic seeds, nanoparticles and resistive implants that apply heat to treat various cancers (8).
So can we benefit from heat therapies?
Heat based therapies such as infrared saunas can provide great benefits and there are multiple lines of evidence to support the use of heat and humidity for the prevention and treatment of viral respiratory infections. Historical and emerging evidence suggests regular sauna bathing enhances cardiovascular, respiratory, and immune function as well as improving mood and quality of life (9).
Saunas have been shown to reduce the risk of all-cause mortality, sudden cardiac death, cardiovascular disease and vascular diseases such as high blood pressure and stroke, along with the risk of neurocognitive diseases, skin conditions and painful conditions such as rheumatic diseases and headache (10). Further evidence suggests that frequent infrared sauna use( is associated with a reduced risk of pneumonia and viral infection, and randomised controlled trial evidence suggests that regular saunas can halve the incidence of respiratory viral infections trials further suggest hot air can treat respiratory infections with humidified air at temperatures above 43°C for 20 to 30 minutes being found to reduce viral shedding, provide immediate relief of symptoms and improve the course of the common cold (11, 12, 13).
Research has also found benefits of saunas to enhance interleukin-2 induced activity of Natural Killer (NK) cells (14), reduce adrenaline and cortisol, increase the cytotoxicity of NK cells, and enhance the proliferative response of B cells (15). Heat-stress also stimulates the release of Heat Shock Proteins (HSPs), (16) which play an important role in antigen presentation and cross-presentation, activation of macrophages and lymphocytes, and activation and maturation of dendritic cells (17) as well as serving a chaperone function and protecting immune cells and proteins from heat-induced damage.
In addition to enhancing cellular responses, heat-stress induces a hormetic stress response that builds physiological resilience and confers tolerance to subsequent stress in a similar way to exercise (18). Furthermore, heat stress improves respiratory function, improves cardiovascular function by modulating the autonomic nervous system, reducing inflammation, oxidative stress and blood pressure, increasing cardiac output, plasma volume and peripheral blood flow, and improving endothelial function, lipid profile, arterial compliance and aids in detoxification via sweating (10).
In addition to offering physiological advantages in the battle against viral infection such as COVID-19, heat-based treatments can support mental wellness and confer many psychological benefits. Sauna bathing and other forms of heat therapy require time and effort devoted towards active relaxation that can help divert attention from anxiety-producing news and/or relieve boredom associated with social confinement. Sauna bathing also enhances sleep (9), which further supports immune function.
Conclusion
The research is overwhelming with how beneficial heat therapy is. It is also cheap, convenient, and a widely accessible therapeutic modality with a long history of traditional use. Although it remains to be seen if heat can be effective in the treatment or prevention of COVID-19. The relatively low cost and wide availability of heat-based treatments such as infrared saunas along with multiple mechanisms of action that include both physical and psychological dimensions, makes heat an attractive option for combating viral infections. The integration of these ancient forms of treatment with modern technology may lead to a greater integration of natural therapies in mainstream healthcare, with the potential to support the wellbeing of both patients and medical staff. This may also lead to a greater convergence between the healthcare and wellness industries, and the development of systems and activities that build the wellbeing and resilience of the wider community, thereby reducing the impact of future pandemics.
(1) Cohen M. (2020). Turning up the heat on COVID-19: heat as a therapeutic intervention. F1000Research, 9, 292. https://doi.org/10.12688/f1000research.23299.2
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