Remarks
Analyzing the epidemiology and the clinic of colds, the first question is, why they appear above all in the cold seasons, or in correspondence of periods of sudden thermal decrease and why they affect the upper respiratory tract, and initially , the mucosa of the nasal and paranasal cavities in particular and/or of the oral cavity and the larynx.
That is, because for the most part they replicate better at temperatures of around 33-35°C or lower, so they could be defined as ”cryophilic” (cold loving ).
Therefore they prefer the mucous membranes of the ”colder” or ”more easily cooled” territories, whose average temperature is lower than the average basal endocorporeal human standard (37°C).3,5,7
Rhinoviruses replicate at a temperature of 33°C, which corresponds to the temperature of the human nasal mucosa. 6
Rhinoviruses replicate more easily at slightly lower temperatures than the body temperature (around 32/33°C, compared to 37°C in our body), which is why they attack the upper airways, and in particular the mucosa of the nose, where they find the ideal climate to multiply.,6,8
Hyperthermic treatment at 43°C for 20 min is effective in reducing the symptoms of common colds by 50%. 8
A 20-min hypertermic treatment at 45°C is effective in suppressing human Rhinoviruses multiplication by more than 90% when applied at specific stages of the virus replication cycle. 9
Coronavirus dried on smooth surfaces retains its vitality for over 5 days at temperatures of 22-25°C and a relative humidity of 40-50%, which is typical for air-conditioned rooms. 10
Conversely, the viability of the virus is rapidly lost (> 3 log10) at higher temperatures and higher relative humidity (for example: 38°C and a relative humidity of > 95%).11
The improved stability of the SARS-CoV at low temperature and low humidity environment can facilitate its transmission in communities in the subtropical area (such as Hong Kong) in air-conditioned environments. 12
In a experimental 2012 study Xiangyug Q. characterized viral (bacteriofage λ ) structure changes upon heating. 13
Moreover, innate immune defenses have a temperature-dependent efficacy in limiting the replication of common cold viruses: it increases with warm temperature 6
In fact, it is documented that hyperthermia induces endogenous production of γ-interferon (IFNγ), with a consequent antiviral effect.14,15,16
It should certainly be mentioned, that a Cochrane review of 2017 does not detect any difference in results in subjects with acute rhinitis in progress, therapeutically using the treatment with humidified and heated air at 40-47°C administered with the RhinoTerm device, compared to the group of control. 17
These conclusions are also reflected in a subsequent 2018 BMJ paper.18
However, these assessments consider the effect of humidified and heated air as a therapy of a disease in progress and not as a preventative treatment.
Vice versa, the Yamaya’s 2019 study on the effect of high temperature on pandemic and seasonal human influenza viral replication19 and the very recent work (February 2020) published by Wang M. are particularly interesting. In this last paper the difference in the diffusion of the Coronavirus SARS-CoV-2 in different chinese cities is detected, correlating with different relative temperatures. As a result, a minimum temperature increase of 1°C correlates with the reduction in the cumulative number of cases by a factor of 0.86 and at a temperature of 38°C the virus quickly loses its activity. 20
Although the spread of the viral airway diseases, in particular of the more serious, such as the recent pandemic from Coronavirus SARS-CoV-2, cannot be influenced only by ambient temperature, the correlations between the spread of virosis and the environmental temperatures are documented. 21
These epidemiological evaluations are consistent with the potential use of hyperthermic treatment not so much as therapy for acute pathologies, but for prophylactic/preventive purposes.