Modeling the impact of indoor relative humidity on the airborne transmission of several respiratory viruses risk using a modified Wells-Riley model

There is good evidence supporting the airborne transmission of many respiratory viruses (measles, influenza A, human rhinovirus and the novel SARS-CoV-2). Relative humidity (RH) is an important factor in understanding airborne transmission as it may impact both airborne survival, inactivation by biological decay, and the gravitational settling of the virus-laden droplets. This study aimed to estimate and compare the impact of indoor relative humidity on the airborne infection risk caused by these viruses using a novel modified version of the Wells-Riley model. To gain insights into the mechanisms by which relative humidity might impact airborne transmission infection risk, we modeled the size distribution and dynamics of airborne viruses emitted from a speaking person in a typical residential setting over a relative humidity (RH) range of 20–80% at a temperature of 20-21 °C. Besides the size transformation of virus-containing droplets due to evaporation and then removal by gravitational settling, the modified model also considers the removal mechanism by ventilation. The direction and magnitude of RH impact depended on the respiratory virus. Measles showed a highly significant RH impact that was as strong as the ventilation impact, as the infection risk was roughly the same at RH of 13.5 % and 6 ACH compared to a higher RH of 70 % and 0.5 ACH. For other viruses, ventilation dominated over RH. In the case of SARS-CoV-2, a very high RH of 83.5% was needed to reduce the infection risk. For rhinovirus, however, the high RH of 80% increased the infection risk. Within the acceptable range of RH of 20-50% indoors, our modeling showed that RH had practically no impact for SARS-CoV-2 and rhinovirus, while the upper RH significantly reduced the infection risk of influenza A at the lowest ventilation rate of 0.5 ACH. This relative impact of RH on infection risk became very weak at higher ventilation rates of 2-6 ACH independently of the virus types (except measles). In conclusion, we showed that in well-ventilated rooms, RH range of 20-50% did not affect the airborne risk of influenza A, SARS-CoV-2, and rhinovirus.