Abstract
A novel modified version of the Wells-Riley model was used to estimate the impact of relative humidity (RH) on the removal of respiratory droplets containing the SARS-CoV-2 virus by deposition through gravitational settling and its inactivation by biological decay; the effect of RH on susceptibility to SARS-CoV-2 was not considered. These effects were compared with the removal achieved by increased ventilation rate with outdoor air. Modeling was performed assuming that the infected person talked continuously for 60 and 120 min. The results of modeling showed that the relative impact of RH on the infection risk depended on the ventilation rate and the size range of virus-laden droplets. A ventilation rate of 0.5 ACH, the change of RH between 20% and 53% was predicted to have a small effect on the infection risk, while at a ventilation rate of 6 ACH this change had nearly no effect. On the contrary, increasing the ventilation rate from 0.5 ACH to 6 ACH was predicted to decrease the infection risk by half which is remarkably larger effect compared with that predicted for RH. It is thus concluded that increasing the ventilation rate is more beneficial for reducing the airborne levels of SARS-CoV-2 than changing indoor RH. Practical implicationsThe present results show that humidification to moderate levels of 40%–60% RH should not be expected to provide a significant reduction in infection risk caused by SARS-CoV-2, hence installing and running humidifiers may not be an efficient solution to reduce the risk of COVID-19 disease in indoor spaces. The results do however confirm that ventilation has a key role in controlling SARS-CoV-2 virus concentration in the air providing considerably higher benefits. The modified model developed in the present work can be used by public health experts, engineers, and epidemiologists when selecting different measures to reduce the infection risk from SARS-CoV-2 indoors allowing informed decisions concerning indoor environmental control.
Highlights
At the start of the pandemic in 2020, the recommendations related to infection control published by the World Health Organization (WHO) [1] acknowledged respiratory droplet transmission (>5μm) as the only mode of SARS-CoV-2 airborne transmission, while possible transmission by aerosols (≤ 5 μm) was disputed considering the limited evidence available
We further modeled the dynamics of droplets emitted from an infected person in an indoor environment to simulate the airborne transmission of SARS-CoV-2 viral load, considering removal by ventilation, deposition by gravitational settling, and biological decay of the SARS-CoV-2 virus in aerosols
We were able to determine and estimate the magnitude by which relative humidity (RH) can affect the airborne transmission of SARS-CoV-2 and the reduction in the infection risk from one infected individual within public indoor spaces
Summary
At the start of the pandemic in 2020, the recommendations related to infection control published by the World Health Organization (WHO) [1] acknowledged respiratory droplet transmission (>5μm) as the only mode of SARS-CoV-2 airborne transmission, while possible transmission by aerosols (≤ 5 μm) was disputed considering the limited evidence available These WHO recommendations have repeatedly come under strong criticism from numerous studies, arguing that microdroplets or aerosols ≤ 5 μm in size are small enough to remain suspended in the air and expose individuals at distances beyond 2 m from an infected person [2,3,4,5,6].
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