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 implications The 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.

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Effective ventilation in general hospital wards is important for controlling the transmission of airborne infectious agents that may cause respiratory diseases. This study investigates the potential of protected occupied zone ventilation (POV) to reduce the risk of cross-infection in hospital isolation wards. Two life-size breathing thermal manikins were used to simulate an infected patient lying in a bed and a sitting, receiving health care worker. N2O was used as a tracer gas to simulate the droplet nuclei exhaled by patients. The contaminant exposure index [Formula: see text] was used to assess the risk of cross-infection for different configurations of the supply velocity, the patient’s lying position and the exhaust openings. The contaminant exposure index [Formula: see text] shows that the ventilation strategy is effective, but this is also highly dependent on the supply velocity and the location of the exhaust relative to the patient’s breathing zone. The patient’s posterior position does not affect the personal exposure of the receiving patient. Compared to traditional ventilation systems, the POV system can reduce the risk of cross-infection in hospital isolation rooms. The full potential of a POV system can be achieved in an isolation ward where movement between the infected and protected zones is restricted or prohibited.

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