Effects of face masks and ventilation on the risk of SARS-CoV-2 respiratory transmission in public toilets: a quantitative microbial risk assessment

Abstract

Public toilets could increase the risk of COVID-19 infection via airborne transmission; however, related research is limited. We aimed to estimate SARS-CoV-2 infection risk through respiratory transmission using a quantitative microbial risk assessment framework by retrieving SARS-CoV-2 concentrations from the swab tests of 251 Thai patients. Three virus-generating scenarios were investigated: an infector breathing, breathing with a cough, and breathing with a sneeze. Infection risk (97.5th percentile) was as high as 10−3 with breathing and increased to 10−1 with a cough or sneeze, thus all higher than the risk benchmark of 5 × 10−5 per event. No significant gender differences for toilet users (receptors) were noted. The highest risk scenario of breathing and a sneeze was further evaluated for risk mitigation measures. Risk mitigation to lower than the benchmark succeeded only when the infector and receptor simultaneously wore an N95 respirator or surgical mask and when the receptor wore an N95 respirator and the infector wore a denim fabric mask. Ventilation up to 20 air changes per hour (ACH), beyond the 12-ACH suggested by the WHO, did not mitigate risk. Virus concentration, volume of expelled droplets, and receptor dwell time were identified as the main contributors to transmission risk. Highlights - The use of public toilets poses a risk of SARS-CoV-2 respiratory transmission - Highest risks generated in the order of sneezing, coughing, and breathing - No gender differences in risk by counteracting dwell times and inhalation rates - Ventilation did not reduce risk even at 20 ACH, beyond the WHO-recommended value - N95 and surgical masks offer the most effective risk mitigation to toilet users Graphical abstract