SARS-CoV-2 airborne transmission: a validated sampling and analytical method
Authors: Robotto Angelo , Civra Andrea , Quaglino Paola , Polato Denis , BRIZIO ENRICO , Lembo David 
Published: May 2021

The most recent scientific studies have finally identified the airborne transmission of SARS-CoV-2 as significant. Therefore, the airborne transmission path for SARS-CoV-2 is of primary scientific and health-related interest as it could actually involve management, accessibility, use and functionality of many activities, including hospitals (where COVID wards represent only a part of the critical issues), schools, workplaces, offices, factories, means of transport, sports venues, and the outdoor environment. It is necessary to develop a sampling and analytical method for virus-laden bioaerosol that could be considered reliable and validated according to ISO/IEC 17025 requirements.The present paper defines samples pretreatments aiming at recover SARS-CoV-2 from glass-fiber and PTFE filters used by low and high-volume air samplers. Recovery test results focused on the sample concentration step carried out by means of ultracentrifugation are reported as well. Human coronavirus strain OC43 (a surrogate β-coronavirus with same SARS-CoV-2 particle structure) was used to validate each step of the recovery tests.We obtained the following results:-the recovery efficiency from glass-fiber filters and quartz filters could be strongly enhanced by using an elution buffer containing up to 40% of fetal calf serum.-the recovery efficiency of coronavirus OC43 (HCoV-OC43) from PTFE filters is much higher and easier than from glass-fiber filters; for glass-fiber filters, we found that a two-step procedure is necessary to elute viral infective particles: a 3 hour-shaking step, followed by a 30 seconds-vortexing step. For PTFE 60 minutes-shaking is enough.-the effect of suction time on filters could be resumed as follows: concerning 10cm glass-fiber filters, sampling durations up to 20 minutes at a flow rate of 500 L per minute do not affect recovery efficiencies. On the contrary, the recovery efficiency of infectious virions from 4.7cm PTFE filters decreases of a factor 2 after 3 hours of sampling at a flow rate of 20 L per minute.-the recovery efficiency of ultracentrifugation is 57%.Furthermore, the effect of the storage temperature of the filters immersed in the transport medium on the infectivity of HCoV-OC43 has been assessed.Based on the sampling techniques and the analytical methods developed as described in the present study, many field tests were carried out reporting virus concentrations up to 50 genomic copies per cubic meter of air in domestic environment with poor ventilation condition, whereas in hospital wards the detectable concentrations of SARS-CoV-2 were generally lower than 10 genomic copies per cubic meter of air.The developed methods, aiming at providing the community with reliable determinations about the presence of SARS-CoV-2 and other airborne pathogens in air, prove essential for the development, during the pandemic, of a coherent management of places (especially of crowded ones) such as means of transport, stations, gyms, theaters, cinemas.