Impact of supplementary air filtration on airborne particulate matter in a UK hospital ward

BackgroundDuring the COVID-19 pandemic, aerosol spread of SARS-CoV-2 has been a major problem in healthcare facilities, resulting in increased use of supplementary HEPA filtration to mitigate transmission. We report here a natural experiment that occurred when an air filtration unit (AFU) on an inpatient ward for older people was accidentally switched off. AimTo assess aerosol transport within the ward and determine whether the AFU reduced particulate matter (PM) levels in the air. MethodsTime-series PM, CO2, temperature and humidity data (at 1 minute intervals) was collected from multiple sensors around the ward over two days in August 2021. During this period, the AFU was accidentally switched off for approximately 7 hours, allowing the impact of the intervention on particulates (PM1-PM10) to be assessed using a Mann-Whitney test. Pearson correlation analysis of the PM and CO2 signals was also undertaken to evaluate the movement of airborne particulates around the ward. FindingsThe AFU greatly reduced PM counts of all sizes throughout the ward space (p<0.001 for all sensors), with PM signals positively correlated with indoor CO2 levels (r = 0.343 - 0.817; all p<0.001). Aerosol particle counts tended to rise and fall simultaneously throughout the ward space when the AFU was off, with PM signals from multiple locations highly correlated (e.g. r = 0.343 - 0.868 (all p<0.001) for PM1). ConclusionAerosols freely migrated between the various sub-compartments of the ward, suggesting that social distancing measures alone cannot prevent nosocomial transmission of SARS-CoV-2. The AFU greatly reduced PM levels throughout the ward space. Practical implicationsO_LIAerosols can freely migrate throughout whole wards, suggesting that social distancing measures alone are not enough to prevent SARS-CoV-2 transmission. C_LIO_LIAppropriately sized supplementary room air filtration, if utilised correctly, can greatly reduce aerosol levels throughout ward spaces. C_LIO_LIAir filtration devices are often placed in rooms without any consideration given to their performance. It is therefore important to commission air filtration devices using PM and CO2 sensors before they are utilised in order to demonstrate that they are effective throughout entire ward spaces. C_LI