Background Aerosol spread of SARS-CoV-2 is a major problem in hospitals, leading to an increase in supplementary HEPA filtration aimed at reducing nosocomial transmission. We report a natural experiment that occurred when an air-cleaning unit (ACU) on a medicine for older people ward was accidentally switched off while being commissioned. Aim To assess aerosol transport within the ward and determine whether the ACU reduced airborne particulate matter (PM) levels. Methods An ACU was placed in a ward comprising two 6-bedded bays plus three single-bed isolation rooms, which had previously experienced several COVID-19 outbreaks. During commissioning, real-time measurements of key indoor air quality parameters (PM1-10, CO2, temperature and humidity) were collected from multiple sensors over two days. During this period, the ACU was accidentally switched off for approximately 7 hours, allowing the impact of the intervention on PM to be assessed. Findings The ACU greatly reduced PM counts (e.g. PM1: 65.5 – 78.2%) throughout the ward (p<0.001 all sizes), with all PM fractions and CO2 positively correlated (r = 0.343 – 0.817; all p<0.001). PM counts rose/fell simultaneously when ACU was off, with PM signals from multiple locations correlated (e.g., r = 0.343 – 0.868 (all p<0.001) for particulates <1m). Conclusion Aerosols rapidly migrated between the various ward sub-compartments, suggesting that social distancing alone cannot prevent nosocomial transmission of SARS-CoV-2, as it fails to mitigate longer-range (>2m) transmission. The ACU greatly reduced PM levels throughout the ward space indicating its potential as an effective intervention to reduce the risk posed by infectious airborne particles.