Can ventilation combat airborne infection risks in schools?

The COVID-19 pandemic caught policy makers, government agencies, school management and facility managers largely off-guard, prompting a review of existing ventilation standards in relation to the mitigating of airborne viral diseases. In light of new guidance in ASHRAE standard 241-2023 ‘Control of Infectious Aerosols’ this paper examines the relative airborne viral prophylaxis benefit of seven different ventilation scenarios, involving natural, mechanical and hybrid ventilation systems. The research aims to assess the relative merits of each approach considering current guidance in European Norm (EN 16798-1) and International Standard (ISO 17772-1) in contrast to the increased minimum equivalent clean airflow rates for classrooms in ASHRAE 241.

The results of this analysis show that increased minimum equivalent clean airflow rates have a marked effect on reducing the airborne viral transmission risk. However, even with the best performing ventilation system the risk of at least one individual becoming infected with the SARS-CoV-2 Omicron variant (after an 8 h long exposure period) was 30% (assuming that one infectious individual was present in a classroom of 20 unmasked, immunologically naïve students). By using a combined strategy involving universal FFP2 masking and ventilation the group infection risk level was dramatically reduced to 2–7% (depending on the ventilation type and flowrate) highlighting the importance of layered prophylaxis strategies at times of elevated community transmission.