There is an interest in impact of national COVID-19 measures and ventilation regimes on potential SARS-CoV-2 exposure in school environments. Since quantitative SARS-CoV-2 exposure information is lacking, we explored associations between total bacterial load (16S), as a marker of microbial exposure, and national COVID-19 measures, ventilation regimes and occupancy.

Longitudinal air monitoring for presence of 16S was conducted through repeated collection of airborne settling dust samples in classrooms of 18 secondary schools between 10-2020 until 06-2021. Total bacterial load was assessed through 16S rRNA qPCR. Information on occupancy and ventilation was collected through questionnaires. A Bayesian linear regression model was used to analyse the data.

In the 335 samples collected, differences in 16S levels between ventilation regimes were small and not statistically significant, possibly due to additional natural ventilation by opening windows in classrooms. A trend for lower 16S levels with mechanical supply and exhaust compared to natural ventilation was observed. Repeated measures in pre- and post-lockdown, with looser versus stricter national COVID-19 measures at schools, showed significantly higher 16S levels pre- compared to post-lockdown. 16S levels were positively associated with higher occupancy, independently from COVID-19 measures. Occupancy was also higher during pre-lockdown when less national school COVID-19 measures were applied.

Total bacterial load was associated with higher classroom occupancy and less COVID-19 measures, suggesting that both variables may improve air quality. Whether associations observed for 16S also exist for SARS-CoV-2 in school air needs to be confirmed in separate studies.