Summary
In two Swiss classes over 7 weeks, HEPA air cleaners cut particle concentrations by 77%. Respiratory-infection absences were 22 without and 13 with air cleaners, but the relative risk estimate was wide (0.73, 95% credible interval 0.44 to 1.18).
Key points
- With HEPA air cleaners running, mean particle concentration in the classrooms fell by 77%; CO2 levels did not change.
- Absences from respiratory infections were 13 with air cleaners and 22 without; the model gave a relative risk of 0.73 (95% credible interval 0.44 to 1.18), a 91% probability of lower risk.
- Coughing also tended to be less frequent with air cleaners.
- Viruses, mostly not SARS-CoV-2, were found in 50 of 448 saliva samples but rarely in air samples (2 of 105) or on the filters (4 of 160).
- Design: crossover study over 7 weeks in 2 Swiss secondary school classes, 38 students aged 13 to 15.
- Funding: NIAID, Swiss National Science Foundation and University of Bern; no conflicts reported.