Indoor aerosol transmission of SARS-CoV-2 has been widely recognized, especially in schools where children remain in close proximity and largely unvaccinated. Measures such as strategic natural ventilation and high-efficiency particulate air (HEPA) filtration remain poorly implemented and mask mandates are often progressively lifted as vaccination rollout is enhanced.
We adapted a previously developed aerosol transmission model to study the effect of interventions (natural ventilation, face masks, HEPA filtration, and their combinations) on the concentration of virus particles in a classroom of 160 m3 containing one infectious individual. The cumulative dose of viruses absorbed by exposed occupants was calculated.
The most effective single intervention was natural ventilation through the full opening of six windows all day during the winter (14-fold decrease in cumulative dose), The universal use of surgical face masks (8-fold decrease).
In the spring/summer, natural ventilation was only effective (≥ 2-fold decrease) when windows were fully open all day. In the winter, partly opening two windows all day or fully opening six windows at the end of each class was effective as well (≥ 2-fold decrease). Opening windows during yard and lunch breaks only had a minimal effect (≤ 1.2-fold decrease).
One HEPA filter was as effective as two windows partly open all day during the winter (2.5-fold decrease) while two filters were more effective (4-fold decrease). Combined interventions (i.e., natural ventilation, masks, and HEPA filtration) were the most effective (≥ 30-fold decrease). Combined interventions remained highly effective in the presence of a super-spreader.
Natural ventilation, face masks, and HEPA filtration are effective interventions to reduce SARS-CoV-2 aerosol transmission. These measures should be combined and complemented by additional interventions (e.g., physical distancing, hygiene, testing, contact tracing, and vaccination) to maximize benefit.
Air purifiers provide a significant benefit, along with proper ventilation, to improve air quality and eliminate polluting factors. Today, air-cleaning devices developed with state-of-the-art filtration systems filter viruses, bacteria and microorganisms with HEPA filters and then purify the indoor air with air purification technologies such as UV and Ioniser. Even in devices with the best filtration system, HEPA filters need to be changed every 1-3 months.
These devices filter or neutralize pollutants such as viruses, bacteria, pollen, particles, etc. in the air. They do not affect increasing the amount of oxygen in the air. If possible, the CO2 values in the classrooms should be measured continuously and fresh air intake should be provided to the classrooms.