2023 Yes https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1004226 Banholzer, Nicolas Europe Observational Students Middle/Jr High (6-8) Carbon Dioxide (CO2) Particulate Matter (PM) Absenteeism Molecular analysis of saliva (21/262 positive) and airborne samples (10/130) detected SARS-CoV-2 throughout the study (weekly average viral concentration 0.6 copies/L) and occasionally other respiratory viruses. Overall daily average CO2 levels were 1,064 +/- 232 ppm (+/- standard deviation). Daily average aerosol number concentrations without interventions were 177 +/- 109 1/cm3 and decreased by 69% (95% CrI 42% to 86%) with mask mandates and 39% (95% CrI 4% to 69%) with air cleaners. Compared to no intervention, the transmission risk was lower with mask mandates (adjusted odds ratio 0.19, 95% CrI 0.09 to 0.38) and comparable with air cleaners (1.00, 95% CrI 0.15 to 6.51).