2021 Yes https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8003457/ Villanueva, Florentina Europe Observational Epidemiology Study Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Carbon Dioxide (CO2) Particulate Matter (PM) COVID-19 pandemic Particulate matter (PM) SARS-COV-2 transmission risk mitigation School environment Ventilation conditions Briefly, findings suggested that although ventilation promoted through opening windows and doors according to official recommendations is guaranteeing adequate ventilation conditions in most of the studied classrooms, thus minimizing the risk of SARS-CoV-2 airborne transmission, a total of 5 (26%) surveyed classrooms were found to exceed the recommended CO2 concentration limit value (700 ppm). In general, preschool rooms were the educational environments that registered better ventilation conditions, while secondary classrooms exhibited the highest peak and average CO2 concentrations. In turn, for PM2.5, PM10 and UFP, the concentrations assessed in preschools were, on average about 2-fold greater than the levels obtained in both primary and secondary classrooms. In fact, the indoor PM2.5 and PM10 concentrations substantially exceeded the recommended limits of
8hr-exposure, established by WHO, in 63% and 32% of the surveyed classrooms, respectively.