2021 Yes https://www.sciencedirect.com/science/article/pii/S1438463921000614 Vassella Europe Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Ventilation rates Carbon Dioxide (CO2) Natural window ventilation Carbon dioxide (CO2) Indoor air quality School Intervention Awareness-raising In more than two-thirds of those (schools that had natural ventilation), the hygienic limit value of 2000 ppm specified for CO2 in the Swiss Standard SN was exceeded. CO2 was significantly reduced in pilot classrooms (Wilcoxon sign-rank tests, p=3.815e-06). Median CO2 levels decreased from 1600 ppm (control group) to 1097 ppm (intervention group), and the average proportion of teaching time at 400 - 1400 ppm CO2 increased from 40% to 70%.