Ventilation in my lab yesterday. Maintained 22.9 #ACH 2nd hour. First hour 9.88 ACH (air changes per hour). 24 students + 1 instructor in a 450 m³ room. Pre-occupancy CO₂: 538 ppm First hour average peak: 615 ppm Second hour: 585 ppm See tinyurl.com/ACHcalc1 #Aranet4 #SAFEindoorAir #SchoolCO2