A joint GMB, NASUWT, NEU, UNISON and Unite education union guide for union reps and health and safety reps, Good ventilation in schools and how to achieve it (April 2023), advises reps: “To check the situation in your workplace, ask your school or college to use carbon dioxide monitors to check whether ventilation is sufficient to keep CO2 levels below 800 parts per million (ppm). Indoor air at 600-800ppm carbon dioxide indicates a relatively well ventilated room.”