Nearly 85 percent of classrooms across the country don't have their own carbon dioxide monitor.
RNZ's 'Whose Breath Are You Breathing?' project revealed that on a cold day, when classroom windows were shut, carbon dioxide levels exceeded the level at which the Ministry of Education says students should briefly leave the class to reduce the risk of Covid-19 transmission.
Public health professionals want carbon dioxide meters in every classroom, so teachers know when to act.
Farah Hancock has the story.