Physics teacher Alby Reid of Reigate Grammar, a school Reigate, Surrey, had tested levels of carbon dioxide in his classroom during a recent two-hour lesson period and had found that the level of CO₂ was 2,300 parts per million (ppm). A normal amount found outdoors is 400 ppm.

Studies have shown that higher levels of CO₂ can lead to declines in student attendance and academic performance because of fatigue or even cognitive impairment, caused by a low level of oxygen. As well, studies have found that accompanying poor air circulation are high levels of dust, danger, germs, and microbes.

Specifically, a 2016 Harvard University study found that students' cognitive functions decrease when ppm reaches 950, a level that is not uncommon indoors. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers recommends that ppm in enclosed spaces should be no more than 700 above outdoor air; in most cases, that translates to 1,100 ppm indoors.

Reid is fortunate that his classroom has windows that can open. May schools in the U.S. that were built in the 1960s and 1970s have windows that do not open because the school districts wanted to save money on heating and cooling.