New research has found what many Canadian students and their parents probably already know: that hot classrooms make it hard to learn, resulting in worse performance on exams.

A new working paper by the the National Bureau of Economic Research, a non-profit economic research organization, draws a direct line between overly hot classrooms and poor academic performance.

For the paper, called “Heat and Learning,” researchers analyzed PSAT scores, or preliminary SATs, from 10 million U.S. students over a 13-year period and compared them with temperature reports from the same period.

They found a “significant” correlation between higher temperatures during exam periods and lower test results.

After examining the PSAT scores of the students between 2001 and 2014, the researchers found that once temperatures rose above the more-comfortable 16-20 degree Celsius range and moved into the hotter 30-degree Celsius range, test scores started dropping.

Test scores started to fall as outside temperatures rose above 21 Celsius, and accelerated further once temperatures rose above 32 Celsius.

The authors write that heat appears to affect academic achievement by decreasing the “productivity of instructional time.” In other words, hot classrooms made it harder for teachers to teach, and harder for students to learn.

The issue of overheated classrooms is one that Canadian teachers have been complaining about for several years. Last fall, the Elementary Teachers' Federation of Ontario called on the province to take steps to relieve the heat by installing more school-wide air conditioning systems or setting up cooling stations.

The union said the extreme temperatures are affecting children's ability to learn and leading to disruptive health issues. It called for heat stress plans to be developed for all schools and upper indoor temperature limits to be set that would require schools to shut down for the day if those limits were breached.

The authors of this study found that air conditioning in schools is key, because it “almost entirely offsets” the effects of heat on exam performance, noting that the systems have already been installed in almost all schools in the U.S. South.

“Without air conditioning, each 1 (degree) F increase in school year temperature (0.5 degrees Celsius) reduces the amount learned that year by one per cent,” the authors write.

They add that air conditioning not only improves student tests scores and their ability to gain new skills, it can increase a student’s future potential earnings.

Because installing air conditioning results in significant economic benefits to students, “school infrastructure improvements may more than justify their costs,” the authors write.

In a city like Houston, Tex., where the average school day is approximately 29 degrees Celsius – well past the point where heat begins to affect learning – the authors estimate that the value of having air conditioning is approximately US$2,120 per year per student, or $2.1 million per year for each 1,000-student high school.

The authors say more research is needed in the area of heat and learning, particularly given the predictions that climate change will result in more heat waves in the future.

“Understanding the causal relationship between cumulative heat exposure and learning is of heightened policy relevance in light of accelerating warming in most parts of the world,” they write.

NBER says its working papers are not official NBER publications and have not been peer-reviewed but are circulated for “discussion and comment purposes.”