Too Hot to Learn?

Over the past few weeks, Europe has experienced a rare period of extreme heat. These high temperatures have already caused elevated mortality rates. The risk of heat-related illness is not only for those outdoors, exposed to sunlight and heat, but under certain conditions, heat-related illness (and even death) also applies to those indoors, where the temperatures can rise to dangerous levels.

As part of a long-running study on the quality of indoor environment quality in schools, we are continuously monitoring 270 classrooms through a sensor network. The data is also useful to study the risk of heat-stress over the past few weeks. The graph below shows how the average daily peak indoor temperature has changed over the last three weeks. The box and whisker show the average peak temperature across 270 classrooms, every day of the week, with the distribution of peak temperatures across classrooms We compare this result side by side with the peak outdoor temperature over the same period (red line).

During the hottest week, temperatures inside classrooms rose to an average of almost 30 °C, with some classrooms at temperatures between 35 and 40 °C. There were very few days when the indoor peak temperature was below 25 °C.


To assess the risk of heat-related illness in an indoor environment, we must first assess the risk threshold for indoor temperatures. Many studies have attempted to determine what indoor temperature level poses a significant risk of heat-related illness. This threshold ranges from 26 to 32 °C, depending on humidity and air pressure conditions.

We use 28°C as the risk threshold for maximum indoor temperature and 26°C as the risk threshold for prolonged exposure. In the graphs below, we found several classrooms that exceeded the risk threshold each day, even on relatively low-temperature days (July 11th, 15th, 22nd, etc.)


Although this is just an intuitive comparison, it adds to the prospect of an impending climate crisis. Extreme heat brought on by climate change can mean more than temporary discomfort; it can cause real harm. Over the next few years, we will have to invest heavily in improving our buildings' HVAC systems to adapt to the increasingly extreme weather. According to to a study of school buildings across the United States, it is estimated that $4 billion of cooling costs will be needed by 2025 to ensure a safe indoor environment. The Netherlands will face a similar challenge, with the difference that Europe will have to face higher energy prices and a fragile supply system before new energy technologies can be used on a large scale.


Durán, N., Eichholtz, P., Kok, N. and Palacios, J., 2021. Indoor Air Quality and Student Performance: Evidence from A Large Scale Field Study in Primary Schools.

Tham, S., et al. "Indoor temperature and health: a global systematic review." Public Health 179 (2020): 9-17.

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