Publish at June 25 2025 Updated June 25 2025
Any activity generates heat, even intellectual activity, brains and computers alike. Human organisms function best in a narrow range around 37° C. We can survive hypothermia at 35° or hyperthermia at 40° without too much damage, but at 41° and above, some of the body's essential proteins begin to degrade and impair cell function. Beyond this threshold, the damage from severe heatstroke (42° and above) can be irreversible. But long before we reach these temperatures, heat affects our capacities, both physical and mental. Torpor and indolence are often associated with heat.
Teachers and schools have long learned to adapt their activities when it's too hot on their premises, but now it seems that traditional responses are insufficient. Are they really?
The principal means of cooling our bodies and biological systems in general is perspiration: when water evaporates, it absorbs energy. From this simple principle flow two rules: have water and be able to evaporate. While the first is relatively easy to respect - all an organism has to do is drink - the second is less obvious, as air's capacity to absorb moisture is limited: the more humid the air, the less water it can absorb.
In a closed or poorly ventilated room, the atmosphere quickly becomes saturated with humidity, and the impression of heat increases in proportion as evaporation becomes less and less effective. Airy schools are appreciated and can stay cooler, but when a wave of humidity accompanies the heat, even aeration and fans make little difference. That leaves air conditioning, if you can afford it, or shifting activity hours. In many hot countries, school is in the morning and late afternoon or evening.
The heat in the air is transmitted mainly by convection; a warm wind warms us up. Above a certain threshold, the warming effect of convection is greater than the cooling effect of evaporation, so ventilation becomes ineffective and can even amplify the warming.
What is the threshold? This threshold varies according to both air temperature and humidity. For example, at 35°C, if humidity reaches 80%, sweat evaporation is unable to compensate for convective heating. At temperatures above 32°C on a wet bulb thermometer (Humidex Index), humans can no longer cool themselves effectively by evaporation. Other approaches to cooling must therefore be considered.
Even under normal conditions, overly-tight clothing prevents evaporation, which is why workers wearing protective suits for construction or spraying jobs are often prone to heatstroke. Not only that, but some uniforms and suits are almost as uncomfortable. Light, breathable clothing is the normal choice when temperatures rise. School dress regulations normally take this into account.
In very hot, very dry countries, it was customary to provide personal water tanks for each inhabitant of a residence; they could immerse themselves in these tanks as needed when hot, dry winds made the atmosphere unbearable due to intense radiation and convection. Such removable water containers are offered on the market as a "new solution" to heatwaves. More practically speaking, in very hot areas such as a schoolyard, misting areas are effective, easy to install, popular and inexpensive.
Radiation is the other major means of heat transmission, whether directly from the sun or indirectly through re-radiation. For example, a brick wall heated by the sun continues to radiate heat long after the sun has set. Dark clothing absorbs the radiation, while light clothing reflects it. Traditionally, in hot desert countries, clothing is designed to reflect radiation, limit convection and facilitate ventilation. In short, paradoxically, when it's very hot, it's better to cover up and reflect if you're going outdoors!
White roofs, window shutters or awnings, the reduction of mineralized surfaces through the creation of vegetated areas and even more technological reflective solutions can passively and inexpensively reduce the surrounding temperature by several degrees, not to mention the positive psychological effect of such arrangements on students.
The principle behind air conditioners and other cooling devices is quite simple: when a gas is compressed, the energy it contains is concentrated in a smaller space, and this space heats up. If you expand a compressed gas, it cools down relative to its compressed temperature. All you have to do is let a compressed gas cool down to obtain an even cooler gas when you expand it. This phenomenon of compressed matter heating up occurs at just about every level of matter, from icy space to the heart of the sun.
The productivist response to the drop in output during the hottest hours has been to air-condition the working environment. The solution is correct from an economic point of view, but terrible in terms of its global effects:
In short, we need to review our priorities and the way we operate.
The hottest hours of the day are between 13:00 and 16:00. Why should we work so hard at these times? Traditionally, a siesta is an economical and effective way of avoiding the worst effects of the heat.
Establishing the siesta as a social and environmental norm is an inexpensive and, above all, politically significant solution. Promoting airy architecture, creating shaded areas, evaporation areas (brumisation), green roofs or roofs that reflect the transparent frequencies of CO2, encouraging humans' natural ability to cool themselves without air conditioning for the most part - all these approaches can be adopted by schools, teachers and their pupils.
Providing space, reducing pressure and gaining altitude all contribute both physically and intellectually to cooling the atmosphere to acceptable temperatures.
Illustration: Chu Viết Đôn - Pixabay
References
Air conditioning accounts for 7% of global electricity consumption - Le Grand Continent - Groupe d'études géopolitiques
https://legrandcontinent.eu/fr/2025/06/21/la-climatisation-represente-7-de-la-consommation-mondiale-delectricite/
Cooling a Warming Planet: A Global Air Conditioning Surge - Yale Environment 360
https://e360.yale.edu/features/cooling_a_warming_planet_a_global_air_conditioning_surge
IEA - Agence internationale de l'énergie - International Energy Agency
https://www.iea.org/data-and-statistics
Climate cooling - A surprising passive technology to the rescue - Denys Lamontagne - Thot Cursus
https://cursus.edu/fr/11910/refroidissement-climatique-une-etonnante-technologie-passive-a-la-rescousse
Manuel Merrck - Heat stroke
https://www.merckmanuals.com/fr-ca/accueil/les-faits-en-bref-l%C3%A9sions-et-intoxications/troubles-provoqu%C3%A9s-par-la-chaleur/coup-de-chaleur
Evaporation - WikHydro - http://wikhydro.developpement-durable.gouv.fr/index.php/Evaporation_(HU)
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