Heat waves: what the human body can endure and its limits

With record temperatures recorded in Europe at the end of June and heatwave alerts hitting a large part of the United States, doctors are warning about the human body's ability to adapt to heat that is intensifying at an unprecedented rate.

At the end of June, France, Spain, the United Kingdom, Belgium, the Netherlands, Switzerland, and Germany recorded record temperatures. The German national weather service, the DWD, indicated that the country had never experienced such intense heat so early in the year for such a prolonged period. In the United States, large areas of the central and eastern parts of the country were under a heat wave warning as the July 4th weekend approached, with temperatures expected to reach between 37,8 and 46,1 degrees Celsius.

A heatwave with daytime temperatures well above 30°C and nighttime lows rarely dropping below 20°C puts considerable strain on the human body. Young children, pregnant women, the elderly, those with chronic illnesses, and outdoor workers are particularly vulnerable.

“The human body can adapt, and these adaptations are more developed in people who are continuously exposed to heat,” explains Claudia Traidl-Hoffmann, a physician and professor at the University of Augsburg, where she heads the Institute of Environmental Medicine. She is also the director of the Helmholtz Institute of Environmental Medicine in Munich and advises policymakers on the German government’s scientific advisory board.

This adaptation to changing environmental conditions takes time, she explains in her book "Die Medizin der Zukunft — Heilen in einer veränderten Welt". Not a matter of years, but of centuries.

Physiologically, thermoregulation mechanisms activate as soon as the outside temperature reaches 23°C: blood vessels dilate to dissipate heat, and perspiration provides additional cooling. But if these mechanisms fail, the consequences can range from cardiovascular disease and stroke to multiple organ failure.

"When body temperature rises too high, metabolic processes first accelerate until the body loses control at all levels, from the immune system to the nervous system. At around 42°C, the body suffers serious cellular damage, multi-organ failure and, without immediate treatment, death can occur," warns Traidl-Hoffmann.

The lungs are also affected. Inhaling hot air appears to accelerate inflammatory processes, although the precise molecular mechanisms remain partially unknown. "The lungs become more easily inflamed and more vulnerable to infections," the doctor points out.

To prepare, Traidl-Hoffmann advises her patients to start planning as early as January: discuss any necessary medication adjustments with their doctor, treat allergies that may worsen in the heat with specific immunotherapy, and stabilize any chronic conditions before the start of the warm season. Once temperatures rise, she recommends drinking plenty of fluids, favoring a light, plant-based diet, avoiding tobacco and alcohol as much as possible, and getting enough sleep to allow the body to recover from some of the heat stress accumulated during the day.

Adaptation capacity varies greatly from person to person. Traidl-Hoffmann compares this capacity to a barrel: in the elderly, those with chronic illnesses, or those on medication, this barrel fills up much more quickly during a heatwave. Young adults in good physical condition and accustomed to heat have a greater tolerance, but this tolerance has its limits.

"This exponential increase in the number of hot days, this rapid pace of change, exceeds what ecosystems and human beings can adapt to," she concludes.