Heat & Health

HOW DOES THE BODY COOL ITSELF?

Humans and other warm-blooded animals, such as dogs, cats and cows, depend on a constant body temperature of around 36–39 °C. During physical exertion or high temperatures, the human body begins to sweat. The evaporation of sweat removes heat from the skin and provides cooling (evaporative cooling), which keeps the body temperature stable. Without sufficient cooling, overheating can occur, which is dangerous for humans and for animals, who sweat even less efficiently.

OVERHEATING AND CLIMATE CHANGE

It is not just extreme heat that poses the threat of overheating – a crucial factor is the combination of air temperature and humidity. This determines the Wet Bulb Temperature (WBT). The WBT is the lowest temperature that the surface of our skin can reach by sweating in the given air humidity and temperature. If the air is nearly saturated with moisture, sweat hardly evaporates and there is no cooling. We are familiar with this from steam baths and thermal baths.

As a result of climate change, it is not only generally getting warmer; it is also getting more humid, as more water is evaporating and the air can absorb more water vapour at higher temperatures. Critical conditions are already occurring in hot, humid regions such as the Arabian Peninsula or India. If these extreme climatic conditions become the norm, some regions could become uninhabitable in the long term.

Heat affects us more strongly when there is no wind and we are exposed to direct sunlight. The Wet Bulb Globe Temperature (WBGT) is a measure that also takes into account these environmental conditions. On very hot days it is used to determine appropriate cooling breaks for outdoor activities such as playing football or working on construction sites.

LIMITS OF SURVIVAL

The WBT theoretically indicates the lowest temperature down to which we are able to cool ourselves. The limit lies at a WBT of 35 °C (e.g., 48 °C air temperature and 40 % humidity), where, at the very latest, heat becomes life-threatening for everyone. The reason for this is that the body can only maintain a stable temperature as long as the skin is cooler than 35 °C. At a WBT of 35 °C or higher, the air is saturated with humidity. This means the body can no longer release heat by sweating, so it overheats. Without external cooling (e.g., air conditioning), this will lead to organ failure within a few hours.

In practice, however, the limit for overheating is lower and also depends on age, health, activity and clothing. While young, healthy people usually have no problems at 22 °C WBT (e.g., 22 °C air temperature and 100 % relative humidity), others may develop heat stress symptoms such as headaches and fatigue, and elderly people may even be in danger of death.