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With climate change, air temperatures are rising, the number of hot days and heat waves is increasing, and frost days and cold waves are decreasing. When do heat waves become a danger to humans and when is hot even too hot?

Climate change is affecting our environment in many ways – heat is one change that we have particularly felt this year. Heat waves are getting stronger, lasting longer, becoming more frequent and hotter: the past few years have been the warmest since weather records began. Climate models predict that the rise in mean annual air temperature will lead to even hotter summers in the future. In Germany, for example, there are already reliable indications that the maximum air temperature will shift toward extreme heat. In southern Germany, up to 30 hot spells per year are expected by the end of this century (Sustainability Times; Umweltbundesamt; KLUG).  

Heat stress and high ground-level ozone concentrations during heat waves also increase illness and mortality risk: according to the Lancet Countdown (2018), 30,000 additional heat-related deaths are already expected in the EU by 2030 (KLUG). 

aware_ has taken on the topic, looked at how hot is too hot and what can be done about it. 

heat waves

The human body 

To understand why heat is such a big problem, let’s take a look at what happens in the human body when it is exposed to extreme heat. Our organism strives to keep its temperature constant at around 37 degrees Celsius. This temperature may only fluctuate slightly. If the outside temperature rises, the body adjusts accordingly, the blood vessels dilate, and the skin begins to sweat. However, in the case of long and intense heat, this natural temperature regulation no longer functions properly. Blood pressure drops, blood circulation deteriorates, and the body loses fluids and salts. This puts particular strain on the circulatory system. The heart has to work harder to pump blood to the skin to dissipate the heat (t-online; ÄZQ; Sustainability Times). 

Temperature and humidity play a special role here: at very high humidity the sweat on the skin can no longer evaporate. The so-called wet-bulb temperature describes the lowest temperature that can be reached with evaporation – in an environment with a certain temperature and humidity. This value is central to our thermal regulation. Below this limit, the body is able to maintain a relatively stable core temperature over an extended period of time. If the wet-bulb temperature is too high, we can no longer release heat to the environment. And if it continues to rise, the risk of heat-related illness increases, and the body can become fatally overheated (NZZ; Sustainability Times). 

But how hot is too hot? 

Among other studies, a recent study on young healthy people by the Noll Laboratory at Penn State University shows that the upper environmental limit is even lower than the previously assumed 35 degrees. It is more like a wet-bulb temperature of 31 degrees. That would be equivalent to 31 degrees at 100 percent humidity or 38 degrees at 60 percent humidity. Current heat waves around the globe are approaching, if not exceeding, these limits (Sustainability Times). 

The German Weather Service defines a “hot day” as a day whose highest temperature is above 30 degrees, and a “tropical night” as a night whose lowest temperature does not fall below 20 degrees (Umweltbundesamt). 

In addition to the temperature, the length of a heat wave is also significant. If the value is above 32 degrees, physical activity can become dangerous. At 35 degrees or higher, there is an acute danger to life. This is because at this point the human cooling mechanism fails completely. Experts estimate that even a healthy person resting in the shade can survive a wet-bulb temperature of 35 degrees for only about six hours (NZZ). 

In May of this year, temperatures of over 34 degrees were recorded in the Indian state of Kerala. This is a record in this area. The highest values are measured in the Gulf region. There, this value approached 35 degrees in 2015. For reference, a wet-bulb temperature of 35 degrees is reached when the air temperature is 45 degrees, and the relative humidity is 50 percent (NZZ).

heat waves

What happens when it is too hot? 

These temperatures harbor a high potential for damage to humans and the environment. As previously mentioned, the body’s own cooling system can be overloaded in hot conditions. The result is regulatory disorders and circulatory problems with typical symptoms such as headaches, exhaustion and drowsiness. Elderly people and people with chronic pre-existing conditions are particularly affected by these symptoms. Across Western Europe, an estimated 70,000 more people died between June and August in 2003, one of the hottest summers in recent years, than would have occurred in a summer without a heat wave. Italy and France were particularly hard hit, with almost 20,000 heat deaths each (NZZ; Umweltbundesamt). 

In addition, high air temperatures combined with intense solar radiation promote the formation of harmful ground-level ozone. Together with particulate matter and nitrogen oxides, the mixture irritates the mucous membranes of the lungs or eyes and acts as an accelerator for lung disease (ZEIT). 

Extreme heat is also considered a general stress factor that is detrimental not only to physical but also to mental health. This phenomenon is probably based on a mixture of biological factors and psychological influences: everyday life becomes more ponderous; we get less done and are more easily distracted. If we also experience sleep disturbances at night because the environment hardly cools down, the brain lacks important rest phases that are crucial for memory formation. From a biological point of view, there are several theories, for example that heat could lead to the activation of tiny inflammation foci in the brain, which could disrupt or even damage the communication between the nerve cells. But it could also be because heat upsets the metabolism of neurotransmitters such as dopamine or serotonin, whose balance is enormously important for our feeling, thinking and even the body’s own thermostat. In addition, there is the fear of the future. And rightly so: Humanity is in danger. All the more important to take active countermeasures now (ZEIT; NetDoktor). 

What can we do? 

The most important thing, of course, is to tackle the causes by reducing emissions as quickly and effectively as possible. Climate-friendly alternatives to energy-intensive air conditioning systems are used in southern Europe, for example: There, buildings have thick walls and are often overgrown or surrounded by plants that serve as natural air conditioners and cool their immediate surroundings by several degrees (GEO).  

We ourselves can take preventive measures to protect ourselves from extreme heat. These include staying hydrated, avoiding alcohol and caffeine, eating a light diet rich in minerals, taking lukewarm showers, foot baths or damp cloths, wearing airy clothing, and staying in cool, shady places and avoiding direct sunlight (ÄZQ). 

In Germany, the federal government has developed a precautionary climate adaptation strategy with measurable objectives, following on from the 2015 Paris Agreement, which sets targets for adaptation to the consequences of climate change in addition to climate protection measures. Official heat warnings from local weather services can warn the public in advance of prolonged periods of heat (BMBF; Stadt Erkrath).

– by Marie Klimczak