Are the places with higher temperatures the most dangerous during heatwaves?

Humidity, duration, and frequency of heatwaves are other determining factors of heat stress.
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Heatwaves are not always most dangerous in places with the highest temperatures. Heat stress is critical when assessing the health risks people face during the hottest months of the year, particularly now with record-breaking temperatures due to climate change.

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The dangers of heatwaves

It is a common mistake to assume that places with the highest temperatures are always the most dangerous during heatwaves. The reality is more complex and lies in the concept of heat stress.

Heatwaves are not solely hazardous because of their temperature; humidity, duration, and frequency also play a fundamental role. These factors increase the risk, mainly due to global warming. Countries at risk of experiencing atypical heatwaves tend to be unprepared, which amplifies their danger.

A clear example occurred in Europe during the summer of 2019. Heatwaves were particularly severe in Spain, which recorded the highest temperatures on the continent. However, the populations of France, Belgium, and the Netherlands suffered even more. In these countries, there were more than 2,500 heat-related deaths.

A study published by ISGlobal attributes this phenomenon to areas with higher heat stress. High temperatures combined with high humidity can be hazardous for the human body. Humidity hinders sweat evaporation, the body’s natural cooling mechanism. As a result, the ability to regulate internal temperature is compromised, increasing the risk of adverse health effects.

Another factor to consider is the duration of the heatwave. Even in places with moderate temperatures, prolonged exposure to sweltering conditions can have serious health consequences. If high temperatures persist for several days or weeks, the human body may experience constant heat stress, increasing the risk of dehydration, heat exhaustion, and heat strokes.

To raise awareness about heat stress, the team at ISGlobal has developed a heat index calculator. By entering the ambient temperature and humidity (which can be obtained from the Spanish State Meteorological Agency), you can determine the risk of heat stress during heatwaves.


What do we mean when we talk about heat stress?

Heat stress is a phenomenon that has gained significant attention in recent years due to its impact on the environment and human health. As global warming accelerates, it becomes an increasingly relevant challenge.

Heat stress occurs when organisms are exposed to extreme temperatures that exceed their tolerance range. In other words, it happens when the human body is in very hot or very cold environments and cannot maintain its average internal temperature.


“Heat stress occurs when organisms are exposed to extreme temperature conditions that exceed their tolerance range.”


Imagine your body as a house with a heating and air conditioning system. When it’s scorching outside, the air conditioning (in this case, your internal cooling system, like sweating) keeps the house cool. But if the heat is too intense, the air conditioning might not be enough, and the house will start to heat up. Something similar happens when we experience heat stress.


Health problems associated with heat stress

The human body is designed to function within specific temperature ranges. When the temperature exceeds those limits, our bodies begin to face problems. As the US Centers for Disease Control and Prevention points out, one of the most common effects of heat stress is dehydration, as extreme heat increases sweating and fluid loss from the body. This can lead to fatigue, weakness, dizziness, fainting, and heat strokes.

Heat stress can also exacerbate existing health problems. People with cardiovascular, respiratory, or metabolic diseases, such as diabetes, are especially susceptible to the harmful effects of heat stress.

Moreover, heat stress also affects our driving. Experts claim that driving in heat is as dangerous as driving with a blood alcohol level of 0.29 grams per liter. High temperatures inside the vehicle increase driving errors or reaction time to unexpected events by 20%.

The impact of heat stress is not limited to humans; it affects the planet’s biodiversity as a whole. Heat stress can devastate entire ecosystems, as witnessed during the 2021 heatwave along Canada’s Pacific coast, which experts believe resulted in the death of over a billion marine animals.


Causes of heat stress

Heat stress results from a combination of factors that disrupt the natural climate balance and expose humans to extreme temperatures. Climate change is one of the main drivers of this phenomenon, which generates global warming and raises average temperatures worldwide.

Currently, the temperature increase has already surpassed 1°C. The last eight years have been the warmest for the planet since global records began, as the World Meteorological Organization (WMO) reported at the beginning of 2023.

The WMO also warns that global temperatures will likely reach unprecedented levels in the next five years, driven by greenhouse gases that trap heat and the natural phenomenon of El Niño. They predict a high probability of surpassing the 1.5°C threshold (compared to pre-industrial levels) for the first time between 2023 and 2027.


“There is a high probability of surpassing the 1.5°C threshold for the first time between 2023 and 2027.”


Urban growth and the expansion of urban areas also play a crucial role in creating hot microclimates known as “heat islands.” Extensive use of heat-absorbing building materials like asphalt and the lack of green spaces and vegetation contributes to local temperature increases. Consequently, cities and their inhabitants are more exposed to heat stress than rural areas.