Heat stress, when summer doesn’t invite you to enjoy the outdoors

August 4, 2021

Table of contents


  • High temperatures, in addition to exacerbating poor air quality, are a health problem.
  • A key factor in determining risk is ambient humidity, a variable that is particularly useful to monitor in sporting events such as marathons.
  • Kunak monitors air quality and the so-called wet bulb temperature through its cost-effective stations and attached probes.


Poor air quality is not the only threat to the planet.

In addition to floods, tornadoes and fires, in this year’s summer, for example, heat stress is clearly on its way to becoming a concerning health issue. At least that is what shows in the increasing number of people treated in hospitals during episodes of extreme temperatures.

Although it may seem a minor problem that can be fixed by means of hydration and staying in the shade, nothing could be further from the truth, according to the more than 170,000 victims of heat stress only in Europe.


The human body’s reaction to extreme temperatures

Compared to other organisms, human survival is subject to strict environmental constraints. That is to say, our species is very fragile in the face of climatic ups and downs.

However, answering the question of what is the maximum temperature a person can withstand is not so simple. The response depends on multiple factors such as air quality, physical condition, clothing and even race. One of the most important variables is, undoubtedly, environmental humidity.

The body temperature of a healthy human being is around 37º C. When thermometers rise, perspiration maintains this variable stable. Yet, the cooling mechanism only works when sweat evaporates. This physical phenomenon only occurs if environmental humidity is low.

According to examples taken from an interesting infographic published on Live Science, the environmental conditions under which one may suffer from hyperthermia after being outdoors for an hour are the following:

  • About 45-46º C with around 50 % relative humidity.
  • About 35º C with 100 % humidity.

Although it may seem otherwise, these values are not so difficult to reach and are no longer exclusive to tropical areas. In Canada’s last heatwave, temperatures reached 32º C in cities like Vancouver with approximately 70 % humidity (similar to cities like Barcelona), resulting in an apparent temperature of about 40º C. Climate change? Yes, it could be a factor.


What are the consequences of prolonged exposure to these environmental conditions?

Floods and hurricanes are on the front pages and television news but high temperature and humidity combined don’t usually get the same attention beyond newsworthy heatwaves. Perhaps, this occurs because, as Georgia Tech researcher Tim Andersen says, ‘we assume, incorrectly it turns out, that human beings can adapt to moderate increases in global temperature’.

This assumption, however, becomes ineffective as soon as the consequences of high temperatures are observed. Undoubtedly, heatstrokes are one of the most serious outcomes.

Yet, heat stress can also cause other mild symptoms such as fatigue, headaches or dizziness. The people that are most at risk, in addition to the most vulnerable groups (children, the elderly population, etc.), are, for example, those who work outdoors or exercise in high temperatures.

There’s a pop-up hospital beside the marathon finish line. Every five minutes or so, a new athlete arrives on a stretcher. pic.twitter.com/99sljMj3WO

— Cathal Dennehy (@Cathal_Dennehy) September 27, 2019


Kunak’s monitoring work

How can this situation be handled? Measuring what is known as ‘wet-bulb temperature’. This variable measures thermal stress by taking into account the temperature, humidity, wind speed, sun angle and cloud cover (solar radiation).

For example, Kunak monitors this parameter by means of a probe attached to air quality stations.

Probe for wet bulb temperature monitoring Probe (left) placed next to a Kunak Air Pro air quality station.
Bicycle equipped with Kunak Air Mobile air monitoring station and wet bulb temperature probe.


Controlling the risk of heat stress is especially useful in sporting competitions. Therefore, Kunak collaborates with World Athletics on air quality monitoring to detect the concentration of pollutants and suspended particles. Of course, temperature and humidity are also monitored during competitions or training sessions. After all, athletics is a combination of extreme physical exertion and continuous exposure to atmospheric elements.

One example is marathons. A year ago, we had the opportunity to analyse environmental conditions in Sapporo, Japan. Barring cancellation due to the pandemic, this city will host the race walks and marathon events of the Tokyo 2020 Olympic Games.

This work was carried out by means of several electric bicycles equipped with Kunak mobile stations and probes for measuring wet-bulb temperature. The purpose was to identify possible pollution hot spots and risk areas on the testing route. After analysing the data, the conclusion was that environmental variables should not cause a handicap for athletes if recorded conditions are maintained.


Why is it convenient to implement parameter monitoring beyond the sports field?

Undoubtedly, due to athletes’ performance and their health (and that of the public that usually attends the events), elite sports require that these heat stress-related constraints be monitored. However, such monitoring should also be considered in urban areas. The consequences for the health of citizens are not the only reason.

Extreme temperatures also worsen poor air quality. In fact, the human body increases its susceptibility to the presence of several simultaneous stress factors. For example, Jackson et al (2010) analysed mortality rates recorded in the Seattle metropolitan area between 1980 and 2006 as well as extreme temperature episodes. In conclusion, there was a clear correlation between mortality and heat, especially among the population over 65 years old. The results were similar to a study conducted in Moscow that included pollution caused by wildfires that swept across large areas of Russia in 2010 (2).



The human body is a wonderful machine. If Mercury on the thermometer drops, one shiver to generate heat. If the ambient temperature rises, one sweat to keep the body within healthy limits. However, everything depends on environmental constraints and their balance.

Sensor technology helps to make these external factors “visible” to trigger alerts or take corrective measures if necessary.

After all, on an increasingly warmer planet, adaptation and resilience must become our main assets.


Sources consulted: