Heat Illness During Cycling – Who’s at Risk?


Lousy weather at Le Tour aside, summertime is the time for big rides on long and hot days. What are the risk factors for developing heat exhaustion or, even worse, heat stroke in cyclists?


My scientific career has revolved around understanding the impact of environmental stress – and especially thermal stress – on human physiology and performance. So, it was truly ironic that I got a solid dose of heat exhaustion during a ride in late June.

During the depths of Ontario heat and humidity (30+°C air temperature and >40 with the humidex), my wife and I went strawberry picking early morning, then I proceeded to ride home over a 50 km loop. I felt good until about 30 km in, then it felt like my legs were fading and my body was a fire hydrant from the sweating.

The final 10 km were truly ugly and even worse, with my constantly pouring water over my head to cool off and stopping several times in the sparse shade. I was less than 1 km from home and considered calling home for a bailout.

Recovering with a cold shower, I drank about 4 L of different fluids the rest of the day and didn’t really urinate until the next morning.

NB. What I experienced was heat exhaustion, the milder form of heat illness. Heat stroke is the next progression, and is far more dangerous. One primary common differentiator is that sweating often fails with heat stroke due to the thermoregulatory system being overwhelmed. The other common differentiator is actual neurological issues with gait, due to the central nervous system getting overwhelmed by high body temperatures.

So if a thermophysiologist with >30 years in the field can get heat exhaustion, then it’s probably safe to say that we all can. This leads to the question, are there specific risk factors that increase the odds?

Check out the video below on data taken from many years of the large Cape Town Cycle Tour.


Sewry N, M Schwellnus, J Killops, S Swanevelder, DC Janse van Rensburg, E Jordaan. Med Sci Sports Exerc. 53(3):517-523, 2021.



Large mass-participation sporting events can be a tool to study both the actual risks from exercise, and also the individual factors that lead to greater risk from exercise. In today’s episode, we’ll take a look at who ended up in the medical tent over three years of the Cape Town Cycle Tour, a 109 km bike tour with >30,000 riders each year in late summer.

Unlike elite competitions like the Tour de France, which are only open to some of the most fit athletes on the planet, the Cape Town Cycle Tour is ideally suited to looking at the overall athlete population. There is no fitness cutoff or qualification to enter, apart from being 13 years or older. Being so large an event, riders set off in waves over 4 hours. So for each individual rider, their actual start and finish times and also the temperature at 5 weather stations along the course were used to calculate their individual heat load.

For this study, crashes were set aside, and medical cases were only recorded for moderate or worse events, so it didn’t include minor cases like headaches. We see that, overall, cases were pretty low at about 2 cases per 1,000 riders. This graph shows the temperature on the horizontal axis in °C, and the rate of medical cases per 1,000 riders on the vertical axis. This particular graph models the impact of temperature on the average rider for the Tour, who was 41 years old and rode at just under 24 km/h. We see that the risk for a medical event doubled as temperature rose by 8°C. Along with temperature, another major risk factor was slower riding speed, which can also be interpreted as less fit riders. Why might this be? It could be the case that, if you’re slower, you’re exercising for longer and spending more time in the heat and putting more overall stress on your body.

Analyzing the data further, the authors found that being >50 years old was a risk factor for serious or worse cases, again along with slower speed. Higher temperatures and age, along with slower speed were risk factors for fluid or electrolyte issues, which could mean either drinking too little OR too much. Meanwhile, age was again a risk factor for cardiovascular issues.

Besides a “gee whiz” factor, why are studies like these interesting and important? Before you can deal with a problem, you must first know what the problem is. This study demonstrates the risk from exercise in the heat, and gives insights into who might be most at risk. This can help organizers provide better medical support by knowing who might be at greatest risk for which types of problems. In turn, this can help prevent problems from ever arising by helping to screen and educate at-risk participants before the events.

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