Working out when it’s hot outside: is it a good idea?

My workout

Published on 1 July 2018
by Philippe Morvan

Lately, the temperature outside has increased dramatically, bringing along with it a number of heat-related risks when engaging in physical activity outdoors. Although the sun’s warm rays can do wonders for our bodies, hearts and minds, it’s important to be aware of the dangers it can also entail.

First, it’s important to note that when exercising, nearly 75%-90% of the energy we use is converted into heat or is used to maintain the body’s physiological balance.

In order to keep its internal temperature at around 37 C, the body dissipates its heat in four different ways:

  • Approximately 60% through radiation (heat emitted in the form of radiation),
  • Approximately 15% through convection (evacuation of heat through air displacement),
  • Approximately 3% through conduction (heat exchanged between two surfaces in contact),
  • Approximately 20%-80% through evaporation (evacuation of heat through breathing and perspiration); depending on whether the person is resting or active.


When the temperature outside is high, the body has a harder time evacuating its heat. In fact, sun rays and hot air prevent the body from efficiently dissipating heat. The body then redirects the blood to the skin and extremities in order to evacuate the heat through perspiration. It is important to note that perspiration does not evacuate heat. Rather, the evaporation of sweat is what helps lower body temperature. If you tend to wipe your perspiration regularly, you are preventing your body from adequately evacuating its excess heat. Moreover, high levels of humidity outdoors greatly affect the body’s ability to dissipate its heat through perspiration.

Naturally, the hotter it gets, the more the heat is likely to hinder your athletic performance. Additionally, if your workout is not done properly, it can cause significant problems such as cramps, heat stroke, exhaustion and sunstroke, the latter which can cause coma, or in extreme cases, even death.

If you must practise physical activities outdoors on a hot day, here are some tips to follow:

  • Plan your intense training sessions early in the morning when the temperature is lower, and your less intense sessions when it is hotter.
  • Reduce the length or intensity of your workouts and take periodic breaks as needed (ideally, choose a shady spot).
  • Don’t try to beat your records or push your limits when it is very hot. Learn to respect your limits.
  • Take care to hydrate properly. Before your outdoor training session, drink some water and continue to drink small amounts of water frequently (approximately every 5 to 10 minutes). Choose a cool drink (5 to 10°C) with low concentrations of sugar and salt. A diluted sports drink may be a good choice.
  • Wear lightweight, well ventilated, light-coloured clothing; these will evacuate heat and allow air to circulate.
  • Don’t forget to wear sunscreen, but choose water-based, rather than oil-based ones. Oil-based sunscreens can hinder the heat evaporation capacity.
  • Caffeinated and alcoholic drinks as well as certain medications (diuretics, Aspirin, beta blockers, etc.) can also reduce your body’s ability to evacuate heat.

I hope these tips are useful to you!
Don’t forget that it is always pleasant to work out in our 21 C gyms, where the water fountain is never far!
Philippe Morvan
Kinesiologist, training supervisor