Heat wave and sport: dangers and recommendations

When the temperature exceeds 30 degrees, practicing intense physical activity increases the risk of dehydration, cramps and heatstroke. What precautions should athletes take? What sports to avoid?

[Mise à jour le 16 juin 2022 à 15h20] Summer and high temperatures do not necessarily exempt us from practicing physical activity. But in case of strong heat you have to follow certain rules to avoid the dehydration, heat stroke… and not getting exhausted. Know thatabove 30°C and in a heat wave, it is not recommended to exercise too intensely outdoors. In case of strong redness, feeling of intense heat, headache, visual disturbances, abnormal sensations (balance, judgement…): stop exercising, spray yourself with water, drink and stay in the shade in a ventilated place“ warns the Ministry of Health. What sports do you prefer? What hazards to engage in physical activity during heat wave ? What recommendations and precautions ?

What are the dangers of exercising during a heat wave?

The human body is made to operate at 37 degrees. When it moves away from this temperature, the organism is altered“explains Dr. Roland Krzentowski, sports physician and Founder of Mon Stade Maison Sport Santé Mgen.”When the individual exerts physical effort, 75% of the activity is transformed into heat that must be evacuated. The human body has built a mechanism to evacuate this heat to maintain itself at 37 degrees, this is called the sweat. To promote this, the body under the direction of the brain will decrease the blood supply to the deep vessels (those that bring blood to the organs) to favor the blood supply to the skin.“continues our expert.

It is necessary to take into account the temperature but also the humidity in the air.

In case of heavy sweating, our deep organs are penalized and at a certain point, this is dangerous for the organism. “The hotter and drier the air, the easier it is for sweat to evaporate, the more humid the air, the more difficult it is. It’s necessary take into account the temperature but also the humidity in the air. For example, if you make an effort at 25 degrees with 80% humidity, this will be more restrictive for your body than an effort made at 30 degrees but with dry air. develops our interlocutor. The 3 elements to take into account when doing sport are: temperature, humidity and solar radiation. “These are gathered by the WGBT index available on meteorological information sites. If the index is between 27 and 31, we are in a moderate risk, but if it exceeds 32 the risk is majorwarns Dr. Krzentowski. Even if the effects on the organism depend on each individual (training, fatigue, habit in high climate etc.)we avoid sport in the sun at 30 degrees. Exercising in hot weather, without getting enough hydration or after too intense an effort, can disrupt the body’s mechanisms for cooling itself and train:

► The dehydration : when it is hot or very hot, the body can lose up to 2 liters of water per hour. And when it loses water, the body loses salt and this promotes the risk of dehydration.

► The heat cramps

► One early fatigue : in the absence of sufficient water compensation, the increase in heart rate can cause early fatigue.

sunstroke which is characterized by an increase in body temperature, flushing of the face, excessive sweating, headaches or even dizziness, nausea and vomiting. Sunstroke should never be neglected, because of its potential evolution towards serious disorders. It requires the immediate use of all means to lower body temperature. To do this, you must first practice simple gestures such as: lying down in a cool and shady place, drinking plenty of water and wetting your neck, face and limbs with water.

► The heatstroke : when the body temperature exceeds 40 degrees, the skin is red, hot and the body no longer produces sweat. The body no longer adapts, it loses control of the situation. The pulse quickens, there is confusion, loss of consciousness and impaired cognition (analyzes, hindered decision-making, memory loss, etc.). Immediately dial 18.

What precautions should be taken when it is very hot?

“If you are not trained, do not start or resume physical activity or athletic in periods of high heat” warns the Ministry of Health, which recommends limiting physical activity as much as possible in the event of a heat wave. “His heart rate is monitored, adds Dr. Krzentowski. If for the same intensity of effort your heart rate is higher than usual, we reduce the effort“. Precautions ;

Drink water. Rule n°1: hydration. You have to stay hydrated before, during and after exercise. Do not wait to be thirsty to drink! Be careful, avoid drinking ice cold to avoid creating a thermal shock.

Do not wait to be thirsty to drink!

Choose loose, light and “breathable” clothingso as not to retain perspiration and help the body to cool down, and rather to light colours because the latter absorb less sunlight than dark colors. The shoes must, if possible, allow good heat dissipation with insulation at the level of the sole,

Exercise in the shade or in an air-conditioned gym (Where run in the undergrowth rather than on bitumen for example).

splash water regularly.

► If you play sports outdoors, you must protect your skin with sunscreen with a high factor (even if the sky is overcast), sunglasses, a hat or cap ideally with a back on the neck to protect it.

Avoid excessive physical exertion in direct sunlight/when it is hot or very hot (above 28°C).

► Stay vigilant towards oneself and other athletes.

► On don’t drink alcohol before physical exertion and avoid diuretic drinks such as dehydrating coffee.

► On decrease the intensity and the duration physical activities.

What advice after the sports session in case of heat?

We drink lots of water to compensate for the loss. And we wait a bit for the body to cool down before taking a shower to avoid thermal shock. Above all, no very cold water (for drinking or showering) : the brain would initiate the heating of the body.

To avoid the risk of dehydration, we drink a lot of water before the sports session, 200 to 300ml (2 glasses) every 30 minutes, during and after the session every 15 to 20 minutes. And above all, you have to drink before you feel thirsty. ‘Drinking allows evaporation through sweat and cooling of the bodyexplains Dr. Krzentowski.You have to drink all the more when it’s hot and you make an effort”. For instance, at moderate walking (5km/H) and in 50% humidity and 30 degrees temperature, you should drink 1 liter of water every hour. Under these same weather conditions, but running fast (30KM/H), you should drink 3 liters per hour”. It is advisable to drink regularly in small successive quantities. Still water but rich in mineral salts is preferable since it promotes perspiration: the salt allows water to pass through the skin. In the event of very prolonged efforts, it may be useful to add sugar in water.

Contrary to popular belief, playing sports in the heat does not burn more calories: you sweat more, so you lose more water, but you don’t lose more weight and you don’t melt more fat.

What sport can you do when it’s hot?

Sport has many health benefits (it acts against overweight, improves endurance, reduces cardiovascular risk, prevents osteoporosis, reduces stress and anxiety…) but when it’s hot, some sports are more recommended.

Swimming, aquagym, aquabike, water polo, paddle : “The sports practiced in water rreduce the risks associated with heat, but the parts of the body that are outside the water should not be neglected“says Dr. Roland Krzentowski, sports doctor.

Fitness, zumba, body balance, yoga, pilates, bodybuilding: The sports practiced in a air-conditioned gymsheltered from the sun are to be preferred“adds the doctor.

Avoid competitive sports (tennis, football, rugby) and outdoor sports during a heat wave, since they encourage you to surpass yourself and increase the intensity of your effort. “Finally, from the moment you take precautions (see above), all sports can be practiced. It’s how you practice that counts“recalls Dr. Krzentowski.

What time to exercise during a heat wave?

We avoid the moment when the sun hits the strongest, that is to say between noon and 4 p.m. We choose to plan these sessions in the morning (before 10 a.m.) or early evening (after 6 p.m.), close to sunset. Between 11 a.m. and 5 p.m., sport is not recommended in the event of very high temperatures or heat waves.

Who should avoid sport when it’s very hot?

► Children, people over 65, and pregnant women are advised not to practice sport in the heat because they are at greater risk of dehydration.

Young people are not more protected, they run the same risks in the event of intense practice under the sun.“ warns Dr. Krzentowski.

► The populations most at risk are those said to be fragile : in overweight or obeseaffected by diabetes, high blood pressure or any condition that weakens the body and increases the risks.

► Persons who are not trained or beginnersThe people who recover from illness (cold, gastro, angina…) should not do sports when it’s hot

Trick : “If we want test its adaptation to heat, we weigh ourselves before and after exercise. The less weight corresponds to the amount of water lost. If you have lost 2% of your body weight, mental and physical performance is slightly affected (fatigue, less concentration, etc.). Between 3% and 4% loss, the capacities are significantly reduced. Beyond 10% of body weight, we are in danger alerts Dr. Krzentowski.

Thanks to Dr. Roland Krzentowski, sports doctor and Founder of Mon Stade Maison Sport Santé Mgen.


– Heat waves, Ministry of Solidarity and Health, June 15, 2022

– Heat wave and high temperatures: launch of the seasonal watch and reminder of the actions to be taken by all, Santé Publique France, June 10, 2022

– Recommendations for populations in the event of heat, Ministry of Solidarity and Health, June 10, 2021

– General knowledge on physical activity, physical inactivity and health, HAS, “heat wave” recommendations from the Ministry of Solidarity and Health.

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