Key Takeaways 💡
Exercising during a heatwave exposes you to hyperthermia and dehydration as soon as temperatures reach 30°C (86°F). Prioritize workouts early in the morning (before 10 a.m.) or in the evening (after 8 p.m.), choose water-based sports and moderate-intensity sessions. Regular hydration, gradual heat acclimatization over 7 to 14 days, and suitable thermoregulating gear remain your best allies for staying active without suffering from extreme heat.
Introduction
35°C (95°F) on the thermometer, Saturday morning. Your running session is calling, but the asphalt is radiating suffocating heat. Your muscles overheat after the very first kilometer, your heart rate spikes, sweat pours down your body. The question arises: should you really go out?
A heatwave turns every sports session into an equation with multiple unknowns. Hyperthermia, dehydration, reduced performance: the risks are real, but stopping all physical activity is not inevitable. The human body is not an infinite machine: without thermal compensation, it protects itself by shutting down activity.
At G-Heat, we have been designing thermoregulating solutions for over ten years to help athletes maintain continuity in their activity, even when temperatures soar. In this guide, you’ll find official warning thresholds, the sports to prioritize or avoid, hydration and acclimatization best practices, as well as gear reflexes that truly make a difference in the field.
Heatwaves and sports: understanding risk thresholds before lacing up your shoes
What a heatwave really means according to Météo-France
A heatwave is not just a hot spell. Météo-France defines it as a period of intense heat, day and night, lasting at least three consecutive days. Nighttime temperatures are actually the deciding factor: if your body temperature does not drop during sleep, your body cannot properly recover.
There are four warning levels: green, yellow, orange, and red. From the orange level onward, authorities recommend postponing intense outdoor physical activity.
The critical thresholds: 30°C, 32°C, and beyond
Extreme heat leaves no room for carelessness. Here are the key thresholds to know in order to adapt your training:
- Above 30°C (86°F): sweating intensifies and endurance performance decreases.
- Above 32°C (89.6°F): outdoor sports are officially discouraged by the French Ministry of Sports.
- Above 35°C (95°F): intense outdoor physical activity should be avoided in the afternoon.
In this high-risk zone, your first technical decision should concern clothing: a men’s cooling t-shirt with TechCool technology or its women’s cooling t-shirt equivalent actively evacuates heat generated by the muscles without requiring pre-wetting. A simple compensation mechanism that pushes the overheating threshold back by a few degrees.
The WBGT index: the athlete’s real thermometer
The displayed temperature only tells part of the story. The WBGT (Wet Bulb Globe Temperature) index integrates four parameters: temperature, humidity, solar radiation, and wind. It is the only indicator that truly reflects the thermal stress experienced by the body during exercise.
Also read: Heatstroke or sunstroke: how to tell the difference and react quickly?
What happens inside your body when you run in 35°C heat
Hyperthermia: when the engine overheats
Under normal conditions, your body temperature stays around 37°C (98.6°F). During exercise in extreme heat, it can climb to 39°C (102.2°F) or even 40°C (104°F). At that point, the body can no longer evacuate the heat produced by the muscles. According to Inserm, around 75% of the energy consumed to produce physical effort is transformed into heat. Sweating becomes insufficient, especially when the air is saturated with humidity.
The first warning signs include headaches, dizziness, burning skin, and confusion. Heatstroke itself can occur within minutes and constitutes a medical emergency.
Dehydration: the silent performance killer
Losing just 2% of your body weight in water already reduces physical capacity. Beyond that, you enter the danger zone: cramps, reduced coordination, and digestive disorders may occur. Thirst is never a reliable indicator: by the time you feel thirsty, you are already dehydrated.
Acclimatization: why 7 to 14 days are necessary
The body can adapt, but it needs time. Research from INSEP shows that 75 to 80% of physiological adaptations to heat occur within the first 4 to 7 days, while near-complete acclimatization requires 10 to 14 days of progressive exposure (60 to 90 minutes per day). During this period, your body learns to sweat earlier, distribute blood flow more efficiently, and retain sodium better. Skipping steps only increases the risks.
“Clothing is not a gadget, it’s a tool. Our goal is to ensure that weather is never again a reason to stop before the finish line.”
— Édouard Castaignet, CEO and co-founder of G-Heat
Which sports to prioritize (and which to postpone) during a heatwave
Activities to absolutely avoid in the afternoon
Some sports become dangerous between 12 p.m. and 5 p.m. during periods of extreme heat. Asphalt can easily reach 50°C (122°F), solar radiation is at its peak, and the air barely circulates.
- Road running on asphalt: reflected heat doubles the thermal stress.
- Football, rugby: repeated high-intensity efforts with insufficient hydration breaks.
- Road cycling under direct sunlight: prolonged exposure and rapid dehydration.
- Trail running under blazing sun: a dangerous combination of effort, heat, and technical terrain.
Water sports: your best ally in summer
Water is your best friend in summer. It dissipates body heat much faster than air. Swimming, aqua fitness, paddleboarding, and kayaking allow you to maintain physical activity without excessive thermal overload.
After getting out of the water or finishing a paddle session, keep a cooling towel with JadeCool technology and certified UPF 50+ protection nearby: it extends the refreshing effect of water around the neck during recovery, right when the cooling effect of swimming suddenly stops.
Be careful though: swimming should always remain supervised, especially after intense exercise, as thermal shock may cause dizziness or fainting.
Yoga, mobility, strength training: smart indoor options
When the sun is blazing, an air-conditioned gym becomes a serious playground. Yoga, Pilates, strength training, and indoor cycling allow you to train effectively without suffering from the heat. It is also the perfect time to focus on general physical preparation, often neglected during the rest of the year.
Even indoors with air conditioning, a men’s cooling sports shorts with TechCool technology (or the women’s cooling sports shorts) better supports hot yoga or summer crossfit sessions thanks to its breathable stretch fabric that wicks sweat away without sticking to the skin.
The best times and places to train without suffering from the heat
Morning before 10 a.m.: the ideal window
This is the golden time slot of summer. Between 6 a.m. and 9 a.m., temperatures are still low, nighttime humidity remains manageable, and the sun has not yet heated the asphalt. Your body also benefits from better physiological readiness after waking up and hydrating properly.
Even early in the morning, low sunlight directly hits the top of your head as soon as you leave the shade. A cooling sports cap with PolyCool technology, activated with water before your session, lasts for several hours and helps before the first real rise in temperature.
Evening after 8 p.m.: beware of residual humidity
The evening remains a good option, but with nuances. Asphalt releases the heat stored throughout the day, and the air may feel more humid. Prioritize parks, coastal areas, and wooded trails over dense urban environments.
When residual humidity prevents effective evaporation, conduction takes over: a cooling neck wrap with JadeCool technology acts directly through contact on the cervical area, without depending on dry air. The blood cooled at the source circulates throughout the entire body.
Learn more: How does our body regulate its temperature during summer?
Air-conditioned gyms, forests, seaside: choosing the right environment
Choosing the right location matters just as much as choosing the right time. Here’s a quick comparison to guide your sessions:
| Location | Thermal advantage | Recommended activities |
|---|---|---|
| Forest / shaded trails | -5°C to -8°C compared to the city | Running, hiking, mountain biking |
| Seaside | Sea breeze, balanced humidity | Swimming, paddleboarding, running on wet sand |
| Air-conditioned gym | Constant 21–23°C (70–73°F) | Strength training, indoor cycling, yoga |
| Outdoor swimming pool | Continuous evaporation effect | Swimming, aqua fitness |
| Urban asphalt in direct sunlight | +5°C to +10°C compared to ambient air temperature | Absolutely avoid |
Hydration, equipment, and precautions: the informed athlete’s checklist
Before, during, and after exercise: the winning hydration protocol
Drinking water is essential — but doing it properly matters even more. Hydration management during a heatwave should never be improvised. Here is the three-step protocol based on recommendations from the French High Council for Public Health:
- Before exercise: 200 to 300 ml every 30 minutes during the two hours preceding your session. Avoid excessive fluid loading all at once.
- During exercise: 150 to 200 ml every 15 to 20 minutes. For activities lasting longer than one hour, prioritize isotonic drinks (1 to 1.5 g of salt per liter).
- After exercise: drink 1.5 times the amount of weight lost. Weigh yourself before and after to adjust accurately.
Technical cooling apparel: compensating for active heat
The right equipment makes the difference between mastering a workout and suffering through it. Beyond the t-shirts, shorts, and caps already mentioned throughout this guide, two additional targeted pieces complete the strategy depending on the context.
Under a helmet (cycling, construction work, motorcycling), the cooling skull cap with PolyCool technology fits comfortably without causing discomfort and promotes heat dissipation in one of the body’s most trapped heat zones. For prolonged upper-body exposure under direct sunlight, the men’s long-sleeve cooling Anti-UV t-shirt with JadeCool technology protects the forearms with integrated UPF 50+ protection, instead of combining short sleeves with sunscreen.
To explore the collection by category, discover our cooling t-shirts and our cooling shorts.
Children, seniors, and medical conditions: increased vigilance
Not all bodies react the same way to extreme heat. Children regulate their body temperature less efficiently. Seniors feel thirst less intensely and may become dehydrated without realizing it. People taking medication such as diuretics or beta-blockers are even more sensitive.
For these groups, the rule is simple: gentle physical activity, cooler time slots, mandatory hydration every 30 minutes even without thirst, and supervision from someone nearby. For seniors walking in the early evening, lightweight cooling accessories (a towel around the neck or a neck wrap requiring no additional moisture) help reduce the risk of heatstroke without constraints.
Must-read: Breathable t-shirt vs cooling t-shirt: which one should you choose for running in summer?
“The goal is not to overcool the body, but to compensate precisely for what the environment takes away from it so that it can continue functioning normally.”
— Gwenaël Fournet, Product Expert at G-Heat
Why choose G-Heat for sports during a heatwave
G-Heat is the French technical response to summer thermal stress. We design thermoregulation solutions built to maintain continuity in your physical activity, whatever the weather conditions.
- French thermal expertise: as a leader in thermoregulating technical apparel, we design equipment intended to support your activity, from running to hiking.
- Four cooling technologies: TechCool, JadeCool, PolyCool, and PhaseCool. Four different approaches to evacuating heat, adapted to each sport and environment.
- Coverage for multiple sports disciplines: running, cycling, trail running, crossfit, summer team sports, and recovery after water sports. Every practice finds its technical solution within the range.
- Maintained activity continuity: our solutions push back the overheating threshold to help maintain focus, pace, and recovery even above 32°C (89.6°F) under direct sunlight.
- Targeted body-zone cooling: t-shirts and shorts for active muscle areas, neck wraps for the cervical zone, caps and skull caps for the head, towels for recovery. Every critical area has its dedicated solution.
- Durability and easy maintenance: machine washable at 30°C (86°F) without fabric softener, air dry only, with thermoregulating properties maintained season after season without loss of efficiency.
- Field-tested approach and support: our products are tested by athletes, for athletes. Personalized advice, reliable guarantees, and fast delivery complete the experience.
“In the field, the best equipment is the one you forget about. If you’re no longer thinking about the heat, you’re 100% focused on your goal.”
— Romane Benderradji, Communication Manager at G-Heat
Conclusion
A heatwave is not a reason to stop exercising — it is a parameter that must be managed intelligently. Adjust your training schedule, choose the right disciplines, focus on proper hydration, and equip yourself with thermoregulating solutions: this is the foundation for an active and safe summer. Discover our cooling apparel and turn heat into just another variable in your training. Whatever the weather. G-Heat, whatever the conditions.
FAQ
At what temperature should you really cancel an outdoor workout?
Above 32°C (89.6°F), the French Ministry of Sports officially advises against outdoor sports activities. At 35°C (95°F) under direct afternoon sunlight, canceling or moving your workout to early morning becomes essential. For moderate activities (walking, gentle yoga, mobility work), exercise remains possible up to around 30°C (86°F) in the shade, provided hydration is reinforced.
How can you recognize the first signs of heatstroke during exercise?
The warning signs are clear: headaches, intense sensations of heat, red and dry skin (when sweating stops, that is a bad sign), nausea, dizziness, mental confusion, and balance issues. At the first sign of trouble, stop immediately, move into the shade, hydrate slowly, and call emergency services if symptoms persist beyond a few minutes.
How much water should you drink before, during, and after exercise in a heatwave?
Plan for 200 to 300 ml every 30 minutes during the two hours preceding exercise, then 150 to 200 ml every 15 to 20 minutes during activity. After your session, drink 1.5 times the amount of weight lost (weigh yourself before and after exercise). For efforts lasting longer than one hour, choose an isotonic drink to compensate for sodium losses caused by sweating.
Do G-Heat cooling technical garments really work in 35°C (95°F) heat?
Yes — provided you understand how they work. JadeCool and PolyCool technologies are water-activated and function through evaporation, lowering perceived temperature for several hours. TechCool technology, on the other hand, works dry by improving the evacuation of body heat. The goal is not to cool the body down artificially, but to help it dissipate the active heat generated during exercise.
Can you continue running every day during a prolonged heatwave?
Yes, but with adjustments. Reduce your training volume by 30 to 40%, lower intensity, move sessions to early morning or evening, and alternate with indoor gym or swimming sessions. A heatwave week is also the perfect opportunity to work on often-neglected qualities such as mobility, strength training, and running technique.
Which sports are recommended for children and seniors during extreme heat?
For children: supervised swimming, water games, cycling in shaded forest areas in the morning, or sports in air-conditioned facilities. For seniors: gentle walking early in the morning or evening, aqua fitness, yoga, or light gymnastics. In both cases, hydration should be enforced every 30 minutes even without thirst, and direct sun exposure between 11 a.m. and 5 p.m. should be avoided.
How should thermoregulating gear be maintained and stored between uses?
For JadeCool or PolyCool cooling garments: rinse with clean water after each use, air dry in the shade, and machine wash at a maximum of 30°C (86°F) without fabric softener. Tumble drying should be avoided to preserve the technical fibers. When stored dry, these garments retain their thermoregulating properties over multiple seasons without losing effectiveness.
Sources and References
[1] “Heat and athletic performance: what solutions help reduce risks?”, Bulletin Épidémiologique Hebdomadaire, Santé publique France
[2] “Paris 2024 Olympic Games: heat stress in sports”, French National Institute of Health and Medical Research (Inserm)
[3] “Sports and heatwaves: official recommendations”, French Ministry of Sports
[4] “Guide for a sporty and responsible summer 2025”, French Ministry of Health
[5] “Heatwave: recommendations for athletes and those around them”, French High Council for Public Health (HCSP)
[6] “Climate change, heat, and health”, World Health Organization (WHO)
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