Exertional Heat Exhaustion and Heatstroke: Warning Signs from 38.5°C to 41°C Core Temperature

Why Summer Endurance Sports Carry High Heat-Injury Risk

Taiwan’s average summer temperature from June to September is 30-35°C, with relative humidity frequently above 75%. When endurance athletes train for more than 2 hours in this environment, core body temperature can climb into a dangerous zone. Research shows:

• Under conditions of 32°C ambient temperature and 70% humidity, average core temperature during 1.5 hours of cycling rises from 36.8°C to 39.2°C
• In athletes with inadequate hydration, core temperature may reach 40°C within 2 hours
• Above 40°C, the athlete enters the danger zone for heatstroke

Heatstroke carries a mortality rate of roughly 10-30%, and surviving cases can have severe lasting effects.

Core Temperature Staging

37-38°C: Normal Exercise State

• Sweating and elevated heart rate
• Performance not yet affected

38-38.5°C: Heat Stress

• Increased sweat rate
• Heart rate 5-10 bpm higher than expected
• Rising subjective sense of fatigue

38.5-39.5°C: Heat Exhaustion

Warning signs:

• Heavy sweating combined with cool, clammy skin
• Dizziness, nausea, feeling like vomiting
• Muscle cramps
• Abnormal heart rate elevation (HR drift greater than 20%)
• Slowed reaction time

At this stage, stop exercising immediately, cool the body, and rehydrate.

39.5-40.5°C: Severe Heat Exhaustion

Warning signs:

• Sweating stops (skin becomes dry and hot)
• Confusion or clouded consciousness
• Unsteady gait, blurred vision
• Vomiting
• Failure of thermoregulation

At this stage, core temperature must be brought below 38.5°C within 30 minutes, or the athlete progresses into heatstroke.

Above 40.5°C: Heatstroke

Warning signs:

• Central nervous system dysfunction (loss of consciousness, seizures)
• Risk of multi-organ damage
• Kidney failure, rhabdomyolysis
• Significantly elevated mortality risk

Call emergency medical services immediately and begin aggressive on-site cooling.

Emergency Response SOP

Initial On-Site Response

1. Move the patient to a shaded area immediately
2. Remove excess clothing and loosen cycling gear
3. Spray the body with water (cool, not ice-cold) and use a fan to aid heat dissipation
4. Provide electrolyte drinks in small, frequent sips
5. Assess level of consciousness; if it is clouded, call emergency services immediately

Cold-Water Immersion (Recommended for Heatstroke)

• Immerse in water at 10-15°C
• Continue for 15-20 minutes
• Monitor core temperature simultaneously (ear or rectal temperature)
• The person may get up once temperature drops to 38.5°C

Mistakes to Avoid

• Do not drink large amounts of ice water rapidly (can trigger gastrointestinal cramping)
• Do not rub the skin with alcohol immediately (skin vasoconstriction actually impairs heat dissipation)
• Do not force fluids on someone with impaired consciousness (risk of choking/aspiration)

Prevention Strategies

Strategy 1: Heat Acclimatization Training

Carried out over 10-14 days before an event:

• Train 60-90 minutes daily in an environment above 32°C
• Maintain Zone 2 intensity
• After 7-10 days, the body typically shows:
  • Plasma volume increased by 8-12%
  • Sweat onset temperature lowered by 0.3°C
  • Sweat sodium concentration reduced by 30%

After heat acclimatization, the rate of core temperature rise in the same environment slows by roughly 30%.

Strategy 2: Precise Hydration and Electrolyte Management

• 2 hours before riding: 500ml of an electrolyte drink
• During riding: 150-250ml every 15-20 minutes
• Use a sports drink containing 500-800mg/L of sodium
• Total hourly fluid intake: 750-1000ml

Strategy 3: Choosing Training Time Windows

• Avoid the high-heat window of 11:00-15:00
• Shift training to early morning (5:30-7:30) or evening (17:30-19:30)
• Substitute indoor Zwift sessions or gym training for daytime rides

Strategy 4: Clothing and Gear

• Light-colored, breathable moisture-wicking apparel
• A sweatband under the helmet to divert sweat
• Cooling sports towels (usable mid-race)
• Reapply sunscreen every 90 minutes

Strategy 5: Self-Monitoring of Body Signals

Ask yourself every 30 minutes:

• Am I still sweating? (No sweating is a danger sign)
• What color is my urine? (Dark yellow indicates dehydration)
• How much higher is my heart rate than expected?
• Has my subjective sense of fatigue risen abnormally?

Stop exercising immediately if any of these signs is abnormal.

Higher-Risk Groups

• Less than 6 hours of sleep per night over the past 3 days
• Alcohol consumption the night before an event
• Taking diuretics or antihistamines
• Higher body fat percentage (reduced heat dissipation efficiency)
• History of previous heat injury
• Age 50 and above

Conclusion

Heat exhaustion and heatstroke are preventable, early-recognizable sports injuries. The gap between the first warning signs at 38.5°C and the fatal edge at 41°C is only 2-3 degrees. By mastering the four pillars of hydration, heat acclimatization, training-time selection, and self-monitoring of body signals, summer training can be safe and effective — not a gamble with your life.

Related Reading

• Body Temperature and Athletic Performance: The Overlooked Core Temperature Code
• Heatstroke Warning Signs and Rescue: A Red-Line Guide for Runners’ Summer Training
• Heat Injury in Taiwan’s Summer Running: Prevention and Emergency Care for Heatstroke and Heat Exhaustion
• Heat Injury in Taiwan’s Summer: Emergency Response for Runners’ Heatstroke and Heat Exhaustion