The body has to dump heat to keep working
Working muscles produce heat as a byproduct of ATP use. Core temperature can rise from 37°C to 39°C+ during hard exercise. If the body can't dump this heat fast enough, performance crashes — and at higher temps, heat illness sets in.
How thermoregulation works
Conduction — heat transfers to cooler surfaces (sweat hitting cold water). Convection — moving air carries heat away (fan, breeze, fast cycling). Radiation — heat radiates from skin to cooler surroundings. Evaporation — sweat evaporating off skin removes heat. The most important mechanism during exercise — accounts for ~80% of heat loss in hot conditions.Why humidity matters
Sweat only cools you if it evaporates. In high humidity, sweat drips off without evaporating — heat loss collapses. Core temperature rises even at moderate workloads.
This is why a 75°F/100% humidity day is more dangerous than an 85°F/40% humidity day.
Signs of heat illness (escalating severity)
Heat cramps — painful muscle spasms during exercise. Usually from electrolyte loss. Stop, hydrate, cool down. Heat exhaustion — heavy sweating, nausea, dizziness, weakness, normal-to-slightly-elevated core temp (<40°C). Stop exercise, get to shade/AC, hydrate, cool. Heat stroke — core temp >40°C, altered mental status (confusion, slurred speech, irritability), hot dry skin (or still sweating). Medical emergency — call 911. Cool aggressively with ice water immersion if available.Exercising in cold
Cold-related injuries are less common but real:
Hypothermia — core temp drops below 35°C. Shivering, confusion, slurred speech. Get inside, warm gradually. Frostbite — fingers, toes, nose, ears most at risk. White or grayish patches that feel numb. Warm gradually with body heat or warm (not hot) water. No rubbing. Bronchospasm — cold dry air can trigger airway constriction. Common in winter runners. Breathing through a buff or scarf helps.Programming adjustments
Hot conditions:- Reduce intensity 10-20% for the first 1-2 weeks (heat acclimation)
- Train early morning or evening
- Hydrate before, during, after — add electrolytes for long sessions
- Recognize symptoms in self and clients
- Longer warm-up (muscles and joints need more time)
- Layer clothing so it can shed
- Cover extremities
- Don't sit/stand around outside sweaty
Hydration during exercise
Most clients dehydrate during long or hot sessions:
- Lose 1-2% of body weight in fluid → noticeable performance drop
- Lose 3%+ → significant impairment, heat illness risk
TL;DR
Body dumps heat via evaporation (mostly). Humidity blocks evaporation — a hot humid day is more dangerous than a hot dry one. Heat stroke is a medical emergency. Cold exercise needs longer warm-ups and layered clothing. Adjust intensity 10-20% in extreme conditions.