Preventing hyperthermia comes down to managing three things: your environment, your fluid intake, and your body’s ability to release heat. Most heat-related illness is avoidable with straightforward planning, but the specifics matter more than people realize, especially at certain temperatures, during physical activity, or if you take medications that interfere with sweating or thirst.
Know Your Heat Thresholds
Your risk of overheating doesn’t start when temperatures feel extreme. OSHA’s proposed workplace heat safety rules set the initial trigger at a heat index of just 80°F, the point where employers should encourage rest breaks and access to shade or cooling. At a heat index of 90°F, the risk jumps enough that the guidelines call for mandatory 15-minute rest breaks every two hours.
These thresholds apply to anyone spending time outdoors, not just workers. If the heat index is 80°F or above, you need a plan: shade or air conditioning available, water on hand, and a way to cool down if you start feeling off. At 90°F and above, limit continuous outdoor activity and build in regular breaks in a cool space. Humidity is a major factor here because it slows evaporation of sweat, your body’s primary cooling mechanism. A 90°F day at 60% humidity is far more dangerous than the same temperature in dry air.
How Much Fluid You Actually Need
The general advice to “drink plenty of water” isn’t specific enough when heat is a real threat. During physical activity in the heat, fluid replacement guidelines from the National Athletic Trainers’ Association recommend drinking 7 to 10 ounces every 10 to 20 minutes. That pace keeps you from losing more than 2% of your body weight in sweat, which is the tipping point where your body’s cooling system starts to falter and your core temperature climbs faster.
Most people underestimate how much they sweat. Active adults can easily lose more than a liter of fluid per hour in hot conditions. Water is sufficient for shorter periods, but if you’re exercising or working outdoors for over an hour, adding electrolytes helps your body absorb and retain that fluid more effectively. Don’t wait until you feel thirsty. By the time thirst kicks in, you’re already mildly dehydrated.
Give Your Body Time to Adapt
Your body can adjust to heat, but it takes time. The CDC recommends a gradual acclimatization period of 7 to 14 days. If you’re new to working or exercising in the heat, start with about 20% of your normal exposure on the first day and increase by no more than 20% each day after that. If you’ve done it before but took a break (vacation, illness, a stretch of cool weather), you can ramp up faster: 50% on day one, 60% on day two, 80% on day three, and full exposure by day four.
During acclimatization, your body makes measurable changes. You start sweating earlier and more efficiently, your sweat becomes less salty (conserving electrolytes), and your cardiovascular system gets better at pushing blood to the skin for cooling. Skipping this process is one of the most common reasons otherwise fit people end up with heat illness early in the summer or during the first days of a heat wave.
What to Wear in the Heat
Clothing choices have a bigger effect on heat dissipation than most people expect. The goal is to maximize airflow against your skin and let sweat evaporate, since evaporation is what actually cools you.
Linen is the standout for hot weather. Its hollow fibers and naturally loose weave create a built-in ventilation system, and it can absorb up to 20% of its own weight in moisture before it feels damp. Cotton is a close second in dry heat because its fiber structure lets air circulate freely, and as absorbed sweat evaporates, it creates a cooling effect. In humid conditions, though, cotton can become waterlogged and heavy.
For exercise, moisture-wicking synthetics like polyester work differently. Polyester fibers absorb almost no moisture (less than 1% of their weight), so sweat travels along the surface of the fabric and spreads out where it evaporates quickly. This keeps the fabric light and keeps that cooling evaporation happening at the outer surface of the garment rather than trapping dampness against your skin.
Loose-fitting, light-colored clothing reflects more radiant heat than dark colors. A wide-brimmed hat reduces direct solar radiation on your head and neck, which are critical areas for temperature regulation.
Medications That Raise Your Risk
Several common medication classes interfere with your body’s ability to handle heat. Some reduce sweating, others suppress your sense of thirst, and some increase sun sensitivity, all of which raise your risk of overheating without obvious warning signs.
Antihistamines, certain blood pressure medications, antidepressants, antipsychotics, and drugs used for overactive bladder can all impair sweating or thirst signals. Diuretics (water pills) increase fluid loss. Stimulant medications, including some used for ADHD, can raise your baseline metabolic heat production. If you take any of these, the practical steps are the same but more urgent: seek cool environments more aggressively, drink fluids on a schedule rather than by thirst alone, and apply broad-spectrum sunscreen (SPF 30 or higher) if your medication increases sun sensitivity.
Heat can also damage medications themselves. Never leave medications in a parked car, and be aware that inhalers can malfunction or even burst in extreme heat.
Who Faces the Greatest Risk
Infants and older adults are the most physiologically vulnerable to hyperthermia, but for opposite reasons. Infants have a very high body surface area relative to their volume (about four times the ratio of a 20-kilogram child), which means they gain heat from a hot environment much faster. Combined with a thicker layer of subcutaneous fat and a significant increase in heat production during the first weeks of life, infants can store excess body heat dangerously fast. They also can’t tell you they’re overheating or move themselves to a cooler spot.
Older adults face a different set of problems. Sweat gland function declines with age, reducing the body’s primary cooling mechanism. Many older adults take medications that further impair thermoregulation, and the thirst response weakens, making dehydration more likely. People with chronic conditions like heart disease or diabetes also have reduced capacity to redirect blood flow to the skin for cooling.
For both groups, the most important prevention measure is environmental: keep indoor spaces cool with air conditioning or fans, check on elderly neighbors or family members during heat waves, and never leave an infant or child in a parked car, even briefly. Car interiors can reach lethal temperatures within minutes on a warm day.
Recognize the Warning Signs Early
Heat illness progresses through stages, and catching it early is far easier than treating it late. Heat exhaustion shows up as a body temperature between 101°F and 104°F, along with pale skin, muscle cramps, headache, dizziness, nausea, fatigue, and rapid breathing or heart rate. At this stage, you’re still sweating, and moving to a cool environment, drinking fluids, and resting will usually reverse it.
Heat stroke is what happens when the body’s cooling system fails entirely. Core temperature rises above 104°F, skin becomes dry and red, and sweating stops. The most telling signs are neurological: confusion, slurred speech, aggression, hallucinations, or seizures. Heat stroke is a medical emergency that can cause organ damage and death. If someone shows these symptoms, getting their body temperature down immediately (cold water, ice, wet towels, shade) while waiting for emergency help is critical.
The transition from heat exhaustion to heat stroke can happen quickly. If rest and cooling don’t resolve symptoms of heat exhaustion within 15 to 20 minutes, or if confusion or disorientation appears at any point, that’s an emergency.
A Practical Prevention Checklist
- Check the heat index before going outside. Above 80°F, plan for shade and water access. Above 90°F, build in 15-minute breaks every two hours minimum.
- Drink on a schedule. 7 to 10 ounces every 10 to 20 minutes during activity, not just when thirsty.
- Acclimatize gradually. Give yourself 7 to 14 days of increasing exposure when starting a new outdoor routine in the heat.
- Choose the right clothing. Linen or cotton in casual settings, moisture-wicking synthetics during exercise. Light colors, loose fit.
- Review your medications. Know whether any of your prescriptions affect sweating, thirst, or sun sensitivity.
- Watch high-risk people closely. Infants, older adults, and anyone with chronic illness need more frequent check-ins during heat events.
- Act on early symptoms. Cramps, dizziness, or nausea in the heat are your body telling you to stop, cool down, and hydrate now.