Heat stress is what happens when your body absorbs more heat than it can release, pushing your core temperature above its normal range of about 98.6°F (37°C). It spans a spectrum from mild discomfort and cramping all the way to heat stroke, a life-threatening emergency where body temperature can spike to 106°F or higher in as little as 10 to 15 minutes. Understanding where you fall on that spectrum, and what pushes you further along it, can genuinely save your life or someone else’s.
How Your Body Manages Heat
Your body runs at a core temperature of roughly 98.6°F, give or take about one degree. Staying in that narrow window keeps your metabolism functioning properly. When you start overheating, your body has two main cooling strategies: sweating (which pulls heat away as moisture evaporates from your skin) and dilating blood vessels near the skin’s surface so warm blood can release heat into the air.
Heat stress begins when those systems can’t keep up. High air temperature, humidity, intense physical work, or direct sunlight can all tip the balance. Humidity is especially dangerous because it slows evaporation, which means sweat stays on your skin without actually cooling you. When both cooling mechanisms fail completely, core temperature climbs fast and organs start taking damage.
Heat Exhaustion vs. Heat Stroke
Heat exhaustion is the earlier, less dangerous stage. It happens when heavy sweating drains your body of water and electrolytes, reducing blood volume and dropping blood pressure. You’ll typically notice headache, nausea, dizziness, weakness, heavy sweating, irritability, and intense thirst. Urine output drops. Your temperature may be elevated but hasn’t reached crisis levels. At this point, moving to a cool environment and rehydrating can reverse things relatively quickly.
Heat stroke is the emergency. The body’s temperature-control system essentially shuts down, and core temperature can blow past 104°F. The hallmark symptoms are neurological: confusion, slurred speech, seizures, loss of consciousness. Skin may be hot and dry (because the sweating mechanism has failed) or, in some cases, still drenched in sweat. Without rapid cooling, heat stroke is fatal. Body temperature can rise from dangerous to deadly in 10 to 15 minutes, so speed matters enormously.
The fastest proven method for bringing someone’s temperature down is cold-water immersion. Submerging the torso and limbs in ice water (around 35°F) cools the body at roughly 0.35°C per minute, nearly twice as fast as warmer water. If a tub isn’t available, covering the person with wet towels, fanning them, and applying ice to the neck, armpits, and groin are reasonable alternatives, though they work more slowly.
Who’s Most Vulnerable
Certain people are at higher risk even in moderate heat. Older adults, young children, and anyone with chronic conditions like heart disease or diabetes have a harder time thermoregulating. People with spinal cord injuries, particularly above the mid-back, lose the nerve connections that trigger sweating and blood vessel dilation, leaving them especially exposed.
Several common medications also interfere with cooling. Diuretics (water pills) deplete fluid and electrolytes, making dehydration more likely. Beta-blockers reduce blood flow to the skin and decrease sweating. Antihistamines with anticholinergic effects, including diphenhydramine (Benadryl) and doxylamine, suppress sweating. Many psychiatric medications carry similar risks: antipsychotics, tricyclic antidepressants, and SSRIs can all impair the body’s ability to sweat or sense heat. Even the seizure medication topiramate reduces sweating. If you take any of these, you need a lower threshold for getting out of the heat and staying hydrated.
Measuring Heat Risk Accurately
Most people check the heat index, which combines air temperature and humidity into a “feels like” number. That works fine for everyday activities, but it has a significant blind spot: it assumes you’re in the shade. Direct sunlight can add as much as 15°F to the actual heat load on your body.
For anyone working or exercising outdoors, a more reliable measurement is the Wet Bulb Globe Temperature (WBGT). It factors in temperature, humidity, wind speed, sun angle, and cloud cover, all measured in direct sunlight. OSHA uses WBGT to guide workload decisions, and many college and high school athletic programs rely on it to determine whether practices should be modified or canceled. If your work or training happens in full sun, WBGT gives a much more accurate picture of your actual risk than heat index alone.
Heat Stress in the Workplace
Outdoor workers in construction, agriculture, and landscaping face some of the highest heat exposure, but indoor workers in kitchens, laundries, warehouses, and manufacturing plants are also at serious risk. Occupational heat exposure is linked to acute kidney injury, cardiovascular strain, and a form of chronic kidney disease documented extensively among agricultural workers in Central America, where repeated daily heat stress appears to cause cumulative organ damage over years.
OSHA published a proposed federal heat standard in August 2024 that would require employers in all general industry, construction, maritime, and agriculture sectors to create plans for evaluating and controlling heat hazards. The public hearing process concluded in mid-2025, and a final rule is still in progress. In the meantime, OSHA can cite employers under the General Duty Clause for failing to protect workers from known heat hazards.
How Acclimatization Protects You
One of the most effective defenses against heat stress is gradual acclimatization. A body that’s been slowly introduced to heat becomes better at sweating earlier, sweating more, and maintaining blood volume. OSHA and NIOSH recommend the “Rule of 20 Percent” for new workers: spend only 20 percent of a normal shift in the heat on the first day, then add 20 percent each subsequent day. By the end of the first week, most physically fit workers can handle a full schedule.
Some people need up to 14 days to fully adapt. Workers returning from a vacation of a week or more, or anyone who’s been ill, should restart the acclimatization process rather than jumping back into full exposure. New and returning workers account for a disproportionate share of heat-related deaths on the job, precisely because they haven’t built up tolerance.
Long-Term Health Effects
A single severe episode of heat stroke can leave lasting damage. The kidneys are particularly vulnerable because reduced blood flow during overheating injures the delicate filtering structures. The heart faces strain from trying to pump more blood to the skin while overall blood volume drops. Neurological symptoms from heat stroke, including memory problems and coordination issues, can persist for weeks or months in some cases.
Repeated moderate heat stress, even without a dramatic heat stroke event, carries its own risks. The chronic kidney disease seen in agricultural workers in Mesoamerica is the clearest example: years of daily heat exposure combined with heavy labor and inadequate hydration appear to cause progressive, irreversible kidney damage in people who are otherwise young and healthy. Researchers have also raised concerns that repeated cycles of heat-related cardiac and kidney strain may set the stage for cardiovascular problems later in life, even in otherwise fit individuals like endurance athletes.