Heat stroke occurs when your core body temperature reaches 104°F (40°C) or higher and your body can no longer cool itself down. The risk factors span a wide range, from the medications you take and the shape you’re in, to where you live and how long you’ve been exposed to heat. Understanding these risks matters because heat stroke can damage the brain, heart, kidneys, and muscles within minutes.
How Your Body Loses the Cooling Battle
Your primary cooling system is sweat evaporating off your skin. When that process fails, heat builds up fast. During vigorous exercise, your muscles produce heat at 15 to 18 times the resting rate. Without any cooling response, that level of heat production would push core temperature from a normal 98.6°F to a dangerous 107.6°F in roughly 25 minutes. Even in a mild climate, intense physical effort can overwhelm your thermoregulation if conditions aren’t right.
Humidity is the biggest environmental saboteur. At high humidity levels (above roughly 75 to 85%), sweat doesn’t fully evaporate. It sits on the skin as a liquid residue, generating little to no cooling. This is why a 90°F day at 30% humidity feels manageable while the same temperature at 80% humidity feels suffocating. Your body is working just as hard to produce sweat, but the sweat can’t do its job.
Age: Risks at Both Ends
Older adults face a triple disadvantage in the heat. Their sweat glands release less sweat, their hearts don’t circulate blood as efficiently, and their blood vessels are stiffer and less responsive. That last point is critical: one of the ways your body sheds heat is by pushing warm blood toward the skin’s surface, where it can radiate outward. Stiff, atherosclerotic vessels don’t dilate as well, trapping heat inside.
Infants and toddlers are vulnerable for different reasons. A newborn has a surface area-to-mass ratio of about 648 square centimeters per kilogram, far higher than an adult’s. In moderate heat, that large surface area helps babies release heat through the skin. But when the air temperature exceeds skin temperature, the equation flips: that same large surface area absorbs heat from the environment instead. Infants also produce less sweat per gland and may need to divert a disproportionate share of their blood supply to the skin, leaving less for internal organs. A one-year-old’s surface area ratio drops about 28% from birth, but it’s still much higher than an adult’s.
Heart Disease, Obesity, and Chronic Illness
Cardiovascular disease is one of the strongest medical risk factors. Cooling your body requires your heart to increase its output significantly, pumping more blood to the skin while still supplying muscles and organs. People with underlying heart conditions, valve problems, or chronic heart failure simply cannot ramp up that output enough. Research published in the Canadian Journal of Cardiology found that individuals with ventricular dysfunction or stenotic valve disease are at particular risk of both heat stroke and circulatory collapse during prolonged heat exposure.
Obesity raises risk through multiple pathways. Fat tissue acts as insulation, making it harder to release heat through the skin. Higher body weight also correlates with lower cardiovascular fitness and impaired blood flow at the skin’s surface, both of which blunt the cooling response. Diabetes adds another layer of vulnerability: not because of the disease’s direct effect on thermoregulation, but because common diabetes medications, including metformin and certain newer glucose-lowering drugs, can worsen dehydration and electrolyte imbalances in the heat.
Medications That Impair Cooling
A surprisingly long list of common medications interfere with your body’s heat defenses. According to the CDC, these drugs raise heat risk through three main mechanisms: disrupting the brain’s temperature-control signals, reducing sweat production, or preventing blood vessels near the skin from opening up.
- Diuretics (water pills for blood pressure): cause fluid and electrolyte loss, pushing you toward dehydration before you even step outside.
- Beta blockers (heart and blood pressure medications): reduce sweating and prevent the small blood vessels near the skin from dilating to release heat.
- Antipsychotics: impair both sweating and the brain’s ability to sense and respond to rising body temperature.
- Tricyclic antidepressants: decrease sweat output.
- Antihistamines with anticholinergic effects (certain allergy and sleep medications like diphenhydramine): reduce sweating and disrupt thermoregulation broadly.
- SSRIs and SNRIs (common antidepressants): can increase sweating in normal conditions but may impair the overall cooling response during heat stress.
- Aspirin and antiplatelet drugs: reduce the dilation of surface blood vessels, limiting heat release through the skin.
If you take any of these, you don’t necessarily need to change your medication in summer. But you do need to recognize that your threshold for heat illness is lower than average and plan accordingly.
Dehydration and Fitness Level
Starting physical activity already dehydrated is one of the most consistent triggers for exertional heat stroke, the type that strikes otherwise healthy people during exercise or physical labor. A military study compared soldiers walking for three hours in 120°F heat: those who began in a dehydrated state experienced significantly more heat strain than those who started well hydrated, and hydration status mattered more than aerobic fitness level.
That said, fitness still plays a role. A well-conditioned body produces less heat at the same work intensity and is better at distributing blood between the skin and working muscles. People who are deconditioned or unaccustomed to heat are at higher risk, which is why heat stroke clusters in the early weeks of military basic training, preseason football practices, and the first hot stretch of summer for outdoor workers.
Lack of Acclimatization
Your body can adapt to heat, but it takes time. The earliest changes, like expanded blood volume, lower resting heart rate, and a slight drop in core temperature, begin within about four days of regular heat exposure. Full acclimatization takes 15 days or more of consistent exertion in the heat. During that adjustment window, you’re producing the same internal heat but your cooling systems haven’t yet upregulated.
This is why sudden heat waves are so deadly compared to sustained hot seasons. People in regions that rarely see extreme heat have bodies and infrastructure that aren’t prepared. A study of London’s 2022 heat wave estimated that 370 deaths in Greater London were attributable to heat during an 11-day period, and 38% of those deaths were linked to the urban heat island effect alone.
Where You Live and Work
Cities are measurably hotter than surrounding rural areas because concrete, asphalt, and buildings absorb and re-radiate heat. This urban heat island effect is especially dangerous at night, when temperatures in dense neighborhoods may stay 5 to 10 degrees higher than in less developed areas. Normally, nighttime cooling gives the body a chance to recover. When that overnight relief disappears, heat stress accumulates over days.
People who work outdoors, in kitchens, in warehouses, or in any setting without adequate cooling face compounded exposure. Athletic organizations and OSHA use a measurement called the Wet Bulb Globe Temperature, which accounts for heat, humidity, wind, and sun exposure simultaneously. At the highest risk levels, even 15 minutes of direct exertion can overwhelm the body, and guidelines recommend 45 minutes of rest per hour. Lack of access to air conditioning at home is another major risk factor, particularly for older adults who may not recognize early signs of overheating.
Exertional vs. Classic Heat Stroke
These two forms of heat stroke have different victim profiles. Exertional heat stroke hits people generating enormous internal heat: athletes, soldiers, laborers, weekend warriors pushing through a hot run. It can happen even in moderate temperatures if the pace is high enough and hydration is poor. Men are more commonly affected, likely because they tend to sustain higher metabolic heat production during intense activity.
Classic heat stroke, sometimes called nonexertional, develops over hours or days during heat waves and primarily affects older adults, people with chronic illness, those on multiple medications, and anyone without access to a cool environment. The person doesn’t need to be exercising. Simply being in a hot space for too long, especially overnight, is enough. Both types are medical emergencies with identical potential for organ damage, but recognizing which risk profile fits your situation helps you know which precautions matter most.