Preventing heat illness comes down to managing three things: how much heat your body absorbs, how well it can cool itself, and how much fluid you’re replacing. Heat illness exists on a spectrum, from mild cramping to heat exhaustion to heat stroke, where the brain and organs start to malfunction. The good news is that nearly every case is preventable with straightforward habits.
How Heat Illness Develops
Your body cools itself primarily through sweating and by pushing blood toward the skin’s surface, where heat can radiate away. When the environment is too hot, the air too humid, or your body too dehydrated to keep up, your core temperature rises. At the milder end, this causes heat exhaustion: headache, irritability, heavy sweating, nausea, and feeling faint. If the process continues unchecked, it can progress to heat stroke, which is defined not by a specific temperature but by signs that the brain is affected. Confusion, delirium, slurred speech, loss of coordination, seizures, or loss of consciousness all signal heat stroke and require emergency treatment.
The important thing to understand is that progression can happen quickly. Someone who seems fine but mentions a headache and seems unusually irritable may already be in the early stages of heat exhaustion. Recognizing those subtle cues, in yourself or others, is one of the most effective preventive tools you have.
Drink Before You’re Thirsty
Thirst is a lagging indicator. By the time you feel it, you’re already mildly dehydrated. The National Institute for Occupational Safety and Health recommends drinking one cup (8 ounces) of water every 15 to 20 minutes when working or exercising in the heat. That works out to about 24 to 32 ounces per hour. Staying ahead of fluid loss keeps your blood volume high enough for your heart to efficiently shuttle heat to the skin.
There is an upper limit. Drinking more than 48 ounces (1.5 quarts) per hour can dilute the sodium in your blood, a condition called hyponatremia, which is dangerous in its own right. If you’re sweating heavily for more than an hour or two, plain water alone isn’t enough. You lose 1 to 4 grams of salt per liter of sweat, and that needs to be replaced. A sports drink, salted snacks, or an electrolyte tablet added to your water will cover this. The goal is to replace both the water and the salt you’re losing, not just one or the other.
Build Up Your Heat Tolerance Gradually
Your body can adapt remarkably well to heat, but it needs time. The process, called acclimatization, involves gradual increases in heat exposure over roughly one to two weeks. During this period, your body learns to start sweating earlier, produce more sweat, and maintain better blood flow to the skin.
NIOSH recommends that someone new to working in the heat start at no more than 20% of a full workload on the first day, adding another 20% each subsequent day. If you’ve done the job before but have been away (after a vacation or illness, for example), you can start at 50% on day one, move to 60% on day two, 80% on day three, and full effort by day four. The same logic applies to athletes returning to outdoor training after time off. Jumping straight into a full-intensity workout during a heat wave is one of the most common triggers for serious heat illness.
Time Your Activity Around the Heat
The simplest prevention strategy is avoiding the worst conditions. Heat index, which combines air temperature and humidity into a single number, is a useful guide. At a heat index of 80°F, you should start taking precautions: access to shade, available water, and the option to take breaks. At 90°F or above, those precautions become critical, and OSHA’s proposed standards call for a minimum 15-minute rest break in the shade at least every two hours.
When you can control your schedule, shift outdoor activity to early morning or evening. If you can’t, seek shade during rest breaks. Shade doesn’t need to be anything elaborate. Trees, a canopy, or a pop-up tent all work as long as they block direct sunlight and allow air to circulate.
Choose the Right Clothing
What you wear matters more than most people realize. Cotton feels comfortable at first, but it absorbs sweat like a sponge and holds it against your skin. Cotton has a moisture regain value of 8.5%, meaning it clings to water rather than letting it evaporate. Polyester, by contrast, has a moisture regain of just 0.4%. Treated with a hydrophilic coating, polyester wicks sweat away from the skin and spreads it across the fabric’s surface, where it evaporates much faster.
Loose-fitting, light-colored, moisture-wicking fabrics are your best bet. Light colors reflect sunlight instead of absorbing it. Loose fits allow air to circulate between the fabric and your skin, which helps the evaporative cooling process. A wide-brimmed hat shields your head and face from direct sun, and this makes a noticeable difference in how hot you feel.
Medications That Raise Your Risk
Several common medications interfere with your body’s ability to handle heat, and many people taking them have no idea. Diuretics (often prescribed for blood pressure) increase fluid loss and can deplete electrolytes. Beta-blockers reduce blood flow to the skin and decrease sweating. Antihistamines with anticholinergic properties, including diphenhydramine (Benadryl) and promethazine, impair sweating and thermoregulation directly.
Some psychiatric medications also affect heat tolerance. If you take any of these drug types, you don’t necessarily need to stay indoors all summer, but you do need to be more aggressive about hydration, shade, and rest breaks. Talk with your prescriber about your specific risk, especially before starting a new outdoor exercise routine or job.
Why Older Adults and Young Children Are More Vulnerable
Adults over 50 store 1.3 to 1.8 times more body heat than younger adults when exposed to the same conditions. This happens because sweating capacity and skin blood flow both decline with age. Older adults increase blood flow to the skin two to three times less than younger people during heat exposure, which means their bodies are slower to dump excess heat. On top of that, research shows that older adults often don’t perceive how hot they’re getting. In one study, elderly participants cycling in 95°F heat didn’t report feeling more uncomfortable than when they exercised at 77°F, even though their skin and core temperatures had clearly risen.
This disconnect between how they feel and how their body is actually doing makes older adults particularly vulnerable. They may not seek shade or water because they genuinely don’t feel the need. Checking in on older neighbors, parents, or coworkers during heat waves is one of the most impactful things you can do. Young children face a different version of the same problem: their bodies produce more heat relative to their size and are less efficient at sweating. They also can’t always articulate that they’re feeling unwell.
What to Do If Someone Shows Signs
If you notice headache, irritability, heavy sweating, nausea, or lightheadedness in yourself or someone else, move to a cool area immediately and start drinking fluids. Remove excess clothing and apply cool water to the skin. These steps are usually enough to reverse heat exhaustion within 15 to 30 minutes.
If there are signs of brain involvement (confusion, slurred speech, loss of consciousness, or seizures), this is a medical emergency. Call 911 and begin cooling aggressively while waiting. Cold-water immersion is the most effective method, cooling the body roughly 2.5 times faster than misting or sponging with water. If a tub or pool isn’t available, pack ice around the neck, armpits, and groin, and wet the skin continuously. Speed matters: the faster core temperature drops, the better the outcome.
A Quick-Reference Prevention Checklist
- Hydrate proactively: 8 ounces every 15 to 20 minutes during activity, with electrolytes if sweating for over an hour.
- Acclimatize: build up heat exposure over 5 to 14 days rather than jumping to full intensity.
- Wear light, loose, wicking fabrics and a wide-brimmed hat.
- Take shade breaks: at least 15 minutes every two hours when the heat index is 90°F or above.
- Know your medications: diuretics, beta-blockers, antihistamines, and some psychiatric drugs all impair heat tolerance.
- Watch others: older adults, young children, and newcomers to hot environments are at highest risk and may not recognize the warning signs themselves.