6 things to know about heat illness

Check on your neighbor

When I picture a heat death, I used to imagine someone collapsing outside in the blazing sun while working at a construction site. That definitely happens, but what I learned while working at CDC is that it can also look like Shirley.

Shirley was 70, recently widowed, and living alone in a home, with diabetes and high blood pressure. All winter, the boy next door would knock before a snowstorm to make sure she was okay. But checking tapered off as the weather warmed (which is typical, as more of us check on neighbors during extreme cold than heat). Once the first heat wave of the year came, she didn’t realize the extension cord powering her portable AC was unplugged. By the time anyone reached her, her body temperature had reached 107°F, and it was too late.

Heat is blanketing much of the country this week, and Shirley’s story will play out again and again. Just look to France: More than 1,000 excess deaths in just five days. But it’s not just deaths. Heat-related emergency room visits will also increase, along with other health problems, from heart attacks to pediatric asthma attacks. One heat wave will cost a state up to $900 million dollars.

During my time at CDC, I learned a lot about heat risk from the subject matter experts. Sure, the usual advice matters (stay cool, drink water), but there’s so much more nuance I wish every American knew as we adapt as humans.

So, here they are. Six hot tips you can use this week from a few of your local epidemiologists.

1. Don’t pay attention to just the temperature

120°F in Phoenix isn’t great, but 90°F in New York can be worse for your health.

You can see it in these maps from Dr. Joseph Allen. The left shows today’s temperatures (“not too bad”). The right shows the risk to your health (“it’s going to be very bad.”)

Local temperatures across the U.S. on the left, and local wet bulb temperature on the right, which incorporates humidity. Source: NOAA and Dr. Joseph Allen

The reason is heat imbalance: Your risk depends not just on temperature but on humidity, air quality, wind, solar radiation, and how long you’re exposed. The danger is when your body makes more heat than it can release. Sweat is the main release valve: evaporating a teaspoon of sweat cools your whole bloodstream by about 2°F. But in humid air, sweat can’t evaporate, so your body can’t cool itself, and risk climbs fast.

So, instead of temperature in your weather app, watch two other metrics:

• Heat index is the “feels like” reading in your app. It combines air temperature with relative humidity to estimate how hot it actually feels to your body because when the air is already full of water vapor, sweat can’t evaporate as easily, so heat builds up. But, importantly, this assumes you’re in the shade and resting, not in direct sunlight, so it can underestimate risk.

• Wet temperatures are more precise because it’s literally a thermometer covered with a wet cloth, mimicking our body’s ability to cool itself with sweat.

Where do you go? The HeatRisk tool was developed just for this.

2. Medications matter

Heat is a double-whammy when it comes to medications:

1. Some medications, including over-the-counter drugs, can make you more sensitive to heat in many ways: reduce the sensation of thirst (e.g., ACE inhibitors); interfere with your body’s ability to regulate heat (e.g., antipsychotics); impair sweating and, thus, cooling (e.g., antidepressants); reduce the ability for blood vessels to dilate (e.g., aspirin); make it harder to sense when you’re overheating (e.g. opioid pain medications); and more. If you’re taking any of these, you must be extra careful.

2. Heat degrades or damages certain medications by altering the chemical make-up. This applies, for example, to insulin, EpiPens, and inhalers. Keep them in a cool place — and don’t leave them in your car.

3. Risk is not uniform

The risk of dying from heat illness depends a lot on where you live — and whether you have air conditioning or a cooling center nearby. In Sun Belt cities like Phoenix, 3 out of 4 heat deaths happen outside, mostly among homeless and outdoor workers.

Among indoor heat-related deaths, an air-conditioning (AC) unit was present in 94% of cases. Among deaths where an AC unit was present, the unit was not functioning in 72% of cases. Source: Maricopa County 2025 Final Heat Report.

In northern cities without universal AC, we worry more about people indoors. In New York City, most heat-stress deaths occur at home. In the 1995 Chicago heat wave, over 700 people died; most of them were older people living alone in apartments without AC. Without air conditioning, indoor temperatures can be much higher than outdoor temperatures, especially at night, and can remain elevated for days after a heat wave.

Beyond environmental factors, there are individual factors that increase risk:

• Pregnant women. There is strong evidence of heat impacting the rate of congenital heart defects, an increase in stillbirths, and more pre-term births.

• Kids with asthma. Heat can provoke asthma symptoms, especially because warmer temperatures can worsen air quality, and humidity can worsen lung function.

• People with cardiovascular disease. Extreme heat makes the heart pump faster and faster to get blood to our fingers and toes. While important, blood goes away from critical organs like the heart.

• Older adults. Aging blunts the heat response and reduces the ability to sweat, medications often impair regulation, and isolation means fewer check-ins.

• Pets get heat stroke too. Here are hot-weather safety tips for pets, including recognizing the signs of overheating, checking the sidewalk for hot pavement, and never leaving pets alone in cars.

4. What about fans? There is a bit of nuance here.

A fan cools you in one way and heats you in another. It speeds the evaporation of sweat, which is your body’s main cooling system, but once the air is hotter than your skin (around 95°F), it also blows heat onto you, like a convection oven. Which force wins depends on the humidity:

• In humid heat, sweat evaporates slowly on its own, so the fan’s evaporation boost matters a lot. It keeps helping well past 90°F.

• In hot, dry heat, sweat already evaporates fast, so there’s little left for the fan to add. The heat gain takes over, raising your core temperature and straining your heart.

This is why CDC and EPA guidance says to stop using a fan once indoor temperatures pass 90°F (or the heat index passes 99°F). It’s a good rule of thumb, but it’s also really time to go to a cooling center: a library, mall, museum, or any public building with AC.

5. By the time you feel thirsty, you’re already behind.

Dehydration is a common driver of heat illness. But thirst doesn’t kick in until you’ve lost about 1–2% of your body weight in fluids. In older adults, that signal arrives even later.

• Drink before you’re thirsty. Aim for about 10 cups of water throughout the day, and more on hot days.

• If you’re working or exercising outdoors, target about 8 ounces every 20 minutes.

• Water is usually enough, but if you’re sweating heavily for more than an hour, you lose sodium and other electrolytes along with water. Replacing fluid losses with only plain water can dilute your system, which creates additional problems.

• Watch what you drink. Remember, alcohol works against hydration. Caffeine, despite its reputation, has largely been cleared. Moderate amounts won’t meaningfully dehydrate you.

6. The signs of heat exhaustion and heat stroke are different

Heat-related illness happens on a spectrum. It can start mildly with a headache, then progress to heat exhaustion, which can progress to heat stroke, a medical emergency. Knowing where someone falls on the spectrum tells you whether to cool them down at home or call 911 right now.

This matters a lot in rural areas, especially when the nearest ER is an hour away, and EMS takes far longer to arrive. The first response often falls to family, coworkers, or whoever happens to be there.

Bottom line

It’s going to be a hot summer, and we are going to need to start adapting to extreme heat more and more. But the best tool is still each other. Stay cool out there and go check on your neighbor, like Shirley.

Love, YLE MD and MW

---

Your Local Epidemiologist (YLE) comprises a team of experts, ranging from physicians to immunologists to epidemiologists to nutritionists, working together with one goal: to “translate” ever-evolving public health science so that people are well-equipped to make evidence-based decisions. The YLE suite of newsletters reaches over 475,000 people across more than 132 countries. This newsletter is free to everyone, thanks to the generous support of fellow YLE community members. To support the effort, subscribe or upgrade below: