---
title: "Running in Hot Weather: How Hot Is Too Hot and How to Run Safely"
description: "Running in hot weather: how to read the heat index, slow your pace, drink and dress for heat, adapt in 1–2 weeks and spot heat exhaustion and heat stroke."
url: "https://wisdomrunning.com/running-in-hot-weather/"
language: "en"
author: ["Alex Gorovenko"]
published: "2026-11-06T12:35:00.000-08:00"
modified: "2026-11-06T12:35:00.000-08:00"
tags: ["Training","Running Tips"]
---

# Running in Hot Weather: How Hot Is Too Hot and How to Run Safely

![Runner on an easy early-morning run along a shaded, tree-lined park path as low summer sun lights the field beside it](https://wisdomrunning.com/content/images/2026/11/running-in-hot-weather-shaded-morning-run.webp)

You can keep up summer training while running in hot weather. That means going out earlier and slowing down by effort instead of pace. It means drinking when you're thirsty. It also means giving your body a week or two to adapt. What you shouldn't do is chase your usual paces through a muggy afternoon or push on through dizziness, nausea or confusion.

This guide shows you how to read the heat index before a run, how to adjust your pace, what to wear and drink, and how to spot heat exhaustion and heat stroke. The advice comes from National Weather Service heat guidance and sports-medicine consensus statements. Studies of runners and marathons in the heat also helped shape it.

## How hot is too hot to run?

No single temperature tells you when running becomes unsafe. What matters is the whole mix: air temperature and humidity, sun and wind, how used to the heat you are, and the intensity and length of your planned run. Dry air is possible on a 90°F morning. A run at 90°F feels different in sticky and still air.

The easiest number to check is the heat index, which many weather apps fold into their "feels like" temperature. It estimates how hot it feels once humidity is factored in, and it assumes you're in the shade. Full sun can push it up by as much as 15°F ([National Weather Service](https://www.weather.gov/safety/heat-index)). Using the NWS formula, 90°F with 50% humidity feels like about 95°F, and the same 90°F with 70% humidity feels like about 106°F ([NWS heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)).

The NWS groups heat index values into four levels ([NWS Amarillo](https://www.weather.gov/ama/heatindex)). They're written for the general public, not for runners, so the right-hand column below is our practical suggestion built on those levels and the sports-medicine guidance that follows. It isn't an official running rule.

| Heat index (shade) | NWS level and what it warns about | Suggested change to your run |
| --- | --- | --- |
| Below 80°F | No heat category | Run as planned, but expect warm runs to feel harder than cool ones |
| 80–90°F | Caution: fatigue possible with long exposure or activity | Keep easy runs easy, slow down by effort, move hard workouts to a cooler time |
| 90–103°F | Extreme Caution: heat cramps or heat exhaustion possible, heat stroke possible | Run early or indoors, keep it short and easy, skip hard workouts unless you're used to the heat |
| 103–124°F | Danger: heat cramps or heat exhaustion likely, heat stroke possible | Move your run indoors or take a rest day |
| 125°F and up | Extreme Danger: heat stroke highly likely | Don't run outside |

Some forecasts also show the wet bulb globe temperature (WBGT), which adds sun and wind to temperature and humidity ([NWS](https://www.weather.gov/news/211009-WBGT)). WBGT numbers run lower than heat index numbers, and you can't convert one into the other. The American College of Sports Medicine's 2023 consensus uses WBGT. For people in the hot Southeast and Southwest who aren't used to the heat or are less fit, it advises more rest and shorter sessions from about 72°F WBGT, cutting intensity and duration from about 78°F, and stopping altogether at 86 to 90°F. Above about 90°F, it cancels activity for everyone. In the northern US, each threshold is about 6°F lower because people there are less used to heat ([ACSM consensus](https://pubmed.ncbi.nlm.nih.gov/37036463/)).

If you want one daily check, the NWS [HeatRisk](https://www.wpc.ncep.noaa.gov/heatrisk/) forecast rates each day from 0 to 4. It factors in how unusual the heat is for the time of year and whether nights stay hot.

Cooler-looking days aren't automatically safe. The ACSM consensus notes that exertional heat illness can happen in conditions as mild as 59°F (15°C), and heat stroke can strike seemingly healthy, fit athletes, especially those pushing hard ([ACSM consensus](https://pubmed.ncbi.nlm.nih.gov/37036463/)).

## Why running in the heat feels so much harder

Running produces a lot of heat in your muscles. When the air is hot, the main way your body gets rid of it is by sweat evaporating from your skin ([Périard 2021](https://pubmed.ncbi.nlm.nih.gov/33829868/)). Humid air already holds a lot of water, so sweat evaporates slowly. More of it drips off without cooling you, and your core temperature rises faster. In one small lab study, eight men cycling at a fairly hard, steady effort at 86°F lasted 68 minutes at 24% relative humidity but only 46 minutes at 80% ([Maughan 2012](https://pubmed.ncbi.nlm.nih.gov/22012542/)).

Your heart pays for it too. Blood has to go to your skin to release heat as well as to your working muscles. Over a steady run, your heart rate gradually creeps up even though your pace doesn't change, and that drift is bigger in the heat ([Wingo 2012](https://pubmed.ncbi.nlm.nih.gov/22410803/); [Lafrenz 2008](https://pubmed.ncbi.nlm.nih.gov/18461000/)). The same pace becomes a harder effort ([Cheuvront 2010](https://pubmed.ncbi.nlm.nih.gov/20689090/)). If your heart rate looks high on easy runs, heat is one of the usual reasons. For more, see [why your heart rate is high on easy runs](https://wisdomrunning.com/why-is-my-heart-rate-high-on-easy-runs/).

Race results show how much this adds up. In an analysis of seven North American marathons, the top men finished 1.7% slower than the course record in the coolest conditions and 4.5% slower in the warmest (a WBGT of about 68 to 77°F), and runners further back in the field slowed even more ([Ely 2007](https://pubmed.ncbi.nlm.nih.gov/17473775/)). A study of 1.8 million finishes in six big-city marathons found the best times at cool temperatures, roughly 39 to 50°F depending on the runner's level, with more runners dropping out as it got warmer ([El Helou 2012](https://pubmed.ncbi.nlm.nih.gov/22649525/)). In Stockholm, the heat slowed back-of-the-pack runners, including those finishing in about 4.7 hours, more than the leaders ([Vihma 2010](https://pubmed.ncbi.nlm.nih.gov/19937453/)).

The slower pace reflects how hot it is on that run. It isn't a sign that you've lost fitness.

## How to adjust your pace when running in hot weather

You'll see charts that add a set number of seconds per mile for every few degrees above 60°F. We couldn't find a study that tests any of these formulas for individual runners, and they disagree with each other. A better guide is effort.

-   **Run by feel or heart rate, not pace.** Keep easy runs at a conversational effort and let the watch show whatever pace that turns out to be. Holding effort or heart rate steady naturally slows you down in the heat ([Yoder 2025](https://pubmed.ncbi.nlm.nih.gov/39981303/); [Wingo 2015](https://pubmed.ncbi.nlm.nih.gov/25943659/)). In lab trials where cyclists set their own pace, they slowed early in the heat, before their body temperature climbed very high ([Tucker 2004](https://pubmed.ncbi.nlm.nih.gov/15138825/)).
-   **Start slower than usual.** The CDC's advice for exercising in heat is to start slowly and pick up the pace gradually ([CDC](https://www.cdc.gov/heat-health/risk-factors/heat-and-athletes.html)).
-   **Move or reshape hard workouts.** Working harder produces more heat. Save intervals and tempo runs for the cooler early morning or a cooler day later in the week. You could use a treadmill instead. Make that workout an easy run if the day is hot.
-   **Shorten or split long runs.** Two shorter runs in the cooler parts of the day, or a long run cut short, beats grinding through midday heat.
-   **Take walk breaks.** A short walk can let your heart rate settle. Walking makes sense and doesn't mean the run is a failure.

Here's how an easy run by effort might look. Take a runner whose usual easy pace is around 10:00 per mile. On a humid summer morning, 10:30 or slower might still be the same easy effort that lets them chat. That's the heat showing up in the numbers. There's no need to fight it.

If you move indoors, our [treadmill pace chart](https://wisdomrunning.com/treadmill-pace-chart/) converts belt speed into minutes per mile.

## When and where to run in hot weather

Timing is often the biggest lever you control.

-   **Early morning is usually best.** The coldest part of the day is typically just after sunrise ([Davis 2016](https://pubmed.ncbi.nlm.nih.gov/26636734/)). Relative humidity is highest then ([NWS](https://www.weather.gov/lmk/humidity)), but because the air is cooler, the heat index is usually lower than in the afternoon. For example, 75°F at 100% humidity feels like about 77°F, while 90°F at 60% feels like about 100°F ([NWS heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)).
-   **Avoid the middle of the day.** The CDC advises limiting outdoor activity at midday, when the sun is hottest ([CDC](https://www.cdc.gov/heat-health/risk-factors/heat-and-athletes.html)). UV rays are strongest from about 10 a.m. to 4 p.m. in the continental US ([CDC](https://www.cdc.gov/skin-cancer/sun-safety/index.html)).
-   **Evenings are a trade-off.** The sun is lower, but ground-level ozone, a lung irritant that forms in sunlight on hot days, is usually worst in the afternoon and early evening ([AirNow](https://document.airnow.gov/air-quality-guide-for-ozone.pdf)).
-   **Check the air quality.** For ozone, people who are active outdoors count as a sensitive group. When the Air Quality Index hits 101 to 150, make runs shorter and easier. At 151 to 200, skip long or intense outdoor runs; run in the morning, when ozone is lower, or move indoors ([AirNow](https://document.airnow.gov/air-quality-guide-for-ozone.pdf)).
-   **Pick shade.** Direct sun can make it feel up to 15°F hotter than the shade-based heat index ([NWS](https://www.weather.gov/safety/heat-index)), so a tree-lined path or the shady side of the street helps.
-   **Plan water.** Choose loops that pass a fountain, a store or your front door. You can drink and refill without carrying much.
-   **Go indoors when it's dangerous.** An air-conditioned gym or a treadmill at home takes the outdoor heat out of the equation. The EPA's air quality guide also suggests moving activity indoors when ozone is high ([AirNow](https://document.airnow.gov/air-quality-guide-for-ozone.pdf)).

## What to wear when running in the heat

Wear loose, lightweight, light-colored clothing ([CDC](https://www.cdc.gov/heat-health/risk-factors/heat-and-athletes.html)). The fabric matters more than the color, so choose thin, breathable material. Sweat that soaks into a heavy, absorbent shirt evaporates poorly and does little to cool you ([Périard 2021](https://pubmed.ncbi.nlm.nih.gov/33829868/)).

A hat or visor keeps the sun off your face and scalp. A brim all the way around also shades your ears and neck ([CDC](https://www.cdc.gov/skin-cancer/sun-safety/index.html)). We found no running study showing a cap lowers heat strain, so think of it as sun protection, not a cooling device.

Use a broad-spectrum, water-resistant sunscreen with SPF 30 or higher, applied about 15 minutes before you head out. Reapply every two hours and after heavy sweating ([American Academy of Dermatology](https://www.aad.org/public/everyday-care/sun-protection/shade-clothing-sunscreen/how-to-apply-sunscreen)). A "water resistant" label only promises 40 or 80 minutes of protection while sweating ([FDA](https://www.fda.gov/drugs/understanding-over-counter-medicines/sunscreen-how-help-protect-your-skin-sun)), and no sunscreen is sweatproof ([AAD](https://www.aad.org/public/everyday-care/sun-protection/shade-clothing-sunscreen/how-to-apply-sunscreen)).

Some runners skip sunscreen because they think it makes them overheat. The research is mixed and the effects are small. One study found no change in sweating or heat tolerance with either mineral or chemical sunscreen ([Fisher 2024](https://pubmed.ncbi.nlm.nih.gov/38126091/)), while another found core temperature about 0.4°F (0.2°C) higher after an hour of cycling with a water-resistant SPF 50 ([Garzón-Mosquera 2026](https://pubmed.ncbi.nlm.nih.gov/42120290/)). Sunburn is the bigger, better-established risk, so wear it.

Heavy sweating also makes chafing more likely. Our [chafing guide](https://wisdomrunning.com/chafing-from-running/) covers how to prevent it.

## How much to drink, and when you need salt

The goal is simple: don't lose too much fluid, and don't drink too much either. Both cause problems.

### Before your run

Start your run well hydrated. Normal meals and drinks over the day usually take care of that. If you need a top-up, the American College of Sports Medicine suggests drinking about 5 to 7 mL per kilogram of body weight at least four hours before exercise ([ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/)). For a 150-pound runner, that's about 12 to 16 fluid ounces.

### During your run

For most runners, the safest everyday approach is to drink when you're thirsty. An international consensus on exercise-associated hyponatremia (low blood sodium) calls this the safest individual strategy ([Hew-Butler 2015](https://bjsm.bmj.com/content/49/22/1432)), and the National Athletic Trainers' Association agrees that drinking to thirst is a safe choice when you don't know your sweat rate ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)). One exception: thirst sensitivity drops after about age 50, so older runners may need to drink a little more deliberately ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)).

On short runs, drinking before and after is often enough. On longer or hotter runs, carry water or plan a route with water stops so you can drink when you're thirsty. Aim to finish having lost less than about 2% of your body weight, which is 3 pounds for a 150-pound runner ([ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/)). Dehydration of as little as 2% can start to hurt your performance and your body's ability to control its temperature ([NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/)).

Don't overdo it. Drinking much more than you sweat can dilute the sodium in your blood. Overdrinking is the main cause of exercise-associated hyponatremia, and finishing a run heavier than you started is a red flag ([Hew-Butler 2015](https://bjsm.bmj.com/content/49/22/1432)). The risk is highest in events lasting more than four hours, for slower runners, and for anyone drinking at every opportunity.

### Find your sweat rate

Sweat rates vary widely. Many people lose about 0.5 to 2 liters (roughly 17 to 68 fluid ounces) an hour, and some lose more ([ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/); [NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)). A simple test tells you roughly where you fall ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)):

1.  Use the bathroom, then weigh yourself without clothes before a typical warm-weather run of about an hour.
2.  Run as normal and note how much you drink. Try not to pee during the run; if you do, estimate how much.
3.  Towel off and weigh yourself without clothes again.
4.  Sweat lost = weight lost + fluid you drank − any urine.

In this example you weigh 150.0 pounds before and 148.5 pounds after. You drank a 16-ounce bottle. The 1.5 pounds of water you lost is about 23 fluid ounces. A pound of water is about 15.3 fluid ounces. Add what you drank (16 ounces). You sweated about 39 fluid ounces (roughly 1.15 liters) in that hour. You lost 1% of your body weight and that is comfortably under 2%.

That number is not a target, so treat it as a ceiling. Drinking somewhat less than you sweat is normal. Drinking more than you sweat is how overdrinking happens. Repeat the test in different weather because your sweat rate changes with heat and humidity and pace.

### Sodium and electrolytes

You lose sodium in sweat, but for runs under an hour, water is enough ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)). Sodium matters more for runs longer than about two hours, for heavy sweaters who lose more than about 1.2 liters (about 41 fluid ounces) an hour, and for people with salty sweat ([sports nutrition position statement](https://pubmed.ncbi.nlm.nih.gov/26891166/)). A sports drink or a salty snack covers it.

Salt tablets have little evidence behind them ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/)), and no sports drink or salt prevents hyponatremia if you overdrink ([Hew-Butler 2015](https://bjsm.bmj.com/content/49/22/1432)). Muscle cramps aren't simply a sign of low salt or dehydration either. In a study of 210 Ironman triathletes, those who cramped had similar blood electrolytes and body-weight changes to those who didn't ([Schwellnus 2011](https://pubmed.ncbi.nlm.nih.gov/21148567/)). If you're on a low-sodium diet or have a heart condition, diabetes or high blood pressure, ask your doctor how to replace salt safely ([CDC](https://www.cdc.gov/heat-health/risk-factors/heat-and-chronic-conditions.html)).

### After your run

For most runs, normal meals and drinks restore your fluid before the next one ([ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/)). If you need to recover fast, such as two runs in one day, drink about 1.5 liters for every kilogram you lost, which works out to about 23 fluid ounces per pound ([ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/)).

A pale-yellow first pee in the morning is a reasonable daily hydration check. Right after you've gulped a lot of water, color tells you little ([NATA 2017](https://pubmed.ncbi.nlm.nih.gov/28985128/); [ACSM](https://pubmed.ncbi.nlm.nih.gov/17277604/)).

## How to get used to running in the heat

Your body does adapt to heat, a process called heat acclimatization or heat acclimation, but it takes repeated exposure. Most of the changes happen within about one to two weeks. Athletes may need six to 10 days for near-complete heart and sweating adaptations, and fitter people adapt faster ([Racinais 2015](https://pubmed.ncbi.nlm.nih.gov/26069301/); [ACSM consensus](https://pubmed.ncbi.nlm.nih.gov/37036463/)).

As you adapt, your heart rate and core temperature at a given pace come down, you start sweating sooner and more, your blood plasma volume rises, and your sweat gets less salty ([NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/); [Périard 2021](https://pubmed.ncbi.nlm.nih.gov/33829868/)). The run that felt awful in the first heat wave starts to feel more manageable.

The heat-acclimatization plans used in research call for at least an hour a day of exercise that makes you hot and sweaty ([Racinais 2015](https://pubmed.ncbi.nlm.nih.gov/26069301/)). Most recreational runners don't train that long, and the research doesn't show exactly how much benefit shorter runs give you. Regular training in mild weather gives partial adaptation, but it can't replace consecutive days in the heat. The first hot days of the season are the riskiest: at one Australian triathlon held in unseasonably hot weather at the start of summer, 15 athletes developed heat illness, while a race in similar conditions two months later had none ([Racinais 2015](https://pubmed.ncbi.nlm.nih.gov/26069301/)).

Here's an example of a gradual two-week build for a runner facing the first hot spell of the year. It follows the consensus idea of adding duration and then intensity over one to two weeks, but the exact steps are our suggestion, not a tested protocol.

| Days | Example plan |
| --- | --- |
| 1–4 | Easy runs of 20 to 40 minutes at a conversational effort, at a warm but not peak time of day. Walk breaks welcome. |
| 5–9 | Gradually lengthen easy runs toward your normal duration. Stay easy. |
| 10–14 | Back to your usual easy volume. Add one moderate workout at a cooler hour, run by effort. |

Adaptations fade without heat, by roughly 2.5% a day in one analysis, but they come back faster the second time ([Daanen 2018](https://pubmed.ncbi.nlm.nih.gov/29129022/)). After a cool week or a trip, ease back in for a few days.

## Ways to stay cool during a run

These tricks mostly help you feel cooler. They don't make a dangerous day safe, so use them on top of slowing down, not instead of it.

![Runner splashing cold water from a park drinking fountain over the back of her neck, her cap and shirt soaked](https://wisdomrunning.com/content/images/2026/11/running-in-hot-weather-fountain-dousing.webp)

*Illustrative scene: wetting your skin and cap may not lower your core temperature, but it can make a hot run feel more bearable.*

-   **Pour water on yourself.** In a lab study of 13 trained runners, dousing themselves with water during a 10K at about 87°F and 47% humidity made them about 27 seconds faster (44:11 versus 44:38) and helped them feel cooler, though their core temperature didn't change ([Anderson 2024](https://pubmed.ncbi.nlm.nih.gov/39231499/)). Wetting your hat, neck and arms at a fountain is a cheap way to feel better.
-   **Try cold drinks, ice or cold towels.** Cooling before or during exercise improved endurance performance in the heat in lab studies of trained athletes ([Tyler 2015](https://pubmed.ncbi.nlm.nih.gov/23945034/)). Outdoors, with natural airflow, the benefit may be smaller ([Racinais 2015](https://pubmed.ncbi.nlm.nih.gov/26069301/)). Treat these as optional extras and test them in training before a race.

## Who should be extra careful in the heat

Anyone can get heat illness, but the risk is higher if you ([ACSM consensus](https://pubmed.ncbi.nlm.nih.gov/37036463/); [NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/)):

-   are new to running or haven't run in the heat yet this season
-   are less fit or carry extra body weight
-   had a hard session in hot conditions in the previous one to three days
-   are getting over a cold, fever, vomiting or diarrhea (sports-medicine groups advise not exercising in the heat while sick or feverish. They say risk can stay higher even after symptoms fade)
-   have had heat illness before (after heat stroke, the risk of another one is higher for a couple of years, and returning to running needs a doctor's clearance)
-   have diabetes or heart disease
-   push yourself hard to keep up or hit a goal no matter what

Some medicines change how your body handles heat, including diuretics, some blood pressure drugs such as beta blockers, antihistamines, some antidepressants and antipsychotics, and stimulants used for ADHD ([CDC](https://www.cdc.gov/heat-health/hcp/clinical-guidance/heat-and-medications-guidance-for-clinicians.html)). Ask your doctor or pharmacist how yours affect running in the heat, and don't stop or change a medicine on your own.

Alcohol increases fluid loss through urine and dulls your sense of heat ([CDC](https://www.cdc.gov/heat-health/hcp/clinical-guidance/heat-and-medications-guidance-for-clinicians.html)), so a hot run the morning after a big night out is a bad idea. Being short on sleep is also a good reason to keep a hot run easy. Older adults, children and pregnant people are among the groups the CDC lists as more vulnerable to heat ([CDC](https://www.cdc.gov/heat-health/about/index.html)).

## Warning signs of heat illness and what to do

Before your next hot run, learn the warning signs. Heat illness can get worse quickly, and the person affected often can't judge it themselves.

| Condition | Signs | What to do |
| --- | --- | --- |
| Heat cramps | Painful muscle cramps or spasms during heavy sweating | Stop, get to a cool place, drink water or a sports drink, and wait for the cramps to pass before running again. Get medical help if they last more than an hour, or if you have heart problems or are on a low-sodium diet. |
| Heat exhaustion | Heavy sweating; cold, pale, clammy skin; fast, weak pulse; nausea or vomiting; cramps; tiredness or weakness; dizziness; headache; fainting | Stop running. Move somewhere cool, loosen clothing, cool your skin with wet cloths and sip water. Get medical help right away if you vomit or get worse, or if you aren't clearly better within 30 minutes. |
| Heat stroke (emergency) | Confusion, odd or aggressive behavior, staggering, collapse or seizure, with a very hot body. The skin is often still sweaty. | Call 911. Start cooling at once: cold-water immersion up to the neck if possible. Otherwise, soak the person with cold water, keep swapping fresh ice-cold wet towels over them and put ice packs on the neck, armpits and groin. No drinks if they're confused or drowsy. |
| Low blood sodium (hyponatremia) | Worsening headache, nausea or vomiting, puffy hands or feet, confusion, weight gain during a long run or race | Get urgent medical care. Don't treat it with more water. Call 911 for confusion. |

These signs and actions follow the [CDC](https://www.cdc.gov/extreme-heat/media/pdf/Heat_Related_Illness.pdf), [NIOSH](https://www.cdc.gov/niosh/heat-stress/about/illnesses.html), the [NATA](https://pubmed.ncbi.nlm.nih.gov/26381473/) and [ACSM](https://pubmed.ncbi.nlm.nih.gov/37036463/) heat-illness statements, and the [hyponatremia consensus](https://bjsm.bmj.com/content/49/22/1432).

Pay particular attention to these points.

-   **Sweating doesn't rule out heat stroke.** Most runners with exertional heat stroke have hot, sweaty skin, not the dry skin often described for heat stroke in people who aren't exercising ([NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/)). Changes in behavior or thinking are the key warning sign.
-   **Cooling fast saves lives.** When cooling starts within minutes, survival is close to 100% ([NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/)). At the Falmouth Road Race in Massachusetts, all 274 runners treated for heat stroke over 18 years survived with on-site cold-water immersion ([Demartini 2015](https://pubmed.ncbi.nlm.nih.gov/24983342/)). Call 911 and start cooling at the same time; don't wait for the ambulance to begin.
-   **Home thermometers can't rule it out.** Forehead, ear, mouth and armpit readings aren't accurate after hard exercise in the heat ([NATA 2015](https://pubmed.ncbi.nlm.nih.gov/26381473/)).
-   **Collapsing right after a finish line** is usually a brief drop in blood pressure from stopping suddenly, and it typically passes within 30 minutes of rest with the legs raised. If the runner is confused, treat it as heat stroke ([ACSM consensus](https://pubmed.ncbi.nlm.nih.gov/37036463/)).
-   **Tea- or cola-colored urine** or severe muscle pain after a hot run needs prompt medical care ([NIOSH](https://www.cdc.gov/niosh/heat-stress/about/illnesses.html)).

## Before your next hot run

Run through this quick check:

-   Check the heat index (or WBGT). The forecast should be for the time you'll actually go out to run. Use the table above to adjust.
-   Run at the coolest time you can on a shaded route with water.
-   Wear a hat and sunscreen along with light and loose clothing.
-   Decide in advance to run by effort, not pace, and to cut the run short if you feel dizzy, sick or confused.
-   Give your body time to adapt. Keep the next one to two weeks easy if you're new to the heat.

## Frequently asked questions

### Is 70 degrees too hot to run?

No. Running at 70°F is fine for most runners, though you'll feel it more in humid air or full sun. Race data show marathon times are fastest in cooler air and slow as it warms, so an easy pace may feel a little harder than usual. Heat illness can still happen during hard efforts on mild days, so stop if you feel dizzy, sick or confused.

### Is it okay to run in 90 degree heat?

Humidity and sun make the difference. At 90°F with 50% humidity the heat index is about 95°F. That's the National Weather Service's Extreme Caution level. At 70% humidity the heat index reaches about 106°F (the Danger level). Keep runs short and easy at the Extreme Caution level. Run early in the day or indoors. Hard workouts can wait unless you're used to the heat. When the heat index reaches the Danger level, move indoors or rest for the day.

### Can you run in 100 degree weather?

Even when the air is dry, 100°F means serious heat. At 20% humidity the heat index is about 98°F (the Extreme Caution range). At 40% humidity it's about 109°F (the Danger range). A treadmill or an air-conditioned gym is a better choice for most recreational runners. So is a rest day. If you do run outside, make sure the heat index is below the Danger level, keep it short and easy, carry water and know the signs of heat exhaustion and heat stroke.

### How long does it take to get used to running in the heat?

Fitter runners adapt faster. Most heat adaptations happen within about one to two weeks of regular runs in warm conditions. Your heart rate and body temperature come down at a given pace and you start sweating sooner. The benefits fade over a few weeks without heat but come back faster the second time.

### Is it better to run in the morning or evening when it's hot?

Morning is usually better. Just after sunrise is when the air is coolest. Humidity is higher then, but the heat index is usually lower than it is later in the day. The sun is lower in the evening, but ozone tends to stay high into the early evening.

### Should I take salt tablets when running in the heat?

Usually not. For runs under an hour, water is enough. Sodium matters mainly for heavy sweaters or people with salty sweat or for runs longer than about two hours. A sports drink or a salty snack covers that. Salt tablets have little evidence behind them, though. If you drink far more than you sweat, no salt prevents low blood sodium.

### Why is running in humidity so much harder?

Sweat evaporating from your skin is how your body does most of its cooling. That evaporation slows in humid air. More sweat drips off without cooling you. Your core temperature and heart rate climb faster at the same pace because of it. In one small lab study, cyclists lasted 68 minutes at 24% humidity but only 46 minutes at 80%, in the same 86°F heat.

### What are the first signs of heat exhaustion when running?

Watch for heavy sweating, cold and clammy skin, dizziness, headache, nausea, cramps, weakness or a fast, weak pulse. Stop, get somewhere cool, loosen your clothing, cool your skin and sip water. Get medical help right away if you vomit, get worse or aren't clearly better within 30 minutes. Call 911 if a runner becomes confused or collapses.