Running in the Heat: How to Adjust Your Pace, Hydrate and Stay Safe

The first hot week of summer catches almost every runner the same way. Your usual easy loop feels like a race, your heart rate climbs while your pace drops, and you finish soaked and wondering where your fitness went. If you went looking for tips for running in the heat after a run like that, you're in good company. Running in hot weather is harder for everyone, elite marathoners included.

It's easy to read a slow, hot run as proof that you're out of shape, and then either skip the next one or push harder to make up for it. Neither helps. Pushing harder in the heat is how ordinary runners end up with heat exhaustion, and skipping every run when the forecast turns hot is how a summer of training quietly disappears.

Your fitness didn't vanish. Your body is spending more of its effort on cooling, and that's a problem you can plan around. Below you'll find why heat slows you down, how much to adjust your pace, what heat is too hot, the best time of day, what to wear, a two-week acclimatization plan, hydration and sodium, cooling methods with evidence behind them, and the warning signs that mean you should stop now.

Why Running in the Heat Feels So Much Harder

Running makes a lot of heat. Most of the energy your muscles burn is released as heat rather than movement, so your core temperature rises on every run, hot day or not. That answers a common question: yes, running increases body heat, and your body's job is to get rid of it as fast as you make it.

On a cool day that's easy. Blood carries heat to your skin, sweat evaporates, and your temperature levels off. On a hot day the air gives your skin less room to shed heat, so your body sends more blood to the skin and sweats harder. That blood is no longer helping your working muscles, and the sweat slowly shrinks your blood volume.

Cardiac drift: why your heart rate keeps climbing

This is why your heart rate creeps up on a hot run even when your pace holds steady. Physiologists call it cardiovascular drift. Coyle and González-Alonso (2001) described it as a steady fall in the amount of blood the heart pumps per beat after the first 10 to 20 minutes of exercise, with heart rate rising to make up the difference.

Heat and dehydration make it worse, and together they're more than the sum of their parts. In a lab study by González-Alonso and colleagues (1997), 15 endurance-trained cyclists exercised in the heat. Overheating alone and dehydration alone, at 4% of body weight, each cut the blood pumped per beat by 7 to 8%, and a faster heart rate covered the gap. When the cyclists were both overheated and dehydrated, the drop per beat was more than twice as large, total blood flow from the heart fell 13% and blood pressure dipped. Their hearts could no longer meet the demand for both cooling and exercise.

Your core temperature sets a ceiling

There also seems to be a temperature at which your body calls time. González-Alonso and colleagues (1999) had seven trained cyclists ride in 40°C (104°F) heat after first lowering, keeping or raising their core temperature in a water bath. Every rider stopped at almost the same core temperature, about 40.1 to 40.2°C. The ones who started coolest lasted longest: 63 minutes, against 28 minutes for those who started warm.

That's your body protecting you, and it gives you the practical rule for the rest of this guide. Anything that keeps your core temperature lower for longer, from a slower pace to a cold drink before you start, buys you more comfortable running.

Humidity matters because sweat only cools you when it evaporates. The dew point, which most weather apps show, is the quickest read of how much moisture is in the air. The higher it is, the more sweat drips off you instead of evaporating, and the hotter the same temperature feels. Whether cold or hot weather is easier to run in overall is a separate question, and our post on whether it's easier to run in the cold or the heat answers it with the marathon data.

How Much to Slow Down When Running in Hot Weather

Expect to run slower, and plan for it before you start. The best numbers come from marathons. Ely and colleagues (2007) matched results from seven North American marathons, over as many as 36 years, against race-day weather measured as wet-bulb globe temperature (WBGT). WBGT is a heat-stress reading that combines air temperature, humidity, sun and wind, and it usually reads a few degrees below the thermometer.

The top three men ran 1.7% slower than the course record in the coolest races (WBGT 5 to 10°C, about 41 to 50°F) and 4.5% slower in the warmest (WBGT 20 to 25°C, about 68 to 77°F). Runners further back, from 25th to 300th place, slowed more than the leaders as the heat rose. For a runner whose cool-weather easy pace is 10 minutes per mile, 4.5% is about 27 seconds per mile, and recreational runners tend to lose more than that. So slowing by 30 to 60 seconds a mile on a hot day is roughly what the data predicts.

What about the charts that adjust your pace by dew point? Treat them as rough coaching guides, not research. The largest study of race weather, El Helou and colleagues (2012), analyzed 1,791,972 finishers in six big-city marathons and found that air temperature had by far the largest effect on running speed. Humidity and dew point had no significant effect of their own. Those were spring and autumn races, though, and on a muggy summer morning you'll feel the dew point the moment your sweat stops drying.

Run by effort, not by pace

Because nobody can predict your exact slowdown, the safest way to pace a hot run is to ignore the watch and run by feel:

• Use the talk test. On easy runs you should be able to speak in full sentences. If you can't, slow down or walk for a minute.
• If you use a heart rate monitor, pace by heart rate. Keep easy runs in the same range you use in cool weather and accept the slower pace that comes with it. Expect the number to drift up late in the run, and slow down when it does.
• Move or shorten hard sessions. Put intervals and tempo runs on the coolest day of the week or on a treadmill, or shorten the hard parts and lengthen the recoveries.
• Cut the distance before you cut the habit. A 30-minute easy run on a hot day keeps your routine going. A long run in the heat is the session most worth moving to dawn or to a cooler day.

What Heat Is Too Hot to Run In?

There's no single cut-off, because it depends on humidity, sun, your fitness, how acclimatized you are and how hard you plan to run. The simplest public tool is the heat index, the "feels like" number that combines temperature and humidity. The US National Weather Service heat index chart classes 80 to 90°F as caution, 90 to 103°F as extreme caution, 103 to 124°F as danger and 125°F or above as extreme danger. Those values are for shade, and direct sunlight can add up to 15°F.

So is 80 degrees too hot for a run? Not for most healthy runners who slow down and drink. Is it okay to run in 90 degree weather? It can be if you're acclimatized and the run is short and easy, but that's the point where the weather service starts warning about heat illness. Running in 100 degree heat is a bad idea for most people, because even dry air at that temperature lands in the extreme caution range or higher before you add the sun.

The first hot days of the year are the riskiest, because nobody has adapted yet. Roberts (2010) reviewed marathons held at northern latitudes and found that when a race started on an unexpectedly hot day with a WBGT above 21°C (about 70°F), and runners weren't acclimatized, it often ended in a mass-casualty medical incident or a cancelled race.

Your own risk rises on top of the weather. The National Athletic Trainers' Association position statement on heat illness (Casa and colleagues, 2015) lists a current or recent illness with fever, sleeping badly or in a hot room, alcohol, and medicines that affect heat control, naming stimulants, antihistamines, anticholinergics, antipsychotics and diuretics. Don't stop a prescribed medicine on your own, but ask your doctor or pharmacist how it affects exercise in the heat. And if you'd like to know which temperature is ideal rather than which is too hot, our post on the best temperature for running covers that sweet spot.

The Best Time of Day for Running in the Heat

Early morning is usually best. The air is coolest around sunrise, the sun is low and the pavement hasn't soaked up a day's heat yet. Relative humidity reads higher at dawn, which puts some runners off, but that's mostly because cool air holds less water. The dew point, the measure of the moisture actually in the air, usually changes much less across the day.

Late evening is the second-best slot, once the sun is down, though pavement and buildings in a city keep releasing stored heat for hours. Whatever the clock says, check the hourly heat index forecast before you head out, and pick routes with shade, water fountains and short loops so you can cut the run short without a long walk home.

What to Wear for Running in the Heat

Less, lighter and looser. Clothing traps a layer of warm, damp air against your skin, and the NATA statement notes that excess clothing reduces your body's ability to control its temperature.

• Light colors in thin, loose technical fabric. Pale clothing reflects sunlight, and loose, breathable fabric lets air move over your skin so sweat can evaporate. Cotton soaks up sweat and gets heavy.
• A light, vented cap or visor and sunglasses. They keep direct sun off your face and eyes. A cap you can soak with cold water doubles as a cooling tool.
• Sunscreen on every exposed patch. A long summer run is a long time in the sun.
• Thin socks that wick sweat, because wet feet blister more easily.
• A handheld bottle or waist belt for anything longer than a short loop past a water fountain.

Heat Acclimatization: Give Your Body 10 to 14 Days

The most useful thing you can do for running in the heat is to let your body adapt. With repeated runs in warm conditions you sweat sooner and more effectively, blood flow to your skin improves, your core temperature and heart rate at a given pace come down and your fluid balance improves (Périard, Racinais and Sawka, 2015). Our review of heat acclimation training research covers the meta-analyses and the mechanisms in depth, so here's how to apply them as a runner.

The timeline is fairly predictable. The 2015 consensus recommendations on training and competing in the heat (Racinais and colleagues) say most adaptations, including a lower heart rate and core temperature and a higher sweat rate, develop in the first week. Near-complete heart and sweating changes take 6 to 10 days, and performance in the heat keeps improving for about 2 weeks. Fitter people adapt faster, in up to half the time. The NATA statement recommends acclimatizing gradually over 7 to 14 days.

A two-week plan for the first hot spell

For athletes preparing for a hot race, the consensus suggests heat sessions of at least 60 minutes a day that raise core and skin temperature and make you sweat. If you're newer to running, there's no need to start there. Regular easy runs in the warm part of the year do most of the work, and the extra days matter more than the extra minutes.

Does running in heat build endurance? It clearly makes you better at running in the heat. Whether it also makes you faster in cool weather is still debated, with some studies finding a small benefit and others finding none, so treat it as a bonus rather than a reason to chase hot runs.

Hydration and Sodium Before and During a Hot Run

Before you run

Start hydrated rather than trying to catch up on the road. Most people who eat and drink normally start the day well hydrated, and the consensus recommendations suggest that during a run of hot days you drink about 6 milliliters of water per kilogram of body weight every 2 to 3 hours, plus the same amount 2 to 3 hours before training. For a 70 kg (154 lb) adult that's about 420 milliliters, or 14 ounces. Pale, straw-colored urine is a reasonable sign you're starting in good shape.

During the run

The goal during a run is to keep sweat losses from going past about 2% of your body weight. That's the point at which the American College of Sports Medicine's fluid position stand (Sawka and colleagues, 2007) and the heat consensus say performance starts to suffer, especially in warm weather. For a 70 kg runner, 2% is 1.4 kg, about 3 pounds.

Sweat rates vary a lot between people, so the most useful number is your own. Weigh yourself without clothes before and after a typical hot one-hour run, then add the weight of anything you drank. The difference is your hourly sweat loss, and each kilogram is about a liter of sweat (each pound is about 15 to 16 ounces). For runs under about an hour, water before and after is usually enough. For longer runs, carry fluid and drink regularly, aiming to replace most of what you sweat.

Sodium, and why more water isn't always better

Sweat carries sodium, typically 20 to 70 millimoles per liter according to the consensus statement, which is roughly 460 to 1,600 milligrams of sodium in every liter you sweat. Heavy, salty sweaters, the ones with white salt marks on a dark cap or shirt, lose the most. For long or very sweaty runs, a sports drink or electrolyte drink during the run, or a salty meal beforehand, replaces some of it. If you have high blood pressure or kidney disease, ask your doctor before adding salt.

Don't overdo the water either. In the 2002 Boston Marathon, Almond and colleagues (2005) found that 13% of the 488 runners whose blood was tested at the finish had low blood sodium (hyponatremia), and 0.6% had a critically low level. The strongest risk factors were gaining weight during the race, which means drinking more than you sweat, and finishing in over 4 hours. The type of drink made no difference. The volume did.

Cooling Strategies That Work

Cooling yourself before and during a run is one of the few heat tricks with solid lab evidence. Bongers and colleagues (2015) pooled 28 studies of exercise in heat above 30°C (86°F) and found that cooling improved performance by about 6.7% on average: 5.7% for cooling beforehand and 9.9% for cooling during exercise. The studies were all in men, and combining several methods worked best for pre-cooling. A separate meta-analysis by Tyler, Sunderland and Cheung (2015) also found that pre-cooling helped most in prolonged exercise, and that cooling during exercise helped too.

• Before the run: drink something cold or an ice slushy in the half hour before you start, take a cool shower, or keep a cold, wet towel around your neck while you get ready.
• During the run: pour water over your head and neck, carry a bottle of ice water, soak your cap, and plan a route that passes fountains, sprinklers or shade.
• After the run: get into the shade or air conditioning, sip something cold and cool your skin with water before you head into a hot car.

One caution comes with pre-cooling. Siegel and colleagues (2010) found that drinking an ice slurry let 10 men run 19% longer at 34°C (93°F) than drinking cold water, but their core temperature at exhaustion was also higher. Feeling cooler can let you run hotter than you realize. Use pre-cooling to feel better, not as a reason to ignore the warning signs below.

Warning Signs of Heat Exhaustion and Heat Stroke

Learn these before you need them. The NATA position statement draws the line between the two clearly, and it's the difference between resting in the shade and calling 911.

Heat exhaustion: stop and cool down

Heat exhaustion means you can't keep going. Signs include heavy fatigue, weakness, dizziness or lightheadedness, headache, nausea or vomiting, and sometimes fainting or collapse, while thinking stays mostly clear. Core temperature is usually below 40.5°C (105°F).

• Stop running and get into shade or air conditioning.
• Take off extra clothing so sweat can evaporate, and cool the skin with cold water, wet towels or a fan.
• Lie down with your legs raised above the level of your heart, and sip fluids if you're not vomiting.
• If you don't feel clearly better within about 30 minutes, or you get worse, get medical help.

Heat stroke: a medical emergency

Exertional heat stroke has two defining signs: a core temperature above 40.5°C (105°F) and a brain that isn't working properly. Watch for confusion, disorientation, loss of balance or staggering, irritability, strange or aggressive behavior, collapse or loss of consciousness. Most runners with exertional heat stroke still have hot, sweaty skin, so don't wait for dry skin before you act.

• Call 911 or your local emergency number.
• Start cooling immediately, on the spot. The NATA statement says cold-water immersion up to the neck is the most effective method, and that when medical staff are on site, people with exertional heat stroke should be cooled first and transported second.
• If there's no tub, use what you have: cold water from a hose or bottles poured over the body, ice-water towels rotated over as much of the body as possible, shade and fanning.

Speed is what saves lives here. The NATA statement reports a 100% survival rate with little or no lasting harm when cooling brings body temperature down within 30 minutes of collapse.

Muscle cramps are a different problem

Cramps during a hot run are common, but they aren't a heat illness in the same sense. The NATA statement calls "heat cramps" a misleading name, because exercise-associated cramps aren't directly caused by a high body temperature. The American College of Sports Medicine's heat illness position stand (Armstrong and colleagues, 2007) notes that they usually respond to rest and replacing fluid and salt.

When to Take It to the Treadmill

Some days the right call is to bring the run indoors. A treadmill in air conditioning lets you hold your planned effort and pace, which makes it the better place for intervals and tempo runs during a heat wave. Put a fan in front of you, because indoors there's no breeze from moving forward, and moving air helps sweat evaporate.

Treadmill running isn't identical to running outside, and our comparison of treadmill vs outside running explains the 1% incline rule and how well treadmill training carries over to the road.

No treadmill? Then don't let the heat turn into a week off. The runners who lose fitness over summer usually aren't the ones who ran slower. They're the ones who stopped. On the hottest days an indoor bodyweight or dumbbell session keeps you moving, and our page on the home workout app with no equipment shows how FitCraft's cardio, strength, yoga and mobility workouts fit around a running week. FitCraft's 3D AI coaches demonstrate each exercise, say your name during the workout and keep your streak going while the weather does its worst.

The Bottom Line

Running in the heat is mostly a matter of adjusting your expectations. Slow down to an effort you could talk through, give yourself 10 to 14 days to acclimatize, start hydrated without drinking more than you sweat, cool off before and during the run, dress light and go early. And stop the moment confusion, staggering or collapse enters the picture, because that's an emergency rather than a bad day.

A hot summer doesn't have to cost you your routine. As one FitCraft user, Katie, put it: "I've tried everything. This is the first time I've stuck with something past two weeks." The slower miles still count, and so do the days you take it inside.