Working out in the heat: what heat and humidity change, and how the body adapts

Heat doesn't stop you training. It changes how long a hard effort lasts, how the same pace feels, and which hours of the day make sense outside. This page covers what lab studies and marathon records measured for heat, humidity, energy use and running, how the body adapts to heat and how long that takes, what the American College of Sports Medicine says about drinking, and what South Florida's weather-station numbers mean for a training week.

It's a training page, not a medical one. Heat-related illness is a medical topic: the CDC's page on heat and athletes covers how to spot it and what to do.¹ I'm a fitness coach, not a doctor: if you have a medical condition, or you feel unwell in the heat, stop and talk to a doctor. Heated studios, saunas, cooling gear and supplements aren't covered here.

Working out in the heat at a glance

Is it bad to work out in the heat?

It's harder, and the research is clear on that. A 2015 expert consensus on training and competing in the heat opens with it: exercising in the heat induces thermoregulatory and other physiological strain that can impair endurance exercise capacity.⁷ In one lab study, eight healthy men rode to exhaustion at about 70% of their maximal oxygen uptake four times, at 3.6, 10.5, 20.6 and 30.5 °C (about 38, 51, 69 and 87 F) and 70% humidity. They lasted longest at 10.5 °C, 93.5 minutes on average, and shortest at 30.5 °C, 51.6 minutes; the authors describe the pattern as an inverted U.²

Harder isn't the same as off limits. The same consensus says the most important thing a person can do to reduce the strain and perform better in the heat is to heat acclimatize, which is covered below.⁷ For the safety side, the CDC's own advice to people exercising in the heat includes scheduling workouts earlier or later in the day when it's cooler, and starting slow and picking up the pace gradually.¹

Working out in humidity: does it make a difference?

Yes. Humidity is what makes warm air feel hotter, and it shortened a workout in the lab. The National Weather Service's heat index is a measure of how hot it really feels when relative humidity is factored in with the air temperature; its example is 96 F at 65% humidity, which feels like 121 F.⁸

In a study at Loughborough University, eight men cycled at 70% of their maximal oxygen uptake until exhaustion in a chamber held at 30.2 °C (about 86 F), at 24, 40, 60 and 80% relative humidity. They lasted 68, 60, 54 and 46 minutes on average, and the times at 60% and 80% were significantly shorter than at 24%. Core temperature and heart rate didn't differ between the trials; the sweating rate was higher at 60% and 80%. The authors conclude that exercise capacity at moderate intensity in a warm environment is progressively impaired as humidity rises.³

Is working out in humidity good for anything? No study here shows a benefit of humidity on its own. What the consensus does say is that adapting in dry heat improves exercise in humid heat, and the other way round.⁷ For outdoor athletes, the weather service points to a different index, the wet-bulb globe temperature (WBGT), which uses temperature, humidity, wind and sun, and notes that the American College of Sports Medicine bases its guidelines for the intensity of sport practices on it.⁸

Does working out in the heat burn more calories?

Not by the lab's usual measure. Labs estimate energy use from oxygen uptake, and two 1994 studies from the same group compared trained men cycling about 40 minutes at 70% of their maximal oxygen uptake at 40 °C (104 F) and at 20 °C (68 F), both at 20% humidity:

• In 12 endurance-trained men, mean oxygen uptake didn't differ between the hot and the cool ride; heart rate was higher in the heat.⁴
• In 13 endurance athletes, mean oxygen uptake was 0.13 litres a minute lower in the heat, and heart rate was higher.⁵

So in both, the heart worked harder in the heat while the oxygen used, the lab's usual measure of energy, did not go up. The heat did change the fuel: more muscle glycogen was used in the hot ride, and the first study's authors suggest the muscles' energy use may rise in the heat, with the extra met by processes that don't use oxygen, which oxygen uptake doesn't count.⁴ And the weight that comes off during a session is counted as sweat: the ACSM's position stand on exercise and fluid replacement estimates a person's sweat rate from body weight measured before and after exercise, and after exercise the goal is to replace the fluid lost.⁹ So a lighter reading on the scale right after a hot workout is the measure of sweat to replace, not a result.

Is it better to work out in the heat or cold?

For endurance, cool beat both extremes in the lab: the eight men in Galloway 1997 lasted longest at 10.5 °C and less at 3.6 °C, as well as at 20.6 and 30.5 °C.² The cold has its own cost. In a walking study of 11 people aged 19 to 27 at six temperatures from 20 °C down to −10 °C (68 F to 14 F), oxygen uptake was significantly higher in the cold than in the warm conditions; at −10 °C it rose from 1.60 to 1.89 litres a minute between the first and second unloaded walks.¹⁰ Neither study tested lifting, and for most weekly training the practical answer is on what time to work out: the time you can repeat, at the hours your weather allows.

Running in the heat

Runners slow down as it warms up, and the race records show it. Ely 2007 matched up to 36 years of results from seven North American marathons with the weather on race day, grouped by WBGT from about 5 to 25 °C. The top men finished 1.7% slower than the course record in the coolest races and 4.5% slower in the warmest, and the 25th-, 50th-, 100th- and 300th-place finishers slowed more than the faster runners as it warmed.⁶ A second dataset, about 1.8 million finishing performances in six big marathons in Europe and the U.S. from 2001 to 2010, found that when the temperature rose above the optimal range for a runner's level, running speed fell and more runners dropped out; air temperature mattered above every other weather factor tested, and humidity had no significant effect in those races.¹¹ That is race-day weather across many temperatures; in the lab study above, with the air held at 30.2 °C, humidity made a clear difference.³

Running in the heat: tips (practical, not findings).

• Move the run, not the plan. Earlier or later in the day, when it's cooler, as the CDC suggests for workouts on hot days.¹
• Run by effort, not pace. Expect the same pace to cost more. Go by the talk test on zone 2 cardio on easy days and let the watch read slower.
• Start slow and pick up the pace gradually, as the CDC puts it.¹ Across the first hot weeks of the year, do the same with the length of your runs.
• Have an indoor fallback. On the worst days, a treadmill run is close to a road run in most measures (treadmill vs outdoor running). New to running? Start with running for beginners.

Heat acclimation vs acclimatization: how the body adapts

The two words describe the same process in two settings. The consensus calls adapting by training in naturally hot conditions acclimatization, and training in an artificially hot indoor room acclimation; the adaptations are similar, but training outdoors is more specific to the conditions you'll compete in.⁷

• How it works. By the consensus, the benefits come from more sweating and skin blood flow, an expanded plasma volume and steadier circulation, and better fluid balance.⁷
• How long it takes. Repeated exercise-heat exposures over 1 to 2 weeks. Most adaptations, such as a lower heart rate and a higher sweat rate, develop in the first week, then more slowly over the next 2 weeks. For athletes, the consensus advises sessions of at least 60 minutes a day.⁷
• What it looked like outdoors. Nine trained cyclists trained for 2 weeks in natural heat of about 34 °C (about 93 F) and 18% humidity. Within the first 5 to 6 days their sweat rate rose about 20%, with no further change after; their power in a 43.4 km time trial in the heat rose 11% by the second trial and another 5% by the third.¹²
• Heart rate came down. In the 1994 study of 13 endurance athletes, 7 days of acclimation lowered their mean heart rate during the 40-minute ride in the heat.⁵
• How long it lasts. It fades more slowly than it builds: the consensus says most of the benefits, such as the heart rate and core temperature changes, are kept for 2 to 4 weeks. Exercise in temperate conditions brings partial acclimatization but can't replace consecutive days of training in the heat.⁷

Does heat training help when it's cool?

The two studies disagree. In Lorenzo 2010, 12 trained cyclists did 10 days of exercise at about 50% of their maximal oxygen uptake at 40 °C; their maximal oxygen uptake rose 5% in cool conditions (13 °C) and 8% in the heat (38 °C), and their time-trial work rose 6% and 8%, while 8 matched controls who trained in the cool had no changes.¹³ In Karlsen 2015, 18 trained male cyclists trained for 2 weeks outdoors in about 35 °C heat (9 of them) or in the cool (9): time-trial performance in the heat improved 16%, but maximal oxygen uptake, peak power and a 43.4 km outdoor time trial at about 5 to 13 °C didn't change.¹⁴ The clear benefit of training in the heat, in both, is being better at exercising in the heat.

Drinking: what the ACSM states

The American College of Sports Medicine's position stand on exercise and fluid replacement sets the goal of drinking during exercise as preventing excessive dehydration, more than a 2% loss of body weight from water deficit. Because sweating rates vary a lot between people, it recommends customized fluid replacement, using body weight before and after exercise to estimate a person's sweat rate.⁹ The heat consensus adds: start training in a euhydrated state and minimize dehydration during it.⁷ This page doesn't give an amount: the ACSM says sweating rates vary between people and recommends a customized plan.

Training in South Florida heat

Diogo is based in South Florida and coaches online, so this section looks at the numbers there. NOAA's 1991 to 2020 normals put the Miami International Airport weather station at 91.4 days a year at or above 90 F, with an average July high of 90.6 F.^15,16 Of those 91.4 days, 63.6 fall in June, July and August, 18.8 in September to November and 9.0 in March to May; December to February averages none.¹⁵ Elsewhere in Florida, the Orlando International Airport station averages 109.9 days a year at or above 90 F, Tampa International Airport 94.3 and Jacksonville 80.1.¹⁵

The hottest July isn't what sets South Florida apart. Miami's July high is within half a degree of Atlanta's, Denver's and Charlotte's, but its station averages roughly twice as many days at or above 90 F, 91.4 against 42.2 to 49.2, and the season runs longer: 18.8 of Miami's fall between September and November, against 3.9 to 5.2 at the other three.^15,16 Then add the humidity, which the heat index exists to account for.⁸ The station is one spot at the airport, and your street may run hotter or cooler. Summer heat by metro has the same numbers for 48 metros. Online coaching pages: South Florida and Florida.

A hot-month week (practical)

A practical layout, not a protocol from a trial:

On the worst days, a 30-minute session indoors beats an outdoor session you cut short. The machine setup is on the treadmill workout.

Common questions

Are there benefits to working out in hot weather?

It's harder: in the lab the same effort ran out sooner in the heat and in humid air.^2,3 The benefit the research shows is adaptation: repeated training in the heat makes exercise in the heat easier, over about 1 to 2 weeks.⁷ Whether it also helps in cool weather, the two studies above disagree.^13,14

How do you work out in hot weather?

Schedule the session earlier or later in the day, when it's cooler, and start slow and pick up the pace gradually, as the CDC advises for exercise in the heat.¹ Expect the same effort to run out sooner,^2,3 and give the body 1 to 2 weeks of repeated sessions in the heat to adapt.⁷ On the worst days, move it indoors or into the water (practical): a pool workout or aqua jogging.

How do you run in hot weather?

Earlier or later in the day, starting slow and building up gradually, as the CDC puts it,¹ and by effort rather than pace (practical). The full list is under running in the heat.

Does exercising in the heat burn more calories?

In two lab studies of trained men at 40 °C against 20 °C, oxygen uptake, the usual measure of energy use, was the same or slightly lower in the heat, while heart rate was higher; one study's authors suggest the muscles' energy use may still rise, met without oxygen.^4,5 The weight lost to sweat is fluid.⁹

Is working out in humidity bad?

It shortens the workout. At about 86 F, eight men lasted 46 minutes on average at 80% humidity against 68 at 24%.³ Slow down, move the session to a cooler hour, or take it indoors.

Is running in the heat harder than running in the cold?

In the marathon records, yes: runners slowed as the weather warmed from cool to warm, and slower runners slowed more.⁶ Very cold air has its own cost: oxygen uptake was higher in the cold in a walking study.¹⁰

How long does heat acclimation take?

About 1 to 2 weeks of repeated exercise in the heat, with most of the change in the first week, by the consensus.⁷ In nine trained cyclists in natural heat, the sweat rate had risen within 5 to 6 days.¹²

What's the difference between heat acclimation and acclimatization?

Where it happens: acclimatization is adapting in naturally hot weather, acclimation is adapting in an artificially hot room. The changes are similar.⁷

If you'd rather have the summer planned

Moving runs to the early hours, keeping the lifting where it is and rebuilding the week when a heat wave hits is a planning job. Coaching is the version where your workouts are built around the hours you actually have, and a weekly call looks at the week you just had and rebuilds the next one. See how coaching works, and apply when you're ready. Otherwise: move one outdoor session to the morning this week.