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How Hot is too Hot? The Ultimate Guide to Working Out in the Summer

June 12, 2019, from the blog on

by Matthew Seiltz


Balmy breezes, the sun shining overhead, birds twittering in the sky — with sunny days all around, who wouldn’t want to take their exercise outdoors?

But what about those unbelievably hot days? You know the ones — the sun burns on your neck, the pavement radiates heat, and just stepping outside saturates your armpits.

Now, of course, you know that it’s important to stay hydrated every time you exercise, but days, where the heat is absolutely blistering, come along every year and may make you pause. Maybe you worked out anyway and found you had trouble catching your breath; perhaps you became dizzy, lightheaded, and felt you couldn’t keep up with how much you were sweating — maybe you even quit your workout.

On days like these, you have to wonder: Is this safe?

In 1980, approximately 1,700 people died because they were unprepared for an extreme wave of heat. In 2003, complications related to a heat wave killed 14,800 people in Paris — and those numbers weren’t even directly related to exercising during extreme heat. Clearly, tremendous heat can be dangerous, but add exercise to the equation and the question rapidly becomes even more serious.

Just how hot is too hot?

Your body in the heat

To understand the answer to that question, you need to know how your body reacts to extremes of heat and cold. Through a process known as thermoregulation, your body strives to maintain a temperature between 97.7 to 99.5 degrees Fahrenheit (36.5 to 37.5 degrees Celsius).

Your hypothalamus — a gland within your brain — is in charge of managing the core temperature of your body. If outside weather is extreme enough to cause a change in your body’s core temperature, your hypothalamus will trigger a specific process to heat or cool your body back to within that normal range.

So, when your hypothalamus registers that your body’s core temperature is rising because it’s hot outside, it gets to work. To get rid of extra heat, your hypothalamus increases circulation, moving the blood in your body toward the surface, dilating superficial blood vessels so that the heat can be dissipated through your skin. When this occurs, you may notice your veins “popping” and your skin flushing.

Alongside increasing circulation, your hypothalamus activates the sweat glands and you begin sweating. The evaporation of the water released onto your skin cools it down, helping reduce your temperature. Finally, your thyroid is also stimulated to lower heat created through metabolic processes.

Source: OpenText

Problems in the heat

That’s what is supposed to happen — under normal conditions. But if you’re exposed to high levels of heat and humidity for long enough, don’t drink enough fluids, and sweat excessively, these cooling systems can fail, and you may become ill.

Heat-related illnesses span a spectrum of severity — but if you notice symptoms beginning of even the “lowest-severity” you need to take action and begin correcting the problem before it becomes one.

Heat cramps: When exercising in the heat, you may get painful cramps. The affected muscle(s) may feel hard to the touch and spasm or give you sharp pains. At this point, your body temperature may still be within normal limits.Heat syncope: Syncope refers to a loss of consciousness, what we may recognize as exercise-related collapse. Just before this happens, you may feel lightheaded or faint. This usually happens when temperatures are high and you’ve been standing or exercising for a long time or when you stand quickly after sitting for an extended period.Heat exhaustion: Not exclusive to exercise, heat exhaustion occurs when your body temperature exceeds normal limits and gets as high as 104 degrees Fahrenheit. You may feel nauseous, weak, cold, faint, or experience headaches and vomiting. You continue to sweat at this point, but your skin may feel cold and clammy. Heat stroke/Sunstroke: If you don’t treat heat exhaustion, it leads to heat stroke, or “sunstroke.” At this point, your core body temperature is greater than 104 degrees Fahrenheit and you are in a life-threatening emergency. Although your skin is no longer capable of sweating, it may feel either dry or moist to the touch. You may be confused, irritable, and experience heart arrhythmias, alongside the symptoms of heat exhaustion. You must get immediate emergency treatment or you risk brain damage, organ failure, and death.

So if you’re exercising in hot weather, pay attention to your body’s reaction. Alongside the symptoms listed above, if you notice you’re sweating more than normal, you feel dizzy or lightheaded, or think your heart is racing faster than it should — stop exercising. You need to get somewhere cool and start hydrating immediately. If possible, get someone to stay with you. And if you have signs of the more serious conditions — particularly heat stroke — see a doctor right away.

It’s important to take these signs seriously. For many athletes, whether to take a break or push through symptoms of a heat-related illness isn’t even a question. And if you continue past where you should, you may face drastic consequences. These symptoms don’t necessarily mean you’re out of shape and need to work harder — they could be early signs of a potentially life-threatening condition that needs to be addressed.

How to tell when it’s too hot out

There is no truly hard-and-fast rule of determining when it is too hot out to exercise outside. That said, there are some subjective determining factors that you should call to mind when it comes to deciding whether to exercise outside.

First, how acclimated are you to the area? If you’re from a northern state in the U.S. like Michigan or North Dakota, and you’re in Southern California or Texas — how accustomed is your body to the higher temperatures? Are you used to humid or dry heat? Being used to your environment can reduce the physical stress on your body.

Second, how fit are you? Research suggests that people who are aerobically fitter may adapt to temperature extremes more quickly than less-fit individuals.

Third, what do the pros say? As long as you have internet access, you’ve got a great tool at your disposal: the WetBulb Globe Temperature — or WBGT. The WBGT measures when your body will be stressed by heat when exposed to direct sunlight. Unlike the heat index, which only takes into account temperature and humidity, the WBGT takes into consideration “temperature, humidity, wind speed, sun angle, and cloud cover.” WBGT is used as a tool by the military and OSHA to manage workload for outside personnel.

By using the tool in the link above, you can select your location to see the WBGT score; then, you can compare it to the numbers below (also available through the link) to determine how long it’s safe to exercise outside before needing a break:

Suggested Actions and Impact Prevention

WBGT (F)EffectsPrecautionary Actions<8080-85Working or exercising in direct sunlight will stress your body after 45 minutes.Take at least 15 minutes of breaks each hour if working or exercising in direct sunlight85-88Working or exercising in direct sunlight will stress your body after 30 minutes.Take at least 30 minutes of breaks each hour if working or exercising in direct sunlight88-90Working or exercising in direct sunlight will stress your body after 20 minutes.Take at least 40 minutes of breaks each hour if working or exercising in direct sunlight>90Working or exercising in direct sunlight will stress your body after 15 minutes.Take at least 45 minutes of breaks each hour if working or exercising in direct sunlight

Can I burn more fat if I exercise in the heat?

It makes sense to wonder if exercising on a particularly hot day actually causes you to burn fat — after all, if your body is that much hotter and you’re sweating much more.

Well, it’s a bit more complicated than that. Studies show that when exercise is performed within climates of high ambient temperatures, the heat can actually have an effect on your body’s hormonal and metabolic response to exercise. These same studies actually show a consistent shift from breaking down fat cells for energy, to breaking down carbohydrates for energy.

Essentially, when exercising in extreme heat, the energy demand becomes too high to break down much fat and instead turns towards using carbohydrates. So all that extra sweat is really just water and salt — not fat burn.

And yet heat may still play a positive role in improving your body composition. Two ways it can do this include heat shock proteins (HSP) and Human Growth Hormone (HGH).

Heat shock proteins

Even without exercise, exposure to heat may cause HSPs to activate. Heat shock proteins (HSP) exist inside cells and aid in muscle protein synthesis and repair. When exposed to thermal stress, they increase to meet the demand.

For example, in a study performed on 25 young, healthy adults, the participants were placed in a heat stress chamber with and without heat for 30 minutes on separate days. Blood samples were obtained before and after. Following the test, HSPs were increased by 49%, among other markers. The researchers found that the heat stress had caused physiological responses similar to those obtained during exercise, although to a lower degree.

This is useful, as it suggests that exposing yourself to greater heat can potentially afford you similar benefits as exercise — or, if combined, increase the response already gained by exercising.

Human Growth Hormone

Synthetic HGH has long been used as a performance-enhancing drug, and not for nothing — it increases lean mass, reduces body fat, and improves performance. But it’s actually naturally produced by your body, with many roles. One of the best parts is, you can enhance this natural ability through exercise.

But alongside that, heat may give your HGH a natural boost, too.

In an older study, 10 healthy males and 7 healthy females were exposed to the dry heat of a Finnish sauna twice per day for 7 days. By day 3, their HGH had increased by 16-fold. Although the HGH levels decreased after that 3rd day, that’s a significant jump.

Another study, with similar conditions (Finnish sauna), took a larger sample size (55 volunteers). While in the Sauna, volunteers’ serum HGH increased by 142%. Within an hour of being removed from the heat of the sauna, HGH had decreased to normal levels.

Given the role that HGH plays in maintaining and building muscle mass, as well as burning fat mass, it’s an important hormone for changing your body composition — so if heat can increase it, even for brief periods of time, that’s a positive, and thus — indirectly — heat may not hurt your body composition goals. But that’s a theory that still needs more research.

Tips for exercising on hot days

Now, with all that said — you’ve decided today, you’re going to workout outside and the WBGT score for your area is pretty high. There are quite a few things you can do to mitigate the effects of extreme heat on your workout.

Consider precooling

If it’s especially hot or humid and you plan on going for a run, lifting weights, playing a sport, etc. outside, consider “precooling” ahead of time. A study decided to test 3 precooling protocols on 12 male team-sport athletes performing bicycle sprint intervals in hot, humid conditions (about 33.7 degrees Celsius, with 51.6% relative humidity). The 3 precooling methods to be tested were: ice vest, cold water immersion, and ice packs placed on the legs.

The rate of increase of heat strain was faster in control, as expected, but similar to the vest. Cold water immersion and the ice packs reduced increases in muscle temperature until minute 16 of the 40-minute test and the ice packs for the entirety. The conclusion was that precooling the body part primarily in use (in this case, ice packs on the legs) was the most effective in preserving performance; however, the study showed that cold water immersion is also an effective method for slowing the rise of heat stress.

Because of this, it may an effective way to help ward off heat-related illness.

Get acclimated

If you’re new to the area — remember our example from earlier? — take some time getting acclimated to the weather. Most people’s bodies take approximately 1-2 weeks of daily heat exposure (about 90 minutes) to get used to it. If you’re in better aerobic shape, it may take you less time. But the point is to take it easy during that acclimation period — your body needs to get used to the new stress.

Pick the time of day — and dress right

Another thing you can do is choose the time of day that you go out to exercise. The sun is at its highest in the early afternoon, so foregoing outdoor workouts during that time is a good way to limit your exposure — and risks — to heat. This may mean going out earlier in the morning or later in the evening.

Regardless, if the sun is out, wear sunscreen and dress appropriately — loose, lightweight clothes that are light-colored and made of breathable fabric, like polyester.

Drink fluids

Of course, you know drinking fluids is important. But how much? Does it change with the heat? The American College of Sports Medicine has some excellent guidelines for you:

Before exercising Drink water with each meal you have before exercising and try to time your workout at least 8-12 hours after the previous session. If you’re expecting a long heat wave, tracking your daily weight before and after exercise sessions can help you track fluid loss. And consider drinking 16-20 fluid ounces 4 hours before exercise.During exercise Drink based on thirst, which will likely increase with extreme heat. But be careful of overhydrating — drinking too much water may increase your risk of developing hyponatremia, or “water intoxication.” If you’re planning on a long run or other prolonged exercise sessions, drinking a beverage that’s 6-8% carbohydrate (like a sports drink) may be helpful.After exercise If you find you’ve lost weight after exercising, drink 16-24 fluid ounces for each pound. Any postworkout meals should be sure to include fluids.  

Know yourself

But perhaps most importantly, when considering exercising in extremely hot weather, you need to know yourself. How fit are you? Do you have any medical conditions that might make you more susceptible to the heat when exercising? Are you new to the climate? Are you capable of keeping yourself accountable if you begin to experience symptoms?

What NOT to do

Just as important as knowing what to do are the things you need to avoid. They start out simple: don’t wear heavy, dark clothes or skip out on drinking water; and range to more serious: ignoring signs of heat-related illnesses, or going from Michigan in the winter to SoCal and jumping into a full-speed HIIT workout.

But you should also avoid caffeine and caffeinated soda if you’re exercising in the heat. A study performed on 12 healthy volunteers tested blood samples both pre- and post-workout. Prior to exercise, participants were given either a soft drink or water. Post-workout, the subjects who were given the caffeinated soft drink had lab values that suggested drinking a caffeinated soft drink while exercising worsened dehydration and increased the strain on their cardiovascular system.

So with hot weather already dampening the ability of your body to respond to the stress placed on it, why make it worse by further dehydrating yourself?


So ultimately, it’s perfectly acceptable to exercise outside when it’s hot out — even in extreme heat. How hot is too hot is somewhat personal and comes down to a handful of factors: the WBGT score, your fitness level, and acclimation, and the level of hydration you can reach and maintain.

Theoretically, if “all your ducks are in order” you could exercise in just about any heat — humans are miraculously adaptable creatures. But, since most people aren’t perfect, you need to use common sense. If in doubt, consult the factors mentioned and the WBGT.

When it comes to improving your body composition, consistency is key — so hot weather doesn’t need to stop you from exercising. You can always workout indoors or ease off the intensity for today. Something is still 100% more than nothing.


Matthew Seiltz is a writer and lifelong strength and fitness enthusiast. When not writing or working out, he can be found with a book or spending time with his wife and sons outdoors.

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