How Hot Is Too Hot to Run Outside?

Running is one of the healthiest activities you can pursue. The benefits are well known, from decreasing cardiovascular mortality risk to improving your mental health. But even though running outside provides scenery and fresh air, record-breaking heat waves can quickly put your health at risk.

Being smart about high temperatures doesn’t make you a less committed runner. Even top athletes pay close attention to the heat index, like when the 2021 Oregon Olympic trials suspended competition due to sweltering record highs.

However, there is no hard and fast rule for knowing how hot is too hot to run. As the intensity of activity and the temperature increase, the risk of experiencing a heat-related illness also rises.

The reality is that running in the heat is less about the weather and more about the precautions you take before and during your session. So, let’s explore preparation tactics to equip you for the sun-baked road ahead. Grab your water bottle and sunscreen because we’re turning up the heat!

The Hidden Dangers of Heat-Related Illnesses

Before you lace up your kicks for a summer sprint, you should know how to recognize the causes and signs of heat-related illnesses. The two most common conditions you’ll face as a runner are heat stroke and heat exhaustion, which can be extremely dangerous if not quickly addressed. 

Heat Exhaustion

Heat exhaustion, the precursor to heat stroke, occurs when your body’s natural cooling systems can’t cope with the heat. 

As you run, your internal temperature rises, especially around your legs. Increased blood flow in and out of those muscles warms up the rest of your body, and receptors tell your hypothalamus—the brain’s body monitor and thermostat—to start working to keep your core between 97.5℉ and 99.5℉.

Your body dissipates heat by sweating and moving warm blood toward the surface of your skin. When the mechanisms the hypothalamus triggers aren’t sufficient, you risk heat-related illness. 

If you experience any of the symptoms of heat exhaustion, like dizziness or faintness, immediately stop and find a cool place to recover. Sip water slowly, and use wet cloths to aid in bringing your body temperature down. Seek medical assistance if symptoms worsen or the condition lasts for more than an hour. 

Heat Stroke

Heat stroke is a life-threatening emergency that occurs when your body is no longer able to regulate itself at all, and your temperature begins climbing rapidly. Heat stroke is easily distinguished from exhaustion by the severity of the symptoms: confusion, slurred speech, unconsciousness, and even seizures. Don’t be afraid to call for help if you or someone else experiences a heat-related emergency.

Knowing the signs of heat-related illness will help you react appropriately if the worst happens, but what can you do to prevent heat exhaustion in the first place? Let's take a closer look at the strategies you can use to stay healthy on a high-temp run. 

Precautions for Running in the Heat

Checking the heat index is the first step for deciding how hot is too hot to run. Whereas the temperature is what you read on a thermometer, the heat index tells you how hot it feels when relative humidity combines with the air temperature. The National Weather Service provides a handy tool to determine the heat index here.

High humidity doesn’t just make it seem hotter, either; it also decreases the evaporation rate, making it harder for your body to cool itself with sweat. Because sweating paired with radiation is your body’s primary cooling method, high humidity increases the risks of heat-related illness.

But a high heat index doesn’t signal a blanket ban on running outdoors. Instead, it means you need to take additional precautions if you want to run outside. Here are some of our top recommendations for running with a high heat index:

1. Wear Heat-Resistant Clothing: Halve the heat with loose-fitting, lightweight clothing. Pick socks that keep your feet cool and dry. Low-cut, breathable socks like the CF2 Cushion Gray or the F4 Tech provide excellent venting while offering comfort and protection for your feet and joints.

2. 
   Schedule a Morning Run: Early exercise is great for many reasons, but it’s especially handy to run before the sun’s heat peaks. Also, consider mapping your jogs through shady areas, like paths that go through forests or near tall trees. Trails, bike paths, and public parks can provide ample cover from overhead sunlight.

3. Ease Into It: Running in the heat is no joke, so don’t feel like you must go full speed. When the heat index is high, ending your run a little earlier or taking a slower pace is OK. Start off light before picking up the pace incrementally, and at the first sign of heat overload, stop and cool off. 

4. Hydrate Regularly: Drinking cool water while you run helps your body regulate its temp, but good hydration is an all-day habit. Women and men should drink an average of  2.7 and 3.7 liters of water respectively throughout the day, and that’s before exercising in a high-heat environment.

“Drinking water with amino acids or fats or vitamins or minerals helps the body take up more of the water,” Appalachian State University professor David Nieman said in an interview with TIME. That's one of the reasons milk provides amazing hydration, but it's hardly a smart pre-run choice. Just try to hydrate with meals, and consider an occasional low-sugar sports beverage to replenish electrolytes.

5. Run With a Buddy: There are two compelling reasons to run with a partner. First, you can motivate each other when conditions aren’t great. Second, you’ve got someone to look out for you if the heat is too much. It's also significantly more challenging to respond to the warning signs of heat-related illness alone than with a running buddy. So, consider bringing a pal along for your next summer run!

How to Stay Cool From Head to Toe

So, how hot is too hot to run outside? It’s hard to put a number on it, but if you’re unsure, stay indoors or run in the morning when the temperature is low. Similarly, it’s best to consult a physician or remain inside for people vulnerable to heat-related illnesses.

If you do venture out in high temps, make sure to prepare appropriately, and that starts with your most valuable running asset: your feet. Even when it takes a tank top or sports bra to beat the heat, they’re still wrapped in double layers! A sock that’s engineered to efficiently wick away sweat and keep your feet cool from start to finish is essential for staying healthy while maxing out your performance. That’s why your running sock should not only ensure comfort but also provide protection from hot conditions.

Fitsok creates high-performance socks for runners, by runners. One thing our socks all have in common is that they keep your feet cool and dry no matter how hot it gets. From our popular Pro Trainer series to our boldly patterned Hibiscus Fly June SOM, our designs incorporate innovative technology to wick away sweat and ensure exceptional airflow. 

Explore our collection of heat-resistant socks and find the perfect pair for your next outdoor training session!