Is It Too Hot To Run Outside? Here's How To Tell.

In plenty of places, the start of summer means sweltering heat. According to a study published in Nature, 2023 was the hottest summer in the Northern Hemisphere in more than 2,000 years.

This year is shaping up to be a scorcher, too. Cities in the Midwest, the Northeast and the mid-Atlantic are all experiencing extreme heat and humidity this week, factors that health experts say could lead to to heat exhaustion and heatstroke if you’re outside for too long. Heat-related deaths have been increasing in the U.S.: There were approximately 1,602 in 2021, 1,722 in 2022, and 2,302 in 2023.

Signs of heat illness include excessive sweating, nausea, cramps and confusion, said Meghan Kennihan, a National Academy of Sports Medicine-certified personal trainer and running coach. “Heat illness can escalate to heat exhaustion or heatstroke, which are life-threatening conditions requiring immediate medical attention,” she said.

If you’re a runner, it’s especially important to be aware of extreme heat conditions. “Running in the heat poses risks such as heat exhaustion, heatstroke, dehydration, and electrolyte imbalances,” Kennihan said.

What’s more, you’ll probably be running poorly if you’re overheated and pushing yourself too hard. “Your performance typically declines because your body prioritises cooling over running efficiency,” Kennihan said. “Heat stress can reduce stamina, slow your pace, and increase perceived effort, making your usual run feel significantly harder.”

To reduce your risk of heat stress, it’s important to time your runs for either earlier or later in the day, said JohnEric Smith, an associate professor at the department of kinesiology at Mississippi State University.

“Training earlier and later in the day will result in significantly lower radiant heat loads and air temperatures,” Smith told HuffPost.

If you run in areas that include stretches of grass, you should encounter less heat than you would on asphalt. “[Running] through the woods and forests can provide shade to also reduce heat load,” Smith said.

And if you have to change your planned distance, duration and pace because of the heat, you’re in good company.

“Research has shown that even elite runners in competition run at slower paces in the heat,” Smith said. “As runners continue to train smart and gradually increase their exposure to running in the heat, they will be better able to run safely.”

Of course, it’s also important to know the signs that it’s just too hot to run. Below, our experts share five reasons you may want to put off your regularly scheduled jog.

A forecasting tool tells you it’s too hot. 

Generally speaking, the answer to “Is it too hot to run?” doesn’t come from one distinct numeric temperature, said Amy Rantala, a primary care sports medicine physician in the Mayo Clinic Health System’s department of sports medicine and orthopaedics.

There are several factors that go into answering this question, Rantala said, including temperature, humidity, speed, how acclimated you are to running in the heat, how hydrated you are, your general health and your sweat-loss rate at any given time.

One relatively easy way to check if it’s too hot, though, is to consult the WetBulb Globe Temperature, a free forecasting tool from the National Weather Service that estimates the expected heat stress on the human body when in direct sunlight.

“It estimates the effect of temperature, relative humidity, wind speed, and solar radiation on humans using a combination of temperatures from three different thermometers,” Rantala said. “The [wet bulb temperature] is typically used at running competitions to determine if the conditions are favourable for running and to make recommendations to runners.”

There’s no universal minimum temperature at which heat illness risk is greater for everyone. But when the WBGT is above 82 degrees Fahrenheit, there are usually increases in heat illness, said Rebecca Stearns, chief operating officer of the Korey Stringer Institute in the Department of Kinesiology at the University of Connecticut.

“Generally, if it’s more than roughly 10 degrees hotter than what you have been exercising in for the past one to two weeks,” then the risk of heat illness is greater, she said.

You’re cramping up. 

Listen to your body. If you’re experiencing muscle cramps, especially in the legs or calves, that’s often a sign of heat-related illness, said Roger Adams, a personal trainer and owner of Eatrightfitness, a nutrition and fitness consultation company.

“Elevated temperatures increase sweat rate and can lead to dehydration and loss of electrolytes,” Adams said. “We need electrolytes for muscles to properly function, contract and relax. If you’re having cramps, you may be dehydrated.”

There’s high humidity.

High humidity is a red flag because when the air is saturated with moisture, sweat doesn’t evaporate efficiently, which impedes your body’s cooling mechanism, said Kennihan.

“If you are dehydrated, too, running in the heat can exacerbate these symptoms, which include dark urine, dizziness, and dry mouth,” she said. “Dehydration reduces blood volume, making it harder for your heart to pump blood and regulate temperature.”

You’re dizzy or you feel nauseated.

If you’re running and get dizzy or lightheaded, you’re in the obvious danger zone for heat-related illness.

“When the body’s core temperature gets too high, it can affect the central nervous system, causing a protective trigger of fainting or dizziness to occur to get the runner to stop,” Adams said.

Other, more extreme signs of possible heat illness include nausea and vomiting. 

“When the body struggles to regulate normal temperature, the core temperature gets too high, [and] the body shunts blood flow more to the surface so the blood can cool off easier,” Adams said. “When blood is taken away from the visceral cavity, this can cause stomach cramps, nausea, and even vomiting, as there is now less blood flow to your internal organs.”

You’re not acclimated to running even in middling heat. 

Acclimatisation is important if you’re going to be running in a hotter and more humid climate. It can take seven to 14 days to really acclimate to changes in elevation, temperature and humidity, Adams said.

Your degree of fitness can greatly affect your performance in the heat.

“If you are new to running, it will be much harder to perform and regulate body temperature in warmer temperatures,” he said. “Allow yourself to get in better shape before expecting more heat tolerance. That means preparing for the heat starts before the run, with a proper hydration strategy.”