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 youre 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 youre a runner, its 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.

Whats more, youll probably be running poorly if youre overheated and pushing yourself too hard. Your performance typically declines because your body prioritizes 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, its 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, youre 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, its also important to know the signs that its just too hot to run. Below, our experts share five reasons you may want to put off your regularly scheduled jog.