Sweating safely

If you’re exercising in a hotter environment than usual, be prepared to stop if you notice dizziness, headaches, an inability to urinate, nausea or chills, which can all be symptoms of heat exhaustion. You should also stop and seek emergency medical attention if you experience confusion, which can signal heat stroke.

The American College of Sports Medicine recommends hydrating before starting a workout, which can mean consuming fluids several hours before exercise. During your workout, some experts recommend simply drinking when you’re thirsty, but thirst can be delayed until after you’ve already lost about 2 percent of your body mass in fluid, and can be particularly delayed in older adults.

Floris Wardenaar, a sports scientist who studies hydration and thermoregulation at the College of Health Solutions at Arizona State University, said it can be difficult to estimate how much fluid you lose during exercise. Weighing yourself before and after a workout can give you an idea. If you’re one pound lighter, for example, you should aim to drink about three cups of fluid to rehydrate, ideally over time rather than all at once. You can also check the color of your urine, which should be closer to pale lemonade than straw when you’re hydrated.

In extreme cases, dehydration can decrease your blood pressure and blood flow to the heart, said Dr. Sean Swearingen, a sports cardiologist at Rush University Medical Center in Chicago. (Conversely, drinking too much can dilute the sodium in your blood and lead to a dangerous condition called hyponatremia, which is rare but can be a risk if you’re exercising for a long time and replacing more fluid than you lose.)

Your heart rate is typically a better gauge of intensity than sweating, and will generally be higher during exercise in warmer conditions. To avoid pushing yourself too hard, Dr. Swearingen recommended matching your heart rate to a typical workout, even if that means cutting back on pace or distance in the heat.

“Leave your ego at the door, and be prepared to start out a little slower,” Dr. Millard-Stafford said.