Sweating a lot can mean you’re working hard, but sweat alone isn’t necessarily a great indicator of workout intensity, said Mindy Millard-Stafford, an exercise physiologist at the Georgia Institute of Technology. “You can’t really compare one person’s sweat rate to another and say, ‘This person worked harder,’” she said.

But understanding how much you sweat can help you stay hydrated and safe while working out in warmer conditions. We asked experts, including Millard-Stafford, what to watch out for.