A Hot Time in the City

You know that you tire more quickly when it’s 90 degrees rather than 50 degrees, even if you’re just taking a leisurely stroll. And you know that the harder you work, the more tired heat makes you. But why is that? And what can you do about it? When you run in the heat, your body sends more blood to your skin to cool you via evaporation. As a result, less blood, which carries oxygen, goes to your leg muscles, and they have to work harder just to maintain your usual pace. Also, the warmer it is, the more you sweat. This is good, because sweating helps to cool you as your perspiration evaporates. But it’s also bad, because your blood volume decreases. With less blood returning to your heart, your heart has to pump more often to keep the same amount of blood circulating throughout your body. And when your heart rate increases, you’re working harder.\ Running is even tougher on humid days. You generate all that sweat, and it’s supposed to cool you down, but the air is so moist that it can’t absorb much more water. So your sweat stays on your skin rather than evaporating, and your body just keeps producing more and more sweat, and you just keep getting more and more tired. That’s why it’s a lot tougher to run on a 90-degree day in the humid Northeast part of the country, like Washington, DC, than in someplace like New Mexico. Out in the desert, it’s hot, but dry, and your sweat can cool you like it’s supposed to.