The heat—miserable and oppressive—is not abating. Today, a third of Americans are under a heat alert as temperatures keep breaking records: Phoenix has hit 110 degrees Fahrenheit for two weeks straight, while this weekend Death Valley in California could surpass the all-time high of 130 degrees.
Even less extreme heat than that can be dangerous. Recently, in Texas, Louisiana, part of Arizona, and Florida, there have been reports of deaths from heat, and many more hospitalizations. The toll of a heat wave is not always clear in the moment: A new report suggests that last summer’s historic heat wave in Europe killed more than 60,000 people.
Ideally, you’d stay in the air-conditioned indoors as much as possible. That’s not an option for everyone. The other thing to do is stay hydrated. The importance of getting enough fluid is hard to overstate—and often underappreciated: Last month, the Texas state legislature banned local governments from mandating water breaks for construction workers. In the heat, hydration “impacts everything,” Stavros Kavouras, the director of the Hydration Science Lab at Arizona State University, in Phoenix, told me. And with temperatures continuing to rise, it’s essential to get it right.
Serious dehydration is really, really bad for you. Your blood volume decreases, which makes your heart work less effectively. “Your ability to thermoregulate declines,” Kavouras told me, “so your body temperature is getting higher and higher.” You might feel weak or dizzy. Your heart rate rises; it gets harder to focus. The worst-case scenario is heatstroke, when your body stops being able to cool itself—a potentially fatal medical emergency.
In extreme temperatures, heat injuries can happen quicker than you might think. Given that the human body is mostly water, you might assume that there is some to spare, but inconveniently, this is not the case. “If you lose even 10 percent of [the water] your body has, you are entering the…
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