You Really Don’t Want to Be Thirsty in a Heat Wave


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 zone of serious clinical dehydration,” Kavouras said. “And if you look at optimal health, even losing just 1 percent of your body weight impacts your ability to function.” There are two basic ways your body cools itself when it gets hot. One is to send more blood to the skin, which releases heat from the core of your body, and is the reason you turn red when you’re overheated. The other is to sweat. It evaporates off your body, and in the process, your body loses excess heat. You can’t cool yourself as effectively if you’re not properly hydrated. At the same time, one of your main cooling mechanisms is actively dehydrating, which means the goal is not just to be hydrated, but to stay that way.

What that takes depends on many factors rather than a single universal rule, but in general, the danger zone is “high humidity with anything above 90 degrees,” Kavouras said, at which point, “it’s actually dangerous” just to be outside. The more active you are in the heat, and the hotter and more humid it is, the greater the risk—and the more important proper hydration becomes. The standard water target in the U.S. during non-heat-wave times is 3.7 liters a day for men and 2.7 liters for women. When it’s very, very hot out, you need more. Even if you spend most of the day in the bliss of AC, you are almost certainly leaving the house at some point.

Instead of trying to figure out what that precise amount should be, Kavouras recommends you focus on two things instead. “No. 1, keep water close to you. If you have water close to you, or whatever healthy beverage, you’ll end up drinking more, just because it’s closer,” he said. And second: Keep an eye on how often you pee—pale urine, six to seven times a day, or every two to three hours, is good. You want it to be “basically like a Chablis, a Riesling, Pinot Grigio, or champagne-colored,” John Higgins, a sports cardiologist at McGovern Medical School at UTHealth, in Houston, told me. “If you notice the urine is getting darker, like a Chardonnay- or Sauvignon Blanc–type of thing, that generally means you are dehydrated.”

Certain groups are especially at risk. Older adults are more prone to dehydration, as are young children, people who are pregnant, and people taking certain medications—blood-pressure medications, for example. None of this requires you to take in extra fluids per se, just that you need to be even more careful that you’re getting enough.

As for what to drink, as a go-to beverage, straight water is hard to beat. Water with fruit slices floating in it has the benefit of feeling like something from a luxury hotel. Carbonated water is also good—you might not be able to drink quite as much of it, which is a potential drawback, but “there is no mechanism in your GI system that will make sparkling water less effective at hydrating you,” Kavouras said. You probably want to avoid downing giant buckets of coffee—caffeine is a diuretic in large quantities and Higgins warns against sugary drinks for the same reason. (A daily iced coffee is fine.) If you’re doing hours of heavy sweating, then you might work in some (less sugary) sports drinks. But for the majority of people, water remains the ideal. Food can also be a fluid source: “Make sure you’re eating a diet that’s rich in vegetables and fruits that have water content,” William Adams, the director of the University of North Carolina at Greensboro’s Hydration, Environment and Thermal (H.E.A.T) Stress Lab, advised. Alcohol, which causes you to lose fluid, is definitively unhelpful.

There are lots of water myths out there. Can you go too hard? Technically, it’s possible to over-hydrate, causing an electrolyte imbalance, but all three experts agreed that for most people, this isn’t really a concern. You can find arguments for drinking hot drinks in the summer—the idea being that they increase the amount you sweat, thereby promoting cooling. But Kavouras is emphatic that you’re better off with cold drinks, which cool your body, he said. In the moments before a race, marathon runners will sometimes take it one step further, slurping ice slurries to lower their body temperature. For good old-fashioned drinking water, about 50 degrees Fahrenheit is best—roughly the temperature of cool water from the tap.

One final key to staying hydrated: Start early. A lot of people, Higgins said, are lightly dehydrated all the time, heat wave or not. “So particularly when you first wake up in the morning, typically you are in a dehydrated state.” Accordingly, he recommends that people drink about a standard water bottle’s worth—roughly 17 ounces—as soon as they wake up. The other thing people forget about, he said, is what happens when they come back inside after enduring the outdoors. “You keep sweating,” he pointed out. In other words: hydrate, and then keep hydrating.

As crucial as hydration is, it is not a miracle. “It doesn’t mean that you can say, ‘I hydrate well, so I’ll go out for a run in the 120-degree weather, and I’ll be fine because I’m drinking a lot,’” Kavouras said. “It doesn’t work this way.” Still, it is a simple but effective tool. As heat waves like this one become even more frequent, many more people will need to learn how to become attuned to their hydration. And perhaps adequate water can be a perverse sort of comfort: You can’t control the unrelenting heat, but you likely can control your water intake. In a heat wave, it helps to have a glass-half-full attitude—and an emptied glass of water.

This story is part of the Atlantic Planet series supported by HHMI’s Science and Educational Media Group.

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