Long afternoons at the playground. Swimming at the local pool. Sing-alongs and s’mores at camp. The American summer vacation is an institution—and much of it takes place outdoors. But as climate change makes summer hotter and hotter, some of the season’s traditions are changing. In recent years, more summer camps have moved indoors and parks have closed during heat waves. Even travel is changing: Planes have been grounded when heat makes taking off difficult, and tourism experts warn that destinations like Greece and the Canary Islands will see fewer visitors during peak tourism season. More and more, summer is getting too hot to be, well, summer.

“The knowledge of heat and children is in the dark ages,” Aaron Bernstein, the former interim director of the Center for Climate, Health and the Global Environment at the Harvard T. H. Chan School of Public Health, told me last month. (After our interview, Bernstein accepted a position as the director of the CDC agency that handles environmental hazards). Studies on the effects of heat have mainly focused on adults, primarily those in wealthy countries. “There were huge assumptions about what heat meant to children, which were largely wrong,” Bernstein added. According to Bernstein, doctors have assumed for decades that extreme heat is a problem only for kids who are exerting themselves through sport or children with a health condition that is aggravated by heat, such as asthma—but this isn’t the full picture. During a heat wave, all children are more likely to be affected by heat illness, which can cause respiratory and kidney disease, as well as inhibit cognitive function.

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Some of these problems can be addressed by learning skills to minimize heat. In Texas, which has among the hottest summers in the country, with average seasonal temperatures above 80 degrees Fahrenheit, children still regularly go to the park, play sports, and attend camps. But Doreen Bruton, the owner of Ride with Pride, a horseback-riding school that offers summer camps in Southlake, Texas, will distribute bandannas that have been soaking in ice buckets to the children before they ride on hot days. On particularly sweltering ones, she’s had to move camp to earlier in the day. Bernstein points out that all kinds of human adaptations—for example, wearing light-colored fabrics—have long been common practice in places around the world where extreme heat is a way of life. Still, he told me, even with adaptive behaviors, at certain temperatures or humidity levels, no one is safe outside for extended periods of time. “The challenge that keeps me up at night is that we really don’t understand what kind of temperatures those are,” he said.

The question How hot is too hot? presents two challenges. The first is that there is no national, or even state, standard for when kids should abandon outdoor activities, which leaves camp directors, sports organizers, and parents making these decisions for themselves. When I asked Tony Deis, who runs Trackers, one of the largest outdoor camp programs in Oregon, he said 105 degrees. The American Academy of Pediatrics pointed me toward the National Weather Service’s Heat Index, which provides a range of warnings based on temperature and humidity. For example, on an average July day in Los Angeles with 53 percent humidity and a temperature of 88 degrees, the chart advises “extreme caution.” In the U.S. over the past 60 years, heat-wave season has increased by 49 days, the frequency of heat waves has increased from two to six a year, and the heat waves themselves have gotten hotter and last longer.

The second challenge is that children respond differently to heat depending on their age, body size, acclimation to high temperatures, and preexisting medical conditions. Children taking certain antibiotics, as well as medication for ADHD or allergies, are potentially more at risk of overheating. Getting heatstroke also makes you more susceptible to getting it again. In more extreme cases of heat sensitivity, summer can require intense levels of risk management. Joline Scott-Roller, a professor who lives in Ashland, Ohio, has a 10-year-old son with a medical condition that means he does not regulate heat properly. He can play outside if the temperature is under 90 degrees—but only in the shade. If the temperature rises above that, he has to stay indoors. “There are days that we have to tell him he is not allowed to leave the house. We cannot risk even him getting in the car,” she told me. For the Scott-Roller family, every summer activity requires extensive planning. “If we’re going to an amusement park or a zoo, I have to think about … how far away is their emergency center? Are there plenty of places with shade? Are there indoor places that have air-conditioning?” Scott-Roller said. “When we go to a playground, I take a cooler full of ice packs. I keep a gallon of water in the car.”

We don’t fully understand the long-term effects of having children outdoors in extreme heat, yet we also don’t fully understand what spending summers indoors does to kids. Bernstein says he has no doubt that it is not good. “There’s pretty robust data that putting children in nature is actually incredibly valuable to preventing mental-health disorders,” he told me. Research shows that access to green space has been linked to a stronger immune system, as well as a decrease in aggression, depression, and ADHD symptoms. “The consequences of [being indoors] is not just kids missing out on a good time for eight weeks,” Bernstein said.

On a chilly day in April, Deis walked me through the new Trackers space, which used to be a Marshalls, in a shopping mall in downtown Portland. Deis was initially horrified at the idea of a facility in a former department store, but, after three summers of weather events including a fatal heat dome, record-breaking temperatures, and megafires in Oregon, he came around. He pointed me to where Trackers is building the archery range, the axe-throwing area, the climbing wall. While some camps will be entirely indoors, most of the Trackers camps will use this space only if smoke or sun demands that they have to.

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When I asked Deis—a lifelong outdoor educator—if there is a part of him that flinches at the thought of keeping kids inside a large, fluorescent-lit space in the middle of a shopping mall, he said he’s gotten over his initial negative reaction. But his reasoning made my stomach sink. “It’s space, and we’re just filling a niche like an animal fills a niche,” he said. “A coyote living in the city looks at everything as nature already. So we’re looking at this space as nature.”