August 5, 2022 12:01 AM EDT

Getting kids to be active in a modern world is a tough sell. It can be hard to compete with indoor comforts like video games, television, and air conditioning.

Sweltering weather is another formidable barrier to kids getting enough physical activity, finds a new scientific review published in the journal Temperature that analyzed more than 150 studies. Children today are about 30% less aerobically fit than their parents were at their same age, leaving them less prepared to acclimate to a hotter, more extreme climate as they age, the study concluded. “The outside world is becoming more of an extreme environment for humans all over the world,” including children, says study author Shawnda Morrison, a cardiovascular and exercise physiologist, and assistant professor at the University of Ljubljana in Slovenia. She is also the founder of Active Healthy Kids Slovenia, which conducts research and advocates for children’s physical activity.

Over time, climate change has made extreme weather like heat waves more common. Air quality is also worsening—not just from the pollution caused from burning fossil fuels, but also from climate-change-driven pollen levels and wildfires, the smoke from which can sting the eyes and penetrate deep into people’s lungs. A warming world is also expected to increase the spread of infectious diseases as animals that spread pathogens alter and expand their territory.

All of that climate-related fallout contributes to kids not being as active, Morrison says, which threatens their lifelong health. According to a 2018 report included in the review, which compared kids’ activity levels across 49 countries, only 39% (or less) of children in most of those countries were getting adequate physical activity. Globally, the vast majority of children are also not meeting the World Health Organization’s recommendation that kids get at least an hour of moderate-to-vigorous intensity exercise a day on average, according to a meta-analysis published in July and included in the review. Many children became even more sedentary during the COVID-19 pandemic, the review says.

This kickstarts an unhealthy cycle, Morrison says, since children who aren’t in the habit of doing enough physical activity tend not to be active as adults. The consequence is that today’s children are not only “extremely unfit”—“they don’t like to move, either,” she says.

Such children may not be prepared for a hotter future. Aerobically fit people are often better able to withstand hotter temperatures, because they tend to have lower resting core temperatures and the ability to sweat more efficiently, says Morrison. Because fitter people tend to have better circulation, their hearts don’t need to work as hard to move their blood around to cool them down. Unfit adults are also more vulnerable to chronic diseases, including cardiovascular disease, diabetes, and cancer, which in turn make them more likely to experience the negative health effects of heat, like heat strokes and heart attacks.

Read More: Why Extreme Heat Is So Bad for the Human Body

Climate experts also expect extreme events—such as hurricanes, heat waves, wildfires, and droughts—to occur more often and to be more likely to happen at the same time. Fitter people may be more prepared to survive these types of emergencies. “I know that people are uncomfortable in the heat,” Morrison says. “But I think it’s important to stress that this is now a survival situation.”

However, childhood provides an opportunity to build up healthy habits kids can maintain throughout their lives. Although many societal factors outside of a family’s control influence a child’s fitness—like living near green space and getting adequate physical education classes at school—Morrison says that parents also help kids build healthy habits by encouraging them to play outside or sometimes choosing more active types of transportation such as biking, walking, or scootering instead of driving.

As temperatures rise, parents have a responsibility to keep children safe in the heat, says Morrison. Younger children may not be able to take steps to cool themselves down, which means that it’s up to parents to keep them hydrated or in the shade if they get flushed, seem tired, or show other signs of being too hot.

Still, parents shouldn’t be afraid to take their kids outside when it’s hot. Being outdoors more often can help acclimate kids’ bodies to a warmer environment, which will make them less vulnerable to dangers like heat stroke, Morrison says. She suggests encouraging kids to play outside in the spring, as soon as the weather starts to get hotter, in order to help kids get used to the heat. Taking kids outside in the morning or evening, when the weather is somewhat cooler, will also help keep them safe and enjoy themselves more outside, which is key.

Morrison knows that getting kids outside isn’t always easy. When she picks her three- and five-year-old children up from school every day, she takes them to the playground to play for an hour. “I know it’s hard for parents, but you really do have to make time to do this, especially with younger children,” she says. “It really has to be built into your day as a priority.”

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