The health benefits of exercise are well-established, from better physical and mental health to maintaining a healthy weight. But most adolescents around the world are not getting as much exercise as they should be, according to a sweeping new study.

The report, based on surveys of 1.6 million students ages 11 to 17 in 146 countries, found that more than 80 percent of adolescents in 2016 did not meet the World Health Organization recommendation of 60 minutes of moderate-to-vigorous physical activity per day. Girls were less physically active than boys in nearly all the countries studied. And while the proportion of boys who did not get enough exercise decreased slightly from 2001 to 2016, girls’ physical activity stayed about the same. There was no difference in physical activity in higher- versus lower-income countries. South Korea and the Philippines had the greatest levels of inactivity in girls and boys, respectively, among all countries studied. By contrast, Bangladesh and India had the least inactivity among both sexes.

The findings, published Thursday in The Lancet Child & Adolescent Health, suggest an urgent need to scale up effective policies aimed at boosting physical activity among young adults worldwide, the report’s authors write. They call for investment and leadership at all levels to address the lack of physical activity and sex discrepancy, and further call for engaging young people themselves to help solve the problem.

“It’s an epidemic,” says Abby King, a professor of health research and policy and of medicine at Stanford University, who co-chaired an advisory committee on physical activity guidelines in the United States in 2018 and was not involved in the study. “The article really makes clear that the trends are going in the wrong direction, especially for girls.”

Regina Guthold, an epidemiologist at the WHO’s Maternal, Newborn Child and Adolescent Health Department, and her colleagues conducted the study by combining data from 293 surveys of physical activity in a random sample of at least 100 school-going boys and girls in each country or region involved. The WHO’s definition of physical activity includes walking or biking to school, active play, physical education classes and after-school sports, and the 60-minute recommendation was cumulative for the day. The researchers estimated the prevalence of physical inactivity among four different income groups, nine geographic regions and by country, from 2001 to 2016.

Just under 78 percent of boys did not meet the WHO’s physical activity guideline in 2016, a slight decrease from about 80 percent in 2001. By contrast, a staggering 85 percent of girls did not meet the WHO threshold in 2016—not a statistically significant change from 2001. High-income countries such as Singapore, the U.S. and Ireland had the greatest gap in activity levels between boys and girls; all had a difference of more than 13 percentage points in 2016.

“It’s really time to do something about it, because it really is a worry that we have four out of five adolescents globally who don’t get enough physical activity,” Guthold says. The WHO member states set a global target of reducing physical inactivity by 15 percent by 2030. At this rate, “we are not at all on track to meet this target,” she says.

Pedro Hallal, an epidemiologist at the Federal University of Pelotas in Brazil, and colleagues published the first global estimate of adolescent physical activity in 2012 (Guthold was a co-author on that study), finding that 80.3 percent of students ages 13 to 15 in 2008 were not meeting the WHO recommendation. They published an updated estimate in 2010 showing similar findings. “This new evidence supports pretty much everything we found at that time,” Hallal says. “We are facing a very serious public health threat, and people underestimate the importance of physical activity.” The situation, he says, “is not normal.”

Guthold and colleagues speculate that these trends are driven by an increasingly sedentary lifestyle in many countries due to technology use as well as the urban environment. Instead of doing something active, many young people now spend time on computers or their smartphones, Guthold says. They also do not walk or bike to school as much as they used to, in part because of increased traffic and safety concerns in big cities. “How can you be active in these settings if it’s too dangerous to walk or ride your bike to school?” she says. Safety is likely an even bigger concern for girls, she adds.

Physical education programs in schools may also be to blame, and many focus heavily on sports and activities that are more popular with boys than girls. Policy makers should ask adolescent girls about what activities appeal to them, and PE programs should take those preferences into account, Guthold says. Schools are one of the most important places to intervene, she adds. She cites a program in Finland that has tried to integrate physical activity into the school day by scheduling four or five short activity breaks throughout the day, as well as including more physicality in classes themselves.

Culture also plays a role, as social norms in some parts of the world discourage adolescent girls or women from participating in sports. In countries such as the U.S., India and Bangladesh there is a strong national tradition of sports such as football, soccer and cricket—which are often male-dominated—Guthold notes. Giving more attention to women’s sports leagues and promoting female athletes may help close the gender gap among adolescents.

The lower activity level in girls was not surprising, King says. Her own research has shown a drop in physical activity among girls in adolescence, and it often continues into adulthood. The trend “could be related to women’s social roles and the work roles women are juggling,” she says. Additionally, her work has shown that women appear to be more sensitive to the walkability of their environment. “That’s one thing that I think really deserves further investigation,” she adds.

While smartphones and other technological devices are likely contributing to the decline in physical activity, King says, they could also be part of the solution. She and her colleagues have found that smartphone apps, in particular, can effectively nudge kids to be more active.

The new study had a number of limitations, including the fact that it was only based on data for adolescents in school. Non-school-going going kids may be poorer and have different activity levels, but there is very little data available for these children, Guthold says. In addition, the report lacked data on the type of physical activities adolescents were pursuing—whether it was walking, sports or active play, for example. Incorporating data from devices such as step counters and accelerometers could provide more quantitative measures of physical activity, rather than relying on students’ memories and self-report.

“These data are not perfect,” Guthold says, but the implications are clear: “It’s time to act.”