GUANGZHOU, China — The children at the Bayi Xiwang elementary and middle school are doing something revolutionary by current Chinese standards: They're playing outside.

Singing and skipping in the dizzying southern Chinese humidity, these students have been given 45 minutes a day to frolic under the sun while peers across the nation remain indoors, hunched over books or squinting at blackboards.

By forcing youngsters to put down their pencils and expose their eyes to natural light, researchers think they can stem an explosion of nearsightedness in China.

By the time they complete high school, as many as 90 percent of urban Chinese youth are afflicted by the condition known as myopia, in which close objects can be seen clearly but things just a few feet or inches away start to blur.

That's about three times the rate among U.S. children. Even more troubling is the severity of the Chinese cases. Between 10 and 20 percent of nearsighted Chinese children are expected to develop "high myopia," which is largely untreatable and may lead to blindness.

"The problem for China is really quite massive," said Ian Morgan, a visiting professor at the Zhongshan Ophthalmic Center at Sun Yat-sen University who helped organize the three-year clinical trial in Guangzhou. "Their best-educated kids — kids who are going to be the intellectuals or political leaders — are going to be progressively losing vision as they get older."

China's market for spectacles and contact lenses is expected to double to $5.3 billion this year from just four years ago, according to Euromonitor International, a consumer research firm.

Mason, Ohio-based LensCrafters recently expanded its network of 200 outlets in China by opening special luxury-branded stores named LC+ in Hong Kong, Shanghai and Beijing.

For those on a budget, state-owned optical shops can still be found in any city charging as little as $10 for a new pair of glasses.

China has also become one of the world's fastest-growing markets for foreign contact-lens makers. The same can be said for laser corrective surgery, which was introduced to China in the mid-1990s and has grown steadily to 1 million procedures a year.

Experts remain divided on how much genetics is to blame for China's struggle with myopia. Scientists have found more than two dozen genes linked to the problem, especially for the most severe forms of the condition. Children whose parents have myopia are also more likely to develop nearsightedness.

But Morgan, an Australian, is trying to prove its origins are largely environmental and linked to schooling. Nearsightedness is rampant in the Asian Tiger economies of Hong Kong, Taiwan and Singapore, where academics are similarly rigorous, even among children who aren't ethnic Chinese.

For example, research has shown that students of Indian ancestry living in Singapore have rates of myopia eight to nine times higher than their peers living in India, Morgan said. Myopia is also prevalent in Orthodox Jewish schools, where reading requirements are intense.

Fishing for older records in China, Morgan and his colleagues found studies that showed the condition affected about 20 to 30 percent of young adults in Guangzhou in the early 1970s, near the end of Mao Zedong's violent Cultural Revolution.

Myopia has steadily increased in concert with China's urbanization and intensified academic competition. It's not uncommon for children to study four hours a day at home on top of a full day of school, as well as attend several hours of tutoring on weekends.

Beijing restaurateur Wang Jiali said she's convinced that too much studying ruined her eyesight.

Growing up in Jianyang in western Sichuan province, she studied in a classroom from 7:30 a.m. to 6:30 p.m., followed by four hours of homework under poor home lighting.

By age 11, Wang strained to read the blackboard. By age 12, she was wearing her first pair of glasses — a pair of cheap red frames whose lenses cracked twice in the first three months. She recently paid $2,134 to receive laser corrective surgery.