About half the men in numerous developing nations use tobacco, and women in those regions are taking up smoking at an earlier age than they used to, according to what is being called the largest-ever international study on tobacco use.

The study, which covered enough representative samples to estimate tobacco use among 3 billion people, "demonstrates an urgent need for policy change in low- and middle-income countries," said lead researcher Gary Giovino, whose report was published in the British medical journal The Lancet.

The figures bolster statements by the World Health Organization that while much of the industrialized world, including the United States, has seen a substantial reduction in smoking in recent years, the opposite trend is under way in parts of the developing world.

The WHO warns that "if current trends continue, it will cause up to one billion deaths in the 21st century."

The new study, the Global Adult Tobacco Survey (GATS), focused on countries in which smoking is known to be a growing problem.

"The burden of tobacco use is moving," says Giovino, who formerly oversaw the Office on Smoking and Health at the Centers for Disease Control and Prevention.

"The tobacco epidemic takes different forms in different countries," he said in an interview with CNN, pointing out that chewing tobacco and other smokeless forms are part of the problem. "But manufactured cigarettes are dominating."

Giovino now runs the University at Buffalo's Department of Community Health and Health Behavior in Buffalo, New York.

The study, conducted between 2008 and 2010, found that across 14 developing nations, 49 percent of men and 11 percent of women used tobacco. Most of them smoked -- 41 percent of men and 5 percent of women.

Numbers were highest in Russia, where 60 percent of men and 22 percent of women used tobacco; China, where 53 percent of men and 2 percent of women were tobacco users; Ukraine, where 50 percent of men and 11 percent of women used tobacco, and Turkey, where 48 percent of men and 15 percent of women used tobacco.

In some countries, smoking rates may now be even higher than they were in 2010, WHO officials say.

"One place where we know it's gone up, unfortunately, is Egypt -- as a result of the revolution," said Edouard Tursan D'Espaignet of WHO''s tobacco control program.

The GATS study found 38 percent of men and less than 1 percent of women smoked in Egypt as of 2010.

However, government regulations limiting smoking in certain places fell apart after Hosni Mubarak's regime was ousted last year, and "the tobacco industry walked in very, very aggressively" to market its product amid the chaos, said Tursan D'Espaignet.

"We are hearing things like 'Smoking is a way to show you're free from the previous regime,'" he said.

The other nations in the new study are India, Bangladesh, Vietnam, the Philippines, Thailand, Poland, Brazil, Mexico, and Uruguay.

In general, marketing is a central reason smoking is on the rise in poorer nations, says Tursan D'Espaignet.

"In many countries, particularly eastern Europe and China, the market is probably saturated" among men, he said. "We can see the tobacco industry is targeting young people, and they're targeting women."

While previous studies done in several countries found that women who smoke generally start later than men, the GATS study found the opposite. "Alarmingly, this study shows that -- in most countries we surveyed -- age of smoking initiation for women might now be approaching the young ages at which men begin," the report says.

Still, the overwhelming majority of tobacco use worldwide is by men.