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.

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