Smokers in their 30s and 40s have five times as many heart attacks as non-smokers the same age, a study released Friday found.

Smoking is responsible for half the heart attacks in younger adults the world over, the study, sponsored by Britain's Medical Research Council and the Imperial Cancer Research Fund, shows.The report in the British Medical Journal - claimed to be the most exhaustive ever into smoking and heart attacks - showed that youth offers no protection from the deadly effects of tobacco.

"This study shows that smoking causes even more premature heart attacks than was previously supposed," said Dr. Rory Collins of the British Heart Foundation.

"The younger you are, the bigger the proportion of heart attacks caused by tobacco. When a habitual smoker in their 30s or 40s has a heart attack, there's an 80 percent chance that tobacco caused it."

The researchers, who looked at 14,000 heart attack survivors in Britain and 32,000 of their relatives, said the problem was the worst in industrialized nations but was also getting serious in the developing world.

"Every year more than half a million people in developing countries have a heart attack in their 30s or 40s and it's getting worse," said Richard Peto of the ICRF.

"Cigarette sales in Asia, Africa and Latin America have risen over the past 10 or 20 years," Peto added.

"More cigarettes means more heart attacks in young adults and more cancers. Worldwide, the only two big causes of premature death that are increasing rapidly are HIV (the virus that causes AIDS) and tobacco."

But even where smoking was on the decline, it caused a significant number of deaths, he added. "In the United States smoking is causing 40,000 heart attacks a year in people in their 30s and 40s," Peto said.

"About half of all smokers eventually get killed by their habit."

Peto said it was especially important that teenagers hear about the findings. "Kids can imagine that one day they might be 30," he said. "And it is those who start smoking in their teenage years who are at greatest risk of having a heart attack in their 30s and 40s."

About 30 million adults die each year around the world from all causes, and 10 percent of these deaths, or three million, are caused by tobacco, the World Health Organization says.

Smoking causes lung cancer, emphysema and heart disease, and using tobacco can cause other cancers that attack the mouth, throat and other parts of the body.

But the study found that stopping smoking, at any age, greatly reduced the risk of death.

"If you happen to survive your first heart attack, then stopping smoking will make you much less likely to have another. Stopping before you have a heart attack is even better," Peto said.