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Smoking the occasional joint does not seem to have any detrimental effects on lung health, according to a U.S. study.

Researchers found that several measures of lung health actually improved slightly as young people reported using more marijuana - at least up to a couple of thousand joints in a lifetime.

"There's no doubt, if you've watched a Harold & Kumar movie, marijuana triggers a cough," said Dr. Stefan Kertesz, from the University of Alabama at Birmingham, who worked on the study. But questions have remained about the drug's longer-term effect on lung functioning.

"Previous studies have had mixed results," Kertesz said. "Some have hinted at an increase in lung air flow rates and lung volume (with marijuana smoking), and others have not found that. Others have found hints of harm."

While marijuana smoke has a lot of the same toxins as cigarette smoke, he said, people who use pot tend to smoke fewer joints each day than tobacco users smoke cigarettes. That and the method of inhaling may offer some relative lung protection, researchers have proposed.

But the findings from the study - which was funded by the National Institute on Drug Abuse, part of the U.S. National Institutes of Health - do not let marijuana off the hook for long-term health consequences.

"I think a lot more work will need to be done to make any blanket statements about safety," said Dr. Jeanette Tetrault, a substance abuse researcher at the Yale School of Medicine in New Haven, Connecticut, who was not tied to the new research.

The new data comes from a long-term study of more than 5,000 young adults in Oakland, California; Chicago; Minneapolis; and Birmingham, Alabama. From 1985 until 2006, researchers regularly asked participants about their past and current use of cigarettes and marijuana. They also tested how much air their lungs could hold and the maximum rate of air flow out of their lungs.

The researchers found that the more cigarettes the participants smoked, the worse their lungs performed on both tests, Kertesz's team reported in the Journal of the American Medical Association.

But moderate levels of pot smoking did not seem to hurt lung function.

Lung volume and air flow rates both increased with each "joint-year" - the equivalent of 365 joints or pipe bowls -- participants said they had smoked, up to about 2,555 joints.

The findings do not mean that people should smoke marijuana to improve lung function, the researchers said.

Pot might irritate the lungs in the short term and cause problems for people with asthma, they said. (Editing by Greg McCune and Mohammad Zargham)