Researchers from the University of Alabama at Birmingham and University of California at San Francisco analyzed marijuana and tobacco use among 5,000 black and white men from the national database, CARDIA (Coronary Artery Risk Development in Young Adults study), which was intended to determine heart disease risk factors over a 20-year period.

Measuring participants’ lung function for air flow and lung volume five times throughout the study period, the researchers found that cigarette smokers saw lung function worsen throughout the 20-year period, but marijuana smokers did not. Only the heaviest pot smokers (more than 20 joints per month) showed decreased lung function throughout the study.

“The more typical amounts of marijuana use among Americans are occasional or low levels,” said Dr. Stefan Kertesz, assistant professor of medicine at the University of Alabama at Birmingham and principle investigator of the study. “From the standpoint of being a scientist, these data suggest that low and moderate range use of marijuana do not do long-term harm.””

Read more: http://abcnews.go.com/Health/smoking-pot-harm-lung-function-time/story?id=15331989