Narrator:        This is Science Today. The health risks associated with smoking tobacco are well-documented. But up until now, little has been known about the long-term effects of marijuana use on the pulmonary system. To try to answer this question, a study led by researchers at the University of California, San Francisco, and the University of Alabama-Birmingham collected data from more than 5,000 tobacco and marijuana users over a 20-year period.

Pletcher:        We found with tobacco exactly what we thought we were going to find. The more you smoke over time and currently, the more adverse affect there is on your pulmonary function.

Narrator:        Dr. Mark Pletcher, an epidemiologist at UCSF and the study's lead author, expected to see similar decreases in pulmonary function for marijuana users. This is because marijuana and tobacco smoke contain many of the same components. But the results surprised him.

Pletcher:        I expected that we would find some of the same types of damage to pulmonary function that we see with tobacco smoke.  But we really couldn't prove that that sort of damage was occurring.

Narrator:        For Science Today, I'm Larissa Branin.