Mark J. Pletcher, MD, MPH, Associate Professor, In Residence, in the Departments of Epidemiology, Biostatistics, and Medicine at the University of California at San Francisco, stated the following in a press release titled "Marijuana Shown to Be Less Damaging to Lungs Than Tobacco," available on ucsf.edu:

“We found exactly what we thought we would find in relation to tobacco exposure: a consistent loss of lung function with increasing exposure. We were, however, surprised that we found such a different pattern of association with marijuana exposure…

An important factor that helps explain the difference in effects from these two substances is the amount of each that is typically smoked. Tobacco users typically smoke ten to 20 cigarettes/day, and some smoke much more than that. Marijuana users, on average, smoke only two to three times a month, so the typical exposure to marijuana is much lower than for tobacco…

Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. On the other hand, our findings do suggest an accelerated decline in pulmonary function with heavier use – either very frequent use or frequent use over many years – and a resulting need for caution and moderation when marijuana use is considered…

I was surprised at these findings. I thought we were going to find some small negative affect on pulmonary function from marijuana use. But we didn’t. At the level of marijuana that most marijuana users smoke, we did not find any appreciable negative consequence on pulmonary function on marijuana use.”

Jan. 11, 2012