Occasional marijuana won't harm lungs, finds study

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Researchers from University of California, San Francisco have found smoking marijuana occasionally doesn't harm the lungs. However, they also found in their study that excessive cannabis smoking does lead to lung damage.

In their study, published in the Journal of the American Medical Association, the scientists followed 5,016 men and women who were part of the Coronary Artery Risk Development in Young Adults (CARDIA) study for a period of 20 years.

Because cannabis has some of the same chemicals as cigarette tobacco, the researchers were interested in finding out if marijuana causes the type of lung damage associated with tobacco.

Fifty-four percent reported they smoked marijuana, tobacco, or both. Tobacco was smoke more often - 8 to 9 cigarettes a day compared with two or 3 episodes of marijuana use in the previous 30 days. 
The authors write:
With up to 7 joint-years of life-time exposure (e.g., 1 joint/day for 7 years or 1 joint/week for 49 years), we found no evidence that increasing exposure to marijuana adversely affects pulmonary function.”
Measurements used to test lung damage included FEV1 - the amount of air you can blow out of your lungs in one second after taking a full, deep breath, and FVC, which is the total amount of air forcibly exhaled after taking full breath in.

In the study FVC actually improved. The researchers suggest it may be from  inhaling deeply that occurs with marijuana smoking that strengthens and stretches the lungs. 

The research team, which was led by Mark J. Pletcher, MD, MPH, notes marijuana has benefits for controlling pain, improving mood and promoting appetite. 

The finding that occasional marijuana smoking doesn't harm the lungs, might be used in support of medicinal use of cannabis to treat chronic health conditions. However, smoking pot more than 20 times a month was shown to cause a substantial decrease in FEV1 - forced exhaled (lung) volume in one-second. 

Pletcher MJ, Vittinghoff E, Kalhan R, Richman J, Safford M, Sidney S, Lin F, Kertesz S. Association between marijuana exposure and pulmonary function over 20 years. JAMA 2012; 307(2): 173-81