Exposure to tobacco smoke causes lung damage with clinical consequences that include respiratory symptoms, chronic obstructive pulmonary disease, and lung cancer. "Marijuana smoke contains many of the same constituents as tobacco smoke, but it is unclear whether smoking marijuana causes pulmonary damage similar to that caused by tobacco," according to background information in the article. "Smoking marijuana is increasingly common in the United States, and understanding whether it causes lasting damage to lung function has important implications for public health messaging and medical use of marijuana."
Mark J. Pletcher, M.D., M.P.H., of the University of California, San Francisco, and colleagues examined associations between marijuana (both current and lifetime exposure) and pulmonary function. The Coronary Artery Risk Development in Young Adults (CARDIA) study is a longitudinal study collecting repeated measurements of pulmonary function and smoking over 20 years (March 1985-August 2006) in a group of 5,115 men and women in 4 U.S. cities. Lifetime exposure to marijuana joints was expressed in joint-years, with 1 joint-year of exposure equivalent to smoking 365 joints or filled pipe bowls. Lung function was assessed by the measurements of forced expiratory volume in the first second of expiration (FEV1) and forced vital capacity (FVC), with lower measures corresponding to poorer lung function.
More than half of participants (54 percent; average age at baseline, 25 years) reported current marijuana smoking, tobacco smoking, or both at 1 or more examinations. The median (midpoint) intensity of tobacco use in tobacco smokers was substantially higher (8-9 cigarettes/day) than the median intensity of marijuana use in marijuana smokers (2-3 episodes in the last 30 days). In fully adjusted models that considered 4-level categorizations of current and lifetime exposure to tobacco and marijuana, tobacco smoking (both current and lifetime) was associated with a lower FEV1 and current smoking with a lower FVC. In contrast, exposure to marijuana (both current and lifetime) was associated with higher FVC and lifetime exposure with higher FEV1. At low lifetime exposure levels, increasing marijuana use was associated with an increase in both FEV1 and FVC. "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," the researchers write.
At more than 10 joint-years of lifetime exposure, there was a non-significant decline in FEV1, but there was a significant decline in FEV1 at more than 20 episodes of marijuana use per month.
"Marijuana may have beneficial effects on pain control, appetite, mood, and management of other chronic symptoms. Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. It is more difficult to estimate the potential effects of regular heavy use, because this pattern of use is relatively rare in our study sample; however, our findings do suggest an accelerated decline in pulmonary function with heavy use and a resulting need for caution and moderation when marijuana use is considered," the researchers conclude.