The peer-reviewed study tracked 5,115 young adults in Oakland, Chicago, Minneapolis, and Birmingham who smoked marijuana or tobacco or both and were given lung function tests repeatedly over the 20 years between 1985 and 2006. Their findings showed that while tobacco smokers clearly had declining lung capacity, marijuana smokers who used marijuana between one "joint year” (365 joints or filled pipe bowls over the 20-year-period) and 7 "joint years" (2,555 joints or filled pipe bowls over the 20-year-period) did not have impaired lung function.
"With up to 7 joint-years of lifetime exposure (eg, 1 joint/d for 7 years or 1 joint/wk for 49 years), we found no evidence that increasing exposure to marijuana adversely affects pulmonary function."
The study was unable to make conclusive statements about the effects of more frequent marijuana use because it lacked a large enough sample of heavy users, however the authors did state "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."
Dr. Donald Tashkin, a UCLA pulmonologist who researches medical marijuana but was not involved in the study, said that marijuana smoke may be less harmful than tobacco smoke despite many similar ingredients because the active ingredient THC (Delta-9-Tetrahydrocannabinol) may have anti-inflammatory effects. "We don't know for sure," he said, "but a very reasonable possibility is that THC may actually interfere with the development of chronic obstructive pulmonary disease."
Dr. Jeanette Tetrault, a substance abuse researcher at the Yale School of Medicine who was also not involved in the study, told Reuters: "I think a lot more work will need to be done to make any blanket statements about safety… These are only two measures of pulmonary function and don't really paint the entire picture."
Sources: Anahad O'Connor, "Marijuana Smoking Does Not Harm Lungs, Study Finds," www.nytimes.com, Jan. 11, 2012
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