Stuff.co.nz (from Reuters) has a reasonably good article about a new research report in the journal JAMA on marijuana, tobacco, and lung function. It’s also worth pointing out that, as usual, there is more information on line that the newspapers don’t tell you about: the abstract, and a video interview (ie, press release) with the main author.
The research report is from the CARDIA study, which is following up 5000 young adults in four US cities, primarily to look at cardiovascular disease risks. This paper uses data on tobacco smoking, marijuana smoking, and lung function, and finds a decrease in lung function in people who took up tobacco smoking, but not in most of those who smoked pot. The study is particularly interesting because the participants were recruited before they started smoking.
The researchers found that lung function of marijuana smokers doesn’t start going down until the cumulative exposure gets up to around 10 joint-years (ie, 1 joint/day for ten years, two/day for five years, and so on), which is a pretty high level, reached by only a few of the CARDIA participants. This is contrasted with tobacco, where negative effects start showing up at exposure levels that are quite common. In fact, and this is whats getting the press, the average lung function is very slightly higher in pot smokers than non-smokers, though not by enough that you would notice without sensitive machinery and thousands of measurements.
Most of this is just that doses of marijuana are much, much lower. In fact, the negative associations at 10 joint-years cumulative exposure to marijuana are pretty similar to those at 10 pack-years cumulative exposure to tobacco. That is, the data are consistent with a single joint doing as much short-term lung damage as a whole pack of cigarettes, and certainly indicate that a joint does more damage than a single cigarette.
The mysterious part is the slightly higher lung function among moderate-level pot smokers. The authors give a number of speculations. My speculation would be that asthmatics, and other with sensitive airways, are slightly less likely to take up smoking pot. I think this would actually be testable in the CARDIA data. In any case, this is careful analysis of pretty high-quality data, done by people with no particular axe to grind, which makes a nice change.