Does Cannabis smoke really cause more lung damage than tobacco smoke? Flaws in a viral study

Earlier this month, researchers published a study in the journal Radiology that was prime fodder for misleading headlines. The study sought to investigate the effect of marijuana smoke on the lungs.

The researchers say they found more lung damage in the marijuana smokers’ group than in the tobacco smokers’ group. That led to a rash of headlines like this one from the Chicago Sun-Times: Smoking marijuana might be more harmful to your lungs than tobacco, study suggests. But here’s the catch: That’s not actually what the study found.

The researchers, Radiologists at the University of Ottowa, note that with the legalization of recreational cannabis use in Canada and the United States, there’s a need to better understand the effects of cannabis smoke on the lungs. For the study, they analyzed the CT chest scans of three groups: marijuana smokers, tobacco-only smokers, and non-smokers, with the latter group serving as a control.

Of the 56 participants in the marijuana smokers’ group, 50 also had a history of smoking tobacco. Among the marijuana and tobacco smokers, the mean number of “pack years” among the marijuana smokers was 25. (“Pack years” are a way of calculating the amount smoked and the number of years a person smoked. For example, if you smoked two packs a day for one year, you would have smoked for two pack years; if you smoked half a pack a day for eight years, you would have smoked for four pack years.) The mean number of pack years among the tobacco-only group was 40.

The study authors concluded that the marijuana and tobacco group had more lung damage than to the lungs than tobacco alone did. That isn’t a terribly surprising conclusion. Inhaling smoke, be it wildfire, marijuana, or cigarette smoke, will damage your lungs. Certainly, some kinds of smoke can be more damaging than others, so it may well be that a history of inhaling two types of smoke might damage a person’s lungs more than inhaling just one kind.

But even that assessment is a reach unsupported by the findings, Peter Grinspoon, a professor of medicine at Harvard Medical School and author of the forthcoming book, Seeing through the Smoke: A Cannabis Specialist Untangles the Truth about Marijuana, tells Inverse.

“The only way they found participants for the marijuana group was screening [medical] charts for ‘marijuana’ and ‘cannabis.’ They didn't even talk to cannabis users. They didn't quantify how much cannabis the participants were using or whether it was illegally sourced cannabis, which can have contaminants, or legally sourced,” Grinspoon says.

The study was also incredibly small, making any conclusions hard to quantify. More importantly, there was no group comprised of marijuana-only smokers. When researchers refer to the tobacco group, they specify “tobacco-only,” but the marijuana plus tobacco group is simply called the “marijuana” group. Granted, six of the 56 people in the marijuana group didn’t have a history of smoking tobacco, but that’s further confounding. Why combine marijuana-only and marijuana-plus-tobacco people in the same group? The corresponding author on the study didn’t respond to Inverse’s request for comment.

Grinspoon says, “It is possible that there could be a synergistic effect between cannabis smoke and tobacco smoke. But to study that, you need a tobacco-only group, a cannabis-only group, and a both-cannabis-and-tobacco group.”

What they have, Grinspoon says, is “this bizarre retrospective study where they just randomly searched the charts for the word marijuana or cannabis.”

The study’s poor design begs the question: What do we know about the effect of marijuana smoke (and only marijuana smoke) on the lungs, and how does that compare to the effects of tobacco-only smoke?

A 2013 literature review published in the Annals of American Thoracic Society concluded that cannabis smoke causes “visible and microscopic injury to the large airways that is consistently associated with an increased likelihood of symptoms of chronic bronchitis that subside after cessation of use.” Cannabis smoke alone didn’t appear to increase a person’s risk of developing emphysema.

The review author, Donald Tashkin, notes that even habitual use of marijuana doesn’t appear to lead to significant abnormalities in lung function, “except for possible increases in lung volumes and modest increases in airway resistance of unclear clinical significance.”

Like cigarette smoke, cannabis smoke does contain carcinogens. Some studies have found that long-term cannabis smokers are more likely to develop cancers of the throat, lung, voicebox, head, and neck than nonsmokers. In those cases, however, the cannabis smokers were also habitual tobacco smokers. When cigarette smoke was removed from the equation, as one study concluded, “the association of these cancers with marijuana, even long-term or heavy use, is not strong and may be below practically detectable limits.”

The researchers at the University of Ottowa posited that the additional airway damage seen in the cannabis and tobacco group could be the result of inhaling unfiltered cannabis smoke, whereas most tobacco smoke is filtered, or that holding the smoke in the lungs longer could be the culprit. Both hypotheses are certainly possible. But unless the stated goal of the study is to analyze the effects of cannabis smoke on the lungs of tobacco smokers, including tobacco smokers in the marijuana group doesn’t make sense. And calling the study group the marijuana group when contrasted with a “tobacco-only” group is misleading.

Grinspoon finds this outrageous.

“The study should be retracted, honestly,” he says.

Inhaling any type of smoke can harm your lungs; that’s uncontested. And the damage different types of smoke cause somewhat depends on factors unrelated to the specific type of smoke, like the amount and duration of exposure. All of these factors are worth further study. But, when assessing the damage caused by cannabis smoke compared to tobacco smoke, all the best available evidence suggests that tobacco smoke is much more harmful to your lungs than marijuana smoke.