A study reported today in The Journal of the American Medical Association claims to present evidence that "e-cigarette use is aggravating rather than ameliorating the tobacco epidemic among youths." The authors, Lauren Dutra and Stanton Glantz of the Center for Tobacco Research and Education at the University of California in San Francisco, claim their results "suggest that e-cigarettes are not discouraging use of conventional cigarettes." They add that their findings "call into question claims that e-cigarettes are effective as smoking cessation aids." But as Boston University public health professor Michael Siegel observes on his tobacco policy blog, Dutra and Glantz "make one of the most cardinal errors in all of epidemiology" by ignoring "the principle that 'correlation does not equal causation.'"
Dutra and Glantz's study is based on data from the National Youth Tobacco Survey, the same study the U.S. Centers for Disease Control and Prevention (CDC) cited last September in sounding the alarm about the recent increase in e-cigarette use among teenagers. The CDC neglected to mention that smoking fell as vaping rose, a trend that might have cast doubt on its warnings that e-cigarettes are a gateway to the real thing. Dutra and Glantz try to make that fear plausible by looking at associations between e-cigarette use and smoking in survey results from 2011 and 2012. They find that "use of e-cigarettes was associated with higher odds of ever or current cigarette smoking, higher odds of established smoking, higher odds of planning to quit smoking among current smokers, and, among experimenters, lower odds of abstinence from conventional cigarettes." In other words, e-cigarette users were more likely to be smokers, tended to smoke more, and were less likely to have stopped smoking, even though they were more likely to say they would like to quit.
The problem, of course, is that a cross-sectional study like this one does not tell us which came first: vaping or smoking. Dutra and Glantz concede that "the cross-sectional nature of our study does not allow us to identify whether most youths are initiating smoking with conventional cigarettes and then moving on to (usually dual use of) e-cigarettes or vice versa." If teenagers try e-cigarettes as a substitute for the conventional kind, it is hardly surprising that vapers are more likely to be smokers. In both 2011 and 2012, half of the current (past-month) e-cigarette users were also current smokers, which is consistent with the hypothesis that vaping is a strategy for cutting down or quitting. It is plausible that smokers with a strong attachment to cigarettes would be especially likely to try that strategy, which could explain why e-cigarette users smoked more and were less likely to have abstained from tobacco.
David Abrams, executive director of the Legacy Foundation's Schroeder Institute for Tobacco Research and Policy Studies, made that point in an interview with The New York Times. "I am quite certain that a survey would find that people who have used nicotine gum are much more likely to be smokers and to have trouble quitting," he said, "but that does not mean that gum is a gateway to smoking or makes it harder to quit." Thomas Glynn of the American Cancer Society likewise cautioned that "the data in this study do not allow many of the broad conclusions that it draws."
Even if we knew that some people start with vaping and move on to smoking, that would not necessarily mean that e-cigarettes made them more likely to smoke. We still would not know what would have happened in the absence of e-cigarettes. Would those same people have started smoking anyway, or did the experience of vaping somehow prime them to like a habit that otherwise would not have attracted them? The same sort of question comes up in discussions of marijuana's purported role as a "gateway" to other drugs. In both cases, symbolism and emotion seem to carry more weight than evidence and logic.