Survey results released last week indicate that use of electronic cigarettes by American teenagers continues to rise, even as their use of conventional cigarettes continues to fall. You might think these diverging trends would give pause to critics who worry that e-cigarettes are "reglamorizing" the old-fashioned, combustible kind. Yet opponents of vaping seem undeterred by reality's failure to match their predictions. Longtime anti-smoking activist Stanton Glantz recently told USA Today "there's no question that e-cigarettes are a gateway to smoking."

If e-cigarettes were "a gateway to smoking," you would expect to see people try them first, then move on to traditional cigarettes. But according to a recent review in the journal Addiction, "Regular use of [e-cigarettes] by non-smokers is rare, and no migration from [e-cigarettes] to smoking has been documented." Such migration would not necessarily show that vaping encourages smoking, since people predisposed to smoke may be especially likely to try e-cigarettes. But if Glantz were right, this sequence would be common.

Furthermore, if Glantz were right, the rising popularity of e-cigarettes would be accompanied by increased consumption of conventional cigarettes. But except for one study showing vaping and smoking rising in tandem among Polish teenagers, there seems to be no evidence this is happening. It certainly is not happening in the United States, where smoking rates among teenagers have reached record lows as more and more of them have tried e-cigarettes.

This year the government-sponsored Monitoring the Future (MTF) Study, which has been asking teenagers about their drug use since 1975, asked them about e-cigarettes for the first time. It found that "more teens use e-cigarettes than traditional, tobacco cigarettes or any other tobacco product." Specifically, MTF numbers published last Tuesday show that 8.7 percent of eighth-graders, 16.2 percent of 10th-graders, and 17.1 percent of 12th-graders reported using e-cigarettes in the previous month.

By comparison, the National Youth Tobacco Survey (NYTS), sponsored by the U.S. Centers for Disease Control and Prevention (CDC), last year put the prevalence of past-month e-cigarette use among all high school students at 4.5 percent. Although the two surveys are not directly comparable, the gap suggests that past-month e-cigarette use by teenagers, which according to the NYTS tripled between 2011 and 2013, continues to climb. We will have a clearer idea of the trend when the 2014 NYTS data are released next year.

"The numbers are stunning," Matthew Myers, president of the Campaign for Tobacco-Free Kids, told The New York Times. The increase in the percentage of teenagers reporting e-cigarette use is indeed dramatic. But Michael Siegel, a professor of public health at Boston University who blogs about tobacco policy, points out that past-month use may be little more than experimentation. In a 2013 survey of high school students in Hawaii, 18 percent reported past-month use, but only 2 percent reported daily use.

In any case, higher vaping rates are not associated with higher smoking rates. The prevalence of past-month e-cigarette use in the Hawaii survey was four times the nationwide rate indicated by the 2013 NYTS. Yet the past-month smoking rate among teenagers in Hawaii was just 7 percent, about half the national average.

In the MTF survey, the "stunning" rates of past-month e-cigarette use that worry Matthew Myers coincided with the lowest past-month smoking rates seen in the history of the study: 4 percent among eighth-graders, 7.2 percent among 10th-graders, and 13.6 percent among 12th-graders. Daily smoking also was less common than ever before. It was reported by 6.7 percent of high school seniors, down from 26.9 percent in 1975, when the study started, and 12.3 percent in 2007, when e-cigarettes were first marketed in the United States. The NYTS and the CDC's National Youth Risk Behavior Survey show similar downward trends in smoking.

Although activists like Glantz and public health officials like CDC Director Tom Frieden worry that e-cigarettes "could become a gateway for young people to take up real cigarettes," notes New York Times science reporter Sabrina Tavernise, "that does not seem to be happening." Far from showing that e-cigarettes are training teenagers for the real thing, the fact that vaping and smoking rates are moving in opposite directions suggests that e-cigarettes may be replacing combustible cigarettes among people who otherwise would be smoking.

The MTF data provide some additional support for that hypothesis. Among the 12th-graders who said they had ever smoked a cigarette, for example, 17.5 percent said they had used only cigarettes in the previous month, 16.7 percent said they had used only e-cigarettes, 21.5 percent said they had used both, and 44.3 percent had used neither. This pattern suggests that e-cigarettes may be helping some adolescent cigarette smokers cut back or quit. Even when teenagers try e-cigarettes first, some of them might otherwise have smoked the conventional kind. The upshot in both cases would be the same: less smoking and less tobacco-related disease, something anti-smoking activists like Glantz ought to welcome.

As Tavernise notes, "most experts agree that e-cigarettes are far less harmful than traditional cigarettes." That is why the crucial question, when it comes to assessing the public health impact of e-cigarettes, is whether they compete with tobacco cigarettes or somehow expand the market for them. The evidence so far indicates that more vaping means less smoking, not the other way around.

This article originally appeared at Forbes.com.