E-cigarettes can have sudden and deadly side-effects, as recent panic over a mysterious vaping illness has shown. But a new study from researchers at the University of California San Francisco demonstrates that vaping’s long-term effects could have significant public health ramifications as well.

The study, published Monday in the American Journal of Preventive Medicine, used data from 32,000 adults collected by the FDA and NIH as part of the Population Assessment of Tobacco and Health Study (PATH). By using statistical modeling to parse out different lifestyle factors that might affect a person’s health, the researchers identified vaping as a clear cause of lung disease in adults, independent of whether or not the user also smoked.

E-cigarettes have been on the market since 2003, but research on their effects is still in its infancy. Data from the PATH study, which is longitudinal (meaning it follows the same people over multiple years) is accessible by anyone, meaning a wide variety of researchers can use it to look at different impacts of vaping. It has been ongoing since 2013, and this paper analyzed data from 2013-2016.

Over that three-year period, the researchers found clear evidence that vapers were developing lung disease. “I was a little bit surprised that we could find a longitudinal association with just two years of followup,” says study author and UCSF public health professor Stanford Glantz.

Chronic lung diseases develop over time, he says, so even if vaping has a negative impact, he wouldn’t have expected the effects to be visible in health data yet. “The fact that we can detect a signal in just two years shows that they’re really bad,” he says.

Adults who’ve used e-cigarettes are 1.3-times more likely to develop lung diseases like chronic bronchitis, emphysema, or asthma than adults who have never vaped, the study found. But the data does suggest that electronic devices are a relatively good move for adults who already smoke: Regular use of cigarettes, pipes, cigars, or other tobacco made lung disease 2.6-times more likely.

But although e-cigarettes were originally touted as cessation aid, the majority of adult vapers are actually “dual users”: they smoke both regular cigarettes and electronic devices. This new study found that dual users are 3.3-times more likely to develop lung disease than someone who has never smoked or vaped. “The risks multiply,” says Glantz.

“I think it’s a critically important paper,” says Harvard University professor of public health Joseph Allen, who was not involved with the study. “We don’t have any information as of now on the potential long-term impacts of e-cigarettes on respiratory health.”

The results represent a first grim step toward producing that information, he says. And they provide a sad confirmation of what researchers already suspected—that e-cigarette use has a dramatic long-term effect on lung health.

“The reality is, millions of e-cigarette users are unwitting participants in an experiment,” Allen says, “and we don’t yet know the results.”