The number of American adults who are trying e-cigarettes for the first time appears to be stabilizing, a new government study finds.

While the proportion of adults who have ever used e-cigarettes increased about 3 percent from 2010 to 2012, there was no significant change in 2013, according to researchers from the Centers for Disease Control and Prevention (CDC).

“The long-term public health impact of these products is uncertain,” the study’s lead author, Brian King, told the Associated Press. He described the plateau in the rate of adults who have ever tried e-cigarettes “a positive note.”

The CDC found about two percent of adults said they had used at least one e-cigarette in the previous 30 days. About 75 percent of current e-cigarette users said they also smoked regular cigarettes. That figure has remained stable for the four years the CDC has been surveying adults about their e-cigarette use.

The findings appear in the journal Nicotine & Tobacco Research.

A study published this summer concluded that allowing e-cigarettes to compete with regular cigarettes might reduce deaths and illness caused by tobacco. The researchers reviewed 81 previous studies on the use and safety of e-cigarettes. The researchers noted that although the long-term health effects of e-cigarette use are unknown, compared with conventional cigarettes they are likely to be much less harmful to users or bystanders.

A group of leading lung health organizations recently urged governments to ban or limit the use of e-cigarettes until more is known about the devices’ health effects. The American Medical Association recently called for reining in the sale and marketing practices of companies that make e-cigarettes.