People who use e-cigarettes indoors may be exposing the people around them to nicotine, a new study suggests. The amount of secondhand nicotine exposure from e-cigarettes is much smaller than from traditional cigarettes, the researchers conclude.

The study evaluated vapor from three brands of e-cigarettes, using a smoking machine in controlled exposure conditions, MedicalXpress reports.

The researchers, from the Roswell Park Cancer Institute in Buffalo, New York, also compared secondhand smoke exposure from conventional cigarettes to secondhand exposure to e-cigarette vapor. They concluded using e-cigarettes in indoor environments may involuntarily expose nonusers to nicotine, but not to toxic tobacco-specific combustion products.

E-cigarettes are designed to produce nicotine vapor without the combustion of tobacco, the article notes. When a person takes a puff of an e-cigarette, the nicotine solution is heated, and the vapor goes into the lungs. No sidestream vapor is generated between puffs, but some of the mainstream vapor is exhaled by the e-cigarette user.

“Our data suggest that secondhand exposure to nicotine from e-cigarettes is on average 10 times less than from tobacco smoke,” lead researcher Maciej Goniewicz, PhD, PharmD, said in a news release. “However, more research is needed to evaluate the health consequences of secondhand exposure to nicotine from e-cigarettes, especially among vulnerable populations including children, pregnant women and people with cardiovascular conditions.”

The study appears in Nicotine and Tobacco Research.

The Food and Drug Administration (FDA) was expected to propose rules on regulating e-cigarettes by October, but the agency has yet to do so. The agency did send a proposed rule in mid-October to the federal Office of Management and Budget, which will review the rule before it is available for public comment.

The FDA is expected to consider e-cigarettes as tobacco products, which will allow the agency to provide the same federal oversight that applies to cigarettes, chewing tobacco, cigarette tobacco, and roll-your-own tobacco.