The health effects of new products known as “heat-not-burn” cigarettes are still unknown, researchers caution in a new study.

The devices mix the electronics behind e-cigarettes with the tobacco-burning properties of regular cigarettes, according to HealthDay. The devices warm up tobacco to about 500 degrees Fahrenheit, producing an inhalable aerosol. Heat-not-burn cigarettes are not approved for sale in the United States. An application for approval was filed with the Food and Drug Administration late last year.

Researchers looked at Google searches about the devices in Japan, where they are available. They found that searches about the devices surged by more than 1,400 percent in 2015, when they were first released in Japan. Searches increased almost 3,000 percent between 2015 and 2017. There are as many as 7.5 million Google searches a month about heat-not-burn devices in Japan, the researchers report in PLOS One.

“We don’t know enough about the health implications of heat-not-burn tobacco products, and that lack of knowledge is extremely dangerous for public health,” said study lead author Theodore Caputi of the Wharton School at the University of Pennsylvania in Philadelphia.