Significantly more Americans are using electronic-cigarettes and other vaporizing devices than a year ago, but most of those consumers are also smoking conventional cigarettes, according to a new Reuters/Ipsos poll.

About 10% of US adults now vape, according to the online Reuters/Ipsos poll of 5,679 Americans conducted between May 19 and June 4 — that’s almost four times higher than a US government estimate that 2.6% of adults used e-cigarettes in 2013.

About 15% of poll participants under the age of 40 now vape.

The surge in use comes as conventional cigarette smoking has declined in the United States to about 19% of adults, prompting tobacco companies like Altria Group Inc, Lorillard Inc and Reynolds American Inc to rush into the e-cigarette market.