Though it's the season to give (and shop), two Boulder head shops say sales of one item in particular has been growing for months, well before the holiday season — vape pens.

Vapor pens, or vapor sticks, are used to smoke a marijuana concentrate made of wax or oil (thus they're commonly described just that way: "wax" or "oil"). By an electric charge, they heat the product until combustion into a vapor.

"It's not just the holiday. They are selling ... exponentially," Aaron Jones said, manager at Mile High Head Shop, on Pearl Street. "It started this year and has been growing ever since."

Although Mile High would not release specific numbers on sales, Jones said pen sales were booming.

"We are seeing a couple vape pen sell a day," Jones said. "The buy-two-and-get-one-free sale is really popular."

The Fitter head shop is seeing a similar popularity in vapor stick sales. The store has seen the biggest jump in sales for the electronic smoking devices in the past four months, said Fitter co-owner Betty Gruskin.

"They are here to stay," Gruskin said. "People have been turned on to them for some time now. Vape sticks are gaining popularity more than anything else."

Gruskin said they seem to be an impulse buy, and that in her experience, older customers are buying them for health reasons, while younger customers are buying them for "discrete smoking."

"People come in and check it out, then end up buying it even though they had no intention to," Gruskin said.

Contact Gavin B. Griffin at gavinbgriffin@gmail.com.