Almost one-third of a sampling of Hawaii ninth- and 10th-graders reported using e-cigarettes last year, more than double the percentage of U.S. high schoolers surveyed in a national study.

According to a University of Hawaii Cancer Center study, 29 percent of 1,941 Hawaii students surveyed reported using e-cigs — more than twice the 12 percent of students grades 9 through 12 surveyed across the country.

The UH study, reported in the journal Pediatrics and announced Monday, showed the use of e-cigs is higher in Hawaii than in other places and its use has exploded in the last three years among adolescents, but why the use is elevated here was unclear.

The study compared three groups of users (e-cig users, tobacco cigarette smokers and dual users) with standard psychosocial risk factors for substance use, which revealed a second surprising result.

"We had thought at first e-cigarette users looked like other smokers," said Thomas Wills, interim director for the Cancer Prevention & Control Program at the UH Cancer Center. "The surprise was they didn’t. What we found was people who only used e-cigarettes were intermediate on the profile of risk factors."

Nonusers had low levels of risk factors whereas dual users and smokers had high levels of risk factors.

Of those surveyed, 17 percent "vaped" e-cigs only, 3 percent smoked cigarettes only, 12 percent were dual users and 68 percent were nonusers of either product.

Teens of Native Hawaiian and Filipino descent had a higher rate of use than other ethnicities.

(Participants were not asked whether they had smoked cigarettes in the past.)

The national study showed 35 percent had smoked cigarettes.

Wills contends aggressive marketing in Hawaii may be behind the trend.

"We’re inclined to suspect that what’s happening is that the e-cigarette (industry) is operating to recruit relatively low-risk teens, who may eventually become cigarette smokers," he said. "It exists in other states, too, but the density (of the Hawaii marketing) seems to be greater than in other states."

Advertising and shops for e-cigs are everywhere, he said. Ads are at movie theaters, on the radio, in shopping malls and commercial districts.

Volcano Fine Electronic Cigarettes is one of the largest e-cigarette retailers on Oahu with shops in every major mall with one exception (Kahala Mall), and two at Ala Moana Center.

According to its website, Volcano is headquartered in Hawaii and is both a manufacturer and a retailer of e-cigs and vaping accessories, with 11 locations in Hawaii and 18 in other states, the U.K. and Southeast Asia.

"The other thing we suspect is flavors," up to 42 at last count, which seem to appeal to adolescents, Wills said.

He said there are "a number of really tantalizing flavors such as pineapple, mango, cotton candy, bubble gum."

The high tax of tobacco cigarettes in Hawaii also may account for the higher use of e-cigs.

Whether e-cigarettes could be considered a "gateway drug" has yet to be answered, but Wills is hoping to obtain grants to conduct such studies, which would follow a group of individuals over time.

He and his researchers surveyed students in both public and private schools, obtaining a snapshot of tobacco and e-cigarette use.

Questions for the risk profile focused on sensation-seeking, rebelliousness and perceptions of smokers — cool, popular, attractive or stupid, ugly and careless. Those with positive perceptions of smokers were more likely to take it up.

Although the Legislature passed a law in 2013 banning the sale of electronic smoking devices to minors, these 14- and 15-year-olds are somehow getting their hands on the devices and the liquid, which may or may not contain nicotine, and is vaporized in the devices.

Hawaii County on July 1 banned the sale of cigarettes and e-cigarettes to anyone under 21.

Although tobacco smoking is known to cause cancer and increases the risk of heart disease mainly from tars and carbon monoxide, the dangers of vaping e-cigarette liquids is unknown and are being studied.

A recent New York Times report painted a picture of a grimy workshop in Shenzhen, China, with boiling vats of chemicals, where factory workers were making tube casings for e-cigs.

It said one study found e-cigarette vapor contained hazardous nickel and chromium at four times the level in traditional cigarette smoke, and another found half the e-cigs sampled malfunctioned and some released vapors tainted with silicon fibers.

In China, there are no regulations and manufacturers operate without oversight. It produces 90 percent of the world’s e-cigarettes and is expected to ship 300 million e-cigarettes to the United States and Europe.

The U.S. Food and Drug Administration is moving toward standards for the e-liquids, so some Chinese manufacturers are moving their production facilities to the West, the New York Times reported.