Estelle Green affectionately refers to her electronic cigarette as her "lifesaver."

The Haiku Plantation grandmother began smoking cigarettes decades ago as a senior in high school. She quit for a few years but returned to her two-plus pack-a-day habit after gaining 25 pounds. Since switching to an e-cigarette three years ago, Green said she’s saved hundreds of dollars, banished the lingering tobacco smell that bothered her grandchildren and says the spots on her lungs and her wheeze have vanished.

But the switch brought something even better.

"I’m able to sing in church," she said, "and I can hit the higher notes."

Green has joined millions of people nationwide who have made the switch from traditional cigarettes that burn tobacco to e-cigarettes, the tobacco-free, battery-powered technology that delivers nicotine in vapor form. For a new generation of smokers — and older ones who have found themselves increasingly ostracized — e-cigarettes are seen as a hipper alternative to smoking and one that is more socially acceptable.

The estimated 3.5 million users nationwide represent a nearly fivefold increase in three years, according to the Tobacco Vapor Electronic Cigarette Association. Electronic cigarettes are a $600 million market, the association says.

But for all the converts to smoke-free puffing, a growing number of health authorities, businesses and anti-tobacco groups remains unconvinced that "vaping," as it’s known among aficionados, is a safe alternative to tobacco. As a result, anti-smoking laws and public-space policies have been evolving to restrict e-cigarette use.

State health officials are grappling with regulating this new breed of cigarette. Allowing use of electronic smoking devices in areas where smoking is restricted by state law would be confusing to consumers and open the door to potential conflicts, said Lila Johnson, the Department of Health’s acting program manager for tobacco prevention and control. She said the department has received numerous calls from businesses, restaurants and agencies that want to create policies banning e-cigarettes in their facilities.

"We’ve been getting calls from businesses like restaurants, asking what do we do with these people who seem to think they have the liberty to light up wherever they want," Johnson said. "We reply that some establishments … have dress codes, so in that light, it’s up to each individual establishment on how they want to address customers using e-cigarettes."

All U.S. domestic airlines have banned their use, and as of May 1, e-cigarettes were rolled into TheBus’ no-smoking policy. Many public venues such as the Stan Sheriff Center at the University of Hawaii don’t allow vaping on the premises, although Aloha Stadium permits use of e-cigarettes in designated smoking areas.

So what are e-cigarettes?

There are a couple of big differences between e-cigarettes and regular smokes. For one thing, they don’t use tobacco and emit a nearly odorless vapor instead of a cloud of smoke containing the toxic byproducts of burning tobacco. While some of the devices look like a traditional cigarette, other versions include a pen-shaped model and a metal tube often worn around the neck on a lanyard.

The devices have three components: a cartridge of liquid nicotine that also serves as the mouthpiece, a battery and an atomizer. As users inhale through the mouthpiece, the battery is activated, switching on the atomizer, which heats the liquid nicotine to produce a vapor that users inhale. A puff of vapor is released but quickly dissipates. (With the devices that resemble an actual cigarette, the "lit" end glows because of an LED light.)

The cartridges come in flavors such as cherry, mint and cappuccino.

E-cigarettes contain no tobacco products; the nicotine is synthetic. Propylene glycol, a common food additive, is mixed into the liquid as a vaporizing agent.

Since the vapor is virtually odorless, many users feel they are not subject to rules governing tobacco products. Even with a no-smoking policy in place, some nightclubs have had to deal with patrons "vaping" indoors. Rob Ramsey, general manager of The Republik on Kapiolani Boulevard, said he is trying to find a workable compromise.

"While we continue to monitor the venue legalities of e-cigarettes, we are encouraged by those who are taking steps not to use them in the club to improve the quality of lives for both themselves and those around them," Ramsey said.

But some legislation is already here. Newly enacted Act 227, signed by Gov. Neil Abercrombie, prohibits the sale of e-cigarettes to anyone under 18. There’s cause for that: The 2011 Hawaii Youth Tobacco Survey reported that 5.1 percent of public high school students admitted to having tried an electronic smoking device.

Underage use also prompted the state Department of Education to classify possession and/or use of e-cigarettes on school property as a contraband offense subject to discipline.

A representative of the American Cancer Society said the focus should be on pushing the U.S. Food and Drug Administration "to exercise any authority over electronic cigarettes."

Cory Chun, government relations director for the society’s Cancer Action Network in Hawaii, said "at this time, electronic cigarettes are so new, there is no science out there to determine if they are safer than cigarettes or that the vapor is not harmful."

And although e-cigarettes are touted as an effective method to kick the nicotine habit, the cancer society continues to stand by previously approved therapies in patch and gum form.

Both the U.S. Centers for Disease Control and Prevention and the FDA, along with other health officials, caution that e-cigarettes haven’t been around long enough for adequate research on their possible health risks.

E-cigarettes were first introduced in China and began appearing in the United States about five years ago. A recent CDC report noted that in 2011, about 21 percent of adults who smoke traditional cigarettes had used e-cigarettes, up from about 10 percent in 2010.

Jessica Yamauchi, executive director of the Coalition for a Tobacco-Free Hawai‘i, said until further research is done, her group believes exposure to e-cigarette products should be limited to ensure the health and safety of the public.

It’s not hard to find fans of electronic cigarettes, though. Users can be seen puffing on the devices in their cars, on the street, and in nightclubs and cafes throughout Honolulu.

Converts say they have extra cash in their pockets after switching from regular cigarettes to the electronic variety. A pack of Marlboro Lights at one Waikiki store was priced at $11.15. Prices for disposable e-cigarettes range from $7 to $12 and from $25 to $100 for starter kits that include charging accessories. Refill cartridge packs vary in price depending on nicotine content, and liquid for do-it-yourself refills are sold, too. Each cartridge is good for several uses.

"The (technology) has been around for eight years," said Scott Rasak, director of marketing for Volcano, the largest e-cigarette retailer in Hawaii. "It just seems new because of increased public awareness and adoption."

Rasak said company sales tripled from last year. Volcano recently opened new locations at Ward Warehouse and in Ewa Beach on Fort Weaver Road, and will be opening in Wahiawa this month, with retail stores on Kauai and Maui in the offing.

"It’s become a fad in Hawaii," he said. "They’re the hipper, cooler alternative to regular cigarettes."

At the Volcano Vapor Cafe in Kapahulu, Shenna Aguilar of Makaha said she smoked a pack and a half of cigarettes a day for 10 years before switching to one of the devices earlier this year.

"Using this is more of a replacement to cigarettes," she said, "but I am using less nicotine because I want to cut back. It’s a gradual weaning process. There’s no more cigarette butts, so it’s better in a ‘green,’ environmental way. Plus there’s no more stinky clothes, I’m more mindful of the people around me, and it’s cut down on my break time at work."

Construction worker Darell Maglinti of Waipahu helped build the Kapahulu cafe. Once he found out what the company had to offer, he tried it, was won over and immediately handed his pack of cigarettes to the cafe’s staff.

"I’ve been using the device for a year and a half now," he said while visiting an e-cigarette kiosk at Ala Moana Center. "I used to do two packs a day, and this is the best thing that has ever come out. I had a longtime habit and my doctor basically told me, ‘quit or die.’ Now, I don’t cough, I don’t smell of cigarettes, and my girlfriend loves me for that.

"I don’t use the liquid nicotine. I don’t inhale. This just permits me to puff. I save big bucks — I used to buy a carton every eight days or so. Now, I just bought these two flavor bottles of banana and watermelon that cost me around $20 that’ll last about two months," he said.

"I just puff away at work or when I’m out drinking."