Invented by a Chinese pharmacist in 2003, electronic cigarettes are now used by millions worldwide as an alternative to smoking. But the devices are not regulated, and the World Health Organization has called for studies on their effects on human health.

In the new study, the researchers compared the heart function of 20 daily smokers before and after smoking one tobacco cigarette to that of 22 e-cigarette users before and after using the device for seven minutes. The people studied were healthy and varied in age from 25 to 45.

Heart function got worse in the tobacco smokers, and their blood pressure and heart rate rose. People using e-cigarettes experienced only a slight elevation in blood pressure.

American Heart Association spokesman Russell Luepker, MD, of the University of Minnesota in Minneapolis, says that because they "light up," electronic cigarettes may be preferred over other smoking cessation aids by some people trying to quit.

It’s not surprising they are less harmful than the real thing, he says. "The e-cigarette has the advantage of not having the thousands of other chemicals, besides nicotine, that a real cigarette has," he says.

"I don't think it's conclusive but there's no doubt if you expose someone to fewer bioactive chemical compounds there is going to be less effect on the heart," Luepker says. But they should only be used as a temporary bridge while quitting smoking, he says.

The e-cigarettes used in the study contained 11 milligrams per milliliter of nicotine (Nobacco, USA Mix) in the liquid. That’s a "moderate" amount, according to Farsalinos, who says they can contain up to 23 milligrams per milliliter of nicotine.

Many laboratory analyses have shown electronic cigarettes do not contain carcinogens, he says. But even in studies where formaldehyde and other carcinogens were found, the levels detected were 500 to 1,400 times less than the amount present in one tobacco cigarette, Farsalinos says.

"You would have to use e-smokes for six to eight months to get the amount of chemical present in a single tobacco cigarette," he says.