Smokers who switch to e-cigarettes - even if it's only some of the time - may dramatically reduce their exposure to air pollutants including carbon monoxide and acrolein, a British study suggests.

Researchers gave e-cigarettes to 40 smokers who said they wanted to quit. After four weeks, the 16 participants using only e-cigarettes had about an 80 percent drop in exposure both to carbon monoxide and to acrolein, a harmful breakdown product that is also in some e-cigarettes' vapor.

The 17 participants who swapped some regular cigarettes for the electronic version had a 52 percent decline in carbon monoxide exposure and a 60 percent decline for acrolein, according to the study published in Cancer Prevention Research.

To get the most benefit from switching to e-cigarettes, smokers need to completely give up traditional cigarettes, noted lead study author Hayden McRobbie, of the Wolfson Institute of Preventive Medicine at Queen Mary University of London.

"This usually happens over a period of time and smokers may get some encouragement from the finding that there is some potential health benefit as soon as they start the process," McRobbie said by email.

Battery-powered e-cigarettes typically feature a glowing tip and a heating element that turns liquid nicotine and other compounds into a cloud of vapor that users inhale.

While tobacco control advocates fear that e-cigarettes may give rise to a new generation of nicotine addicts who eventually transition to conventional cigarettes, the current study adds to a small but growing body of evidence suggesting the devices might benefit the health of people who already smoke.

An international analysis of published research by the Cochrane Review in December concluded the devices could help smokers quit but said much of the existing research on e-cigarettes was thin.

Even though the current study points to another potential benefit of e-cigarettes, more evidence is still needed from longer and larger trials to before scientists can draw firm conclusions about any safety advantages, said Dr. Nancy Rigotti, director of tobacco research at Massachusetts General Hospital in Boston.

"It is exactly the type of incremental, careful work that is needed but it is not yet a definitive study," Rigotti, who wasn't involved in the study, said by email.

Over time, carbon monoxide inhaled with cigarette smoke can decrease the amount of oxygen carried in red blood cells. It can also lead to hardening of the arteries, heart disease and strokes.

Acrolein is present in tobacco smoke and the vapor from e-cigarettes that use vegetable glycerin in their nicotine liquid, according to the study authors. Acrolein is known to irritate exposed tissues and can destroy cilia, the tiny hairs that sweep dirt and bacteria out of the lungs and airways, making smokers more susceptible to lung disease and other illnesses.

Study participants were typically in their 40s and had attempted to quit at least twice before joining the trial. All of them were offered the same type of e-cigarette and encouraged to completely abandon traditional cigarettes.

Researchers measured carbon monoxide in participants' breath one week before switching to e-cigarettes, on the day they switched and again four weeks later. They followed the same schedule for testing participants' urine for exposure to acrolein.

Because the e-cigarette model used in the study did contain vegetable glycerin, the study authors note it was possible dual users of tobacco and e-cigarettes might increase their acrolein exposure. So the significant drop in acrolein in this group was the "headline finding," they write.

One limitation of the study, the authors acknowledge, is it only included people with a desire to quit smoking, making it possible the results would be different for smokers with no intention of kicking the habit. It's also possible that the specific model of e-cigarette used in the study might not be representative of other devices.

Still, the findings suggest smokers should be told e-cigarettes may curb their exposure to toxic chemicals, said Dr. Riccardo Polosa, head of the tobacco research center at the University of Catania in Italy.

"This study adds to the evidence that e-cigarettes are much less harmful compared to conventional cigarettes," Polosa, who wasn't involved in the study, said by email.