A large-scale switch from tobacco to e-cigarettes would cut smoking-related deaths by a quarter in the United States by 2100, even assuming the gadgets are themselves not risk-free, researchers said Tuesday.

Scientists are still unsure about the potential harms of "vaping" as an alternative to traditional cigarettes, though most seem convinced it is at least safer.

Hypothesising that an e-cigarette carries only five percent of the health risk of the real McCoy, and that only a handful of people will still smoke tobacco by 2026, the researchers said 6.6 million premature deaths could be prevented by 2100.

This represented a 25-percent drop from the 26.1 million premature deaths projected under the status quo, with 19.3 percent of American men and 14.1 percent of women smoking in 2016, the study showed.

Per smoker, this amounted to an average gain in life expectancy of about four months, according to findings published in the journal Tobacco Control.

In a more pessimistic scenario which assumes that e-cigarettes come with about 40 percent of the risk of traditional smokes, some 1.6 million premature deaths are avoided, said the research team.

This works out to an average life expectancy gain of just under a month per person.

A death is notched up as premature when a person departs before their expected age -- say 75 or 80 depending on the country -- and is usually preventable through a healthier lifestyle. 

Research is continuing into the risks and benefits of e-cigarettes, with critics fearful the gadget's "safer" image will create a new generation of nicotine addicts and act as a gateway to traditional smoking.

But even under the researchers' pessimistic scenario, there were "gains to a strategy that used e-cigarettes to reduce cigarette smoking," study co-author David Levy of the Georgetown University Medical Centre in Washington told AFP.

The benefits were "massive", commented John Britton of the UK Centre for Tobacco & Alcohol Studies.

The findings, he said via the Science Media Centre, "demonstrate the importance of embracing, rather than rejecting, the potential of this new generation of nicotine products."

E-cigarettes, devices that seek to recreate the experience of tobacco smoking by heating a liquid to release a vapour that is inhaled like smoke, have exploded onto the market in recent years.

An estimated seven million people in Europe alone have taken up "vaping".

According to the World Health Organization, tobacco kills up to half of its users -- more than seven million people per year.

Of these, nearly a million are people exposed to second-hand smoke