We don’t have long-term, population-level data yet, so the exact risk cannot be quantified, but the early signs are strong enough that scientists are warning against repeating the mistakes made with cigarettes.

“Though smoking was once given the benefit of doubt,” write study co-authors Freddy Sitas and Bernard Stewart of the University of New South Wales in Australia in a related commentary, “the same should not now be accorded to vaping given the strength of relevant carcinogenicity data.”

Vaping emerged in the early 2000s, touted as a safer, less smelly delivery system for the addictive chemical nicotine than methods that involve inhaling the smoke of burning tobacco leaves. Instead, a device heats and vaporizes a nicotine-containing liquid, which the user then inhales.