There is an on-going debate over the presence of diacetyl in e-liquids and the level of the risk it could pose. Here is a good article from Vaping360 which gives a good overview of the current state of the research and how it applies to vapers.
If you're still unsure, you can also make an effort to avoid creamy, buttery flavors of e-juice, since diacetyl is normally used in those sorts of flavor profiles.
It's worth noting that levels of diacetyl are much higher in cigarette smoke than in e-cigarette vapor (100 times higher, according to Dr. Farsalinos' article linked above), so the relative risk of smoking is still higher than vaping.
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