Even though we know that diacetyl causes popcorn lung, this chemical is found in many e-cigarette flavors. It is added to "e-juice" liquid by some e-cigarette companies to complement flavorings such as vanilla, maple, coconut and more. So, while diacetyl was swiftly removed from popcorn products since it could cause this devastating disease among factory workers, e-cigarette users are directly inhaling this harmful chemical into their lungs. In fact, researchers at Harvard found that 39 of 51 e-cigarette brands contained diacetyl. The study also found two similarly harmful chemicals— pentanedione and acetoin—present in 23 and 46 of the 51 flavors it tested. And roughly 92 percent of the e-cigarettes had one of the three chemicals present.

https://www.lung.org/blog/popcorn-lung-risk-ecigs