Which chemical in e-cigarette liquid is directly linked to bronchiolitis obliterans ('popcorn lung')?

Diacetyl, a chemical used to add buttery flavor in e-liquids and microwave popcorn, is directly linked to bronchiolitis obliterans ('popcorn lung') u2014 permanent scarring of the small airways. Nicotine is highly addictive but not the cause of this specific lung disease.