Researchers at the University of California, Riverside, found that heating propylene glycol, the main ingredient in most e cigarette liquids, can produce two toxic chemicals, methylglyoxal and acetaldehyde, which damage lung cells. Using 1,2 Propylene Glycol: A Biomarker of Exposure Specific to e Cigarette Consumption E cigarette, or Vaping Product, Use Associated Lung Injury (EVALI) Fact Sheets Yale Medicine Some of the most dangerous e cig flavors are cinnamon, vanilla, raspberry, pineapple, and orange. Besides flavors, e cigarette aerosol contains other dangerous chemicals like metals, formaldehyde