Ingredient/Byproduct	Evidence Strength	Risk Level (Real-World Use)	Consumer Action
Propylene Glycol (PG)	Strong (multiple peer-reviewed studies)	Moderate	Prefer higher VG ratios; avoid dry hits
Vegetable Glycerin (VG)	Strong	Low-Moderate	Lower acrolein than PG's formaldehyde; still produces byproducts
Nicotine (salt form)	Strong (Christen et al. 2025)	High (addiction)	Start at lowest effective concentration
Diacetyl	Strong (occupational studies)	High if present	Choose diacetyl-free brands
Cinnamaldehyde	Moderate (in vitro)	Moderate	Avoid cinnamon-flavored e-liquids
Sucralose	Moderate (2022, 2025 studies)	Moderate	Avoid sweetened e-liquids
Heavy metals (Ni, Cr, Pb)	Strong (2024, 2025 studies)	Moderate-High (pH dependent)	Use neutral-pH liquids; replace coils
Methylglyoxal (accumulated)	Strong (2026 study)	High in used disposables	Don't use depleted devices
Vitamin E acetate	Strong (CDC EVALI investigation)	None in legal nicotine products	Only buy from licensed retailers