Nicotine in e-cigarettes typically is labeled as mg/ml or percent nicotine, but these metrics may be difficult to understand. We examined whether adults who use e-cigarettes accurately can compare nicotine concentrations presented as mg/ml and percent and/or can identify the strength of concentrations presented using these metrics.
310 adults completed an online survey in 2020. Participants viewed nicotine concentration pairs (e.g., 24 mg/ml vs 3%) and indicated which concentration was stronger or if they were equal. Participants also categorized nicotine concentrations presented as mg/ml (0-60 mg/ml) and percent (0-6%) into strength categories (no nicotine, very low, low, medium, high, very high). “Correct” answers for the strength of each concentration (e.g., 30 mg/ml is “high” nicotine) were based on the subjective opinion of e-cigarette researchers.
When making direct comparisons, adults correctly identified that one concentration was stronger or that both were equivalent about half of the time (8.30 [5.08] of 19). Adults answered correctly more often when the stronger concentration was presented in mg/ml (4.75 [2.74] of 8) than percent (2.54 [2.60] of 8), p < .001). Adults "correctly" identified the strengths of 8.90 (3.22) of 18 nicotine concentrations, with more "correct" responses for mg/ml (5.27 [2.15]) than percent (3.63 [1.71], p < .001). Adults classified concentrations presented as percent as weaker than equivalent concentrations presented as mg/ml.
Adults had difficulty understanding nicotine concentrations labeled using the most common metrics, especially percent nicotine. A singular, easy-to-understand labeling system may increase public knowledge about the nicotine concentration/strength of vaping products.

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