E-cigarette use dose measures among e-cigarette only users and dual users.

<p>(a) # days per month was defined as “number of days of e-cigarette use in the past 30 days” (b) # cartridges per day: “number of e-liquid cartridges/refills used per day” (c) # puffs per day: “number of puffs off an e-cigarette per day.” Error bars denote the interquartile range.