AbstractBlood phosphatidylethanol (PEth), a metabolite of ethanol, is emerging as a direct biomarker of choice for characterizing ethanol consumption in clinical, research, and forensic contexts. An accumulating body of evidence, and a recent international consensus conference, supports a cutoff of 20 μg/L of PEth (16:0/18:1) to distinguish abstinence from beverage ethanol consumption. There is a dearth of research, however, on whether exposures to nonbeverage ethanol sources are sufficient to produce PEth concentrations that exceed this cutoff. To explore this possibility, we recruited 30 participants, who indicated past‐90‐day abstinence from beverage alcohol, to characterize their past‐30‐day nonbeverage ethanol exposures (including source, frequency, and intensity of exposures) and to undergo PEth testing. Two of the 30 participants (6.7%) produced PEth concentrations ≥20 μg/L. One of these participants (PEth = 26 μg/L) reported multiple ethanol exposure sources, including near‐daily intensive exposures to ethanol vapor. The other participant (PEth = 49 μg/L) reported only once‐daily use of an ethanol‐containing mouthwash; the veracity of his abstinence claim is refuted. The results of this study support a rebuttable presumption that PEth ≥20 μg/L is indicative of beverage ethanol consumption. They suggest, however, that intensive, incidental alcohol exposures have the potential, under unusual circumstances, to result in PEth concentrations that modestly exceed this threshold.