OBJECTIVE: Preoxygenation decreases morbidity for patients requiring endotracheal intubation. However, rigorous means for determining adequate pre-oxygenation are limited in the emergency department (ED). End-tidal oxygenation (EtO METHODS: Healthy volunteers were randomized to receive supplemental oxygen via a nonrebreather mask (NRBM) or a noninvasive ventilation mask (NIV). Participants underwent 3-minute trials at 3 different settings: NRBM at 15 liters per minute (LPM), 35 LPM, and 55 LPM, or NIV at 40% fraction of inspired oxygen (FiO RESULTS: Complete data were obtained for 104 participants. Beta regression analysis revealed a strong correlation between NC EtO CONCLUSIONS: Supplemental oxygen led to an overestimation of NC EtO