Study objective: Comparing the efficacy of three different preoxygenation strategies, i.e. non-rebreather face mask (NRM), bag-valve-mask (BVM) with and BVM without additional positive end-expiratory pressure (PEEP) in three subgroups of spontaneously breathing volunteers. Study endpoints Primary Endpoint: Difference in expiratory oxygen concentration (FeO2) at the end of the 3 min preoxygenation sessions. Secondary endpoints: * Difference in Oxygen Reserve Index (ORi) at the end of the 3 min preoxygenation sessions. * Changes in regional ventilation from baseline to 1.5 min and 3 min into the preoxygenation sessions, assessed using electrical impedance tomography (EIT) * Time to reaching an Oxygen Reserve Index (ORi) ≥ 0.5 * Time to reaching an Oxygen Reserve Index (ORi) =1 * Time to Oxygen Reserve Index (ORi) back to baseline after the 3 min preoxygenation sessions. Number of participants 15 participants per subgroup, i.e. 45 participants in total. Inclusion criteria * Normal-weight adults (body mass index, BMI 18.5-24.9 kg/m2) with an American Society of Anesthesiologists Physical Status Classification System (ASA) score of I or II * Adults with a BMI 25-39.9 kg/m2 with and ASA score \<3 * Healthy (ASA I) children aged 6-12 years Exclusion criteria ASA 3, Age \< 6 and age 12-18, pregnant women, missing informed consent, signs and symptoms of an acute respiratory illness on the study day. After informed consent and a medical check-up, baseline measurements will be done for 10 minutes (peripheral oxygen saturation SpO2, ORI, regional ventilation). Then the participants will undergo 3 different preoxygenation sessions with the 3 interfaces (i.e. non-rebreather facemask with reservoir and a bag-valve-mask with and without PEEP) in a randomized order and a 20 min washout between the sessions. Each preoxygenation session is conducted in a supine position and will be proceeded until the measured FeO2 (end expiratory O2 fraction) reaches 90% or up to a maximum time of 3 min. Afterwards 10 min of SpO2, ORI and regional ventilation measurements will follow.