['Safety in helicopter emergency medical services (HEMS) settings might be enhanced by the routine use of portable multigas analyzers to indicate hypoxic ambient air mixtures and the presence of hazardous gases.', 'In our HEMS (EC-135, Lifeliner 1, Amsterdam, The Netherlands), we introduced a professional multigas analyzer (Gas Alert Max XT II; Honeywell Analytics, Lincolnshire, IL) to prospectively detect possible hazardous gas mixtures. This analyzer measures ambient oxygen percentage (FO2, 0%-30%), carbon monoxide (CO, 0-1,000 ppm), hydrogen sulfide (H2S, 0-200 ppm), and combustibles (lower explosive limit, %).', 'Before 12 HEMS flights, we measured a stable baseline ambient FO2 of 20.9%. However, in all flights, takeoff to cruising altitude (500-1,000 ft) markedly decreased the measured FO2 to 19.1% ± 0.1% (mean ± standard deviation; Wilcoxon signed rank test, P\xa0= .002) with a minimum of 18.8%. This triggered the optoacoustic hypoxia alarm (preset to 19.5% to comply with international guidelines) in all cases. Maintaining cruise altitude restored measured FO2 slowly back to 20.9%. In contrast, CO, H2S, and combustibles remained unchanged during takeoff and flight.'] Does the Hypoxia Alarm of a Multigas Analyzer be Triggered During Helicopter Takeoffs in Helicopter Emergency Medical Services Operations?