Diving practices in technical divers' community and behaviour towards self-reported unusual symptoms.

Introduction: The use of gas mixtures containing helium for deep recreational diving is increasingly common, involving complex logistics and decision-making compromises. The characteristics and inherent risks of this practice remain poorly documented. This study aims to provide an epidemiological inventory of practices and diving-related incidents within the technical diving community. Methods: An international online survey was disseminated on social networks targeting certified trimix divers. It collected demographic data, diving experience, and dive management practices, along with self-reported unusual symptoms, treatments, and outcomes following trimix dives. Results: A total of 558 questionnaires were analysed, predominantly from males (92%), mostly over 46 years old (61%), with high certification levels and recreational diving purposes. Forty-two percent reported one or more medical risk factors related to diving. Rebreather use was prevalent (79% at least occasionally). Decompression was primarily managed using compartmental models (85%) with gradient-factors adjustment. Dive planning varied significantly among individuals. Gas density at depth frequently exceeded the current recommendations. Ten percent had experienced symptoms suggestive of gas toxicity, mainly related to nitrogen narcosis. Thirty-six percent (199/558) reported experiencing, at least once, symptoms of diving-related incidents, with 61% (n = 121/199) expressing certainty. In 48% (120/261) of incidents involving decompression sickness (DCS) or breathing symptoms, no treatment was initiated. Among episodes involving DCS symptoms (n = 254), 42% received normobaric oxygen, and 23% sought medical advice, while 16% were treated with hyperbaric oxygen. Only 2.5% reported probable long-lasting sequelae. Conclusions: The diversity of practices highlights the lack of robust scientific data supporting them. The accident rate in mixed-gas diving may be higher than in typical scuba air diving, though mostly of mild severity. Treatment appears to be neglected despite divers' high knowledge levels. Continued research into decompression and the physiological effects of these dives is essential, along with ongoing awareness and education efforts in diving first aid within this exposed community.