Inclusion Criteria:  Members of emergency medical services (EMS) and search and rescue (SAR) services with an occupational licence, an age between 18 and 60 years, an American Society of Anaesthesiologists (ASA) physical status class I, providing informed and written consent with no current COVID 19 symptoms and temperature ≤ 37.5°on test days, not being tested positive for COVID-19, ideally COVID-19 vaccinated.  Exclusion Criteria:  Members under the age of 18 years, an ASA physical status class II or more, a medical history of psychiatric disorders and neurological diseases, previous high altitude pulmonary oedema (HAPE) or high altitude cerebral oedema (HACE) or severe acute mountain sickness (AMS) (defined as a Lake Louise Score (LLS) > 9) occurred at altitudes similar to the ones tested in the study, no informed consent, current COVID 19 symptoms, being tested positive for COVID-19 or symptoms and body temperature ≥ 37.5°on test days.