Proposal for defining the severity of environmental hypoxia

Every day, millions of people are exposed to environmental hypoxia, primarily due to living at or travelling to high altitudes for reasons such as work, tourism, business, or sports, e.g., mountaineering. Furthermore, hypoxic exposure at low altitudes (e.g., in hypoxia rooms or tents) is increasingly being used to pre-acclimatize individuals to high altitudes for activities such as athletic training, trekking and high-altitude climbing, as well as for preventive and therapeutic purposes. Especially for these applications a clear definition of environmental hypoxia severity is lacking, impeding risk assessment. Our proposed classification does not address the acclimatization process itself, but rather the degree of hypoxia that healthy individuals can tolerate acutely (for a few hours) without prior acclimatization and chronically (up to years) after acclimatization. Based on real-world levels of hypoxia observed in high-altitude residents and mountaineers, as well as in individuals exposed to simulated hypoxia at low altitude, we propose the following definitions: Altitudes of up to 1,000 m, corresponding to fractional inspired oxygen (FiO₂) levels above 18.5%, should be considered 'normoxia'. Altitudes ranging from 1,000 m to 3,500 m or FiO₂ levels between 18.5% and 13.5% can be classified as mild hypoxia. Altitudes between 3,500 m and 5,500 m or FiO₂ levels between 13.5% and 10.2%, represent moderate hypoxia. While people usually can acclimatize to mild and moderate hypoxia, the severe hypoxia experienced at altitudes above 5,500 m or FiO₂ levels below 10.2% poses considerable health risks to humans, where acclimatization in the long term is no longer possible.