{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,25]],"date-time":"2026-09-25T05:00:09Z","timestamp":1790312409871,"version":"4.1.0"},"reference-count":40,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,9,25]],"date-time":"2026-09-25T00:00:00Z","timestamp":1790294400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Physiol."],"abstract":"<jats:p>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.<\/jats:p>\n                  <jats:p>\n                    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 (FiO\n                    <jats:sub>2<\/jats:sub>\n                    ) levels above 18.5%, should be considered \u2018normoxia\u2019. Altitudes ranging from 1,000 m to 3,500 m or FiO\n                    <jats:sub>2<\/jats:sub>\n                    levels between 18.5% and 13.5% can be classified as mild hypoxia. Altitudes between 3,500 m and 5,500 m or FiO\n                    <jats:sub>2<\/jats:sub>\n                    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 FiO\n                    <jats:sub>2<\/jats:sub>\n                    levels below 10.2% poses considerable health risks to humans, where acclimatization in the long term is no longer possible.\n                  <\/jats:p>","DOI":"10.3389\/fphys.2026.1985822","type":"journal-article","created":{"date-parts":[[2026,9,25]],"date-time":"2026-09-25T04:27:53Z","timestamp":1790310473000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["Proposal for defining the severity of environmental hypoxia"],"prefix":"10.3389","volume":"17","author":[{"given":"Martin","family":"Burtscher","sequence":"first","affiliation":[{"name":"Institute of Sport Science, University of Innsbruck","place":["Innsbruck, Austria"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ricardo","family":"Amaru","sequence":"additional","affiliation":[{"name":"Cell Biology Unit, National Academy of Sciences, Universidad Mayor de San Andres","place":["La Paz, Bolivia"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Johannes","family":"Burtscher","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Psychotherapy, Psychosomatics and Medical Psychology, University Hospital for Psychiatry II, Medical University of Innsbruck","place":["Innsbruck, Austria"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Katharina","family":"H\u00fcfner","sequence":"additional","affiliation":[{"name":"Department of Psychiatry, Psychotherapy, Psychosomatics and Medical Psychology, University Hospital for Psychiatry II, Medical University of Innsbruck","place":["Innsbruck, Austria"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Max","family":"Gassmann","sequence":"additional","affiliation":[{"name":"Institute of Veterinary Physiology, Vetsuisse Faculty, University of Zurich","place":["Zurich, Switzerland"]},{"name":"The Zurich Center for Integrative Human Physiology (ZIHP), University of Zurich","place":["Zurich, Switzerland"]},{"name":"Universidad Peruana Cayetano Heredia","place":["Lima, Peru"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1965","published-online":{"date-parts":[[2026,9,25]]},"reference":[{"key":"B1","doi-asserted-by":"publisher","first-page":"96","DOI":"10.1111\/j.1600-0838.2008.00837.x","article-title":"Consensus statement on playing football at different altitude","volume":"18","author":"B\u00e4rtsch","year":"2008","journal-title":"Scand. J. Med. Sci. Sports."},{"key":"B2","doi-asserted-by":"publisher","first-page":"2294","DOI":"10.1056\/NEJMcp1214870","article-title":"Acute high-altitude illnesses","volume":"368","author":"B\u00e4rtsch","year":"2013","journal-title":"N. Engl. J. Med."},{"key":"B3","doi-asserted-by":"publisher","first-page":"1400","DOI":"10.4187\/respcare.11928","article-title":"Feasibility of delivering 5-day normobaric hypoxia breathing in a hospital setting","volume":"69","author":"Berra","year":"2024","journal-title":"Respir. Care"},{"key":"B4","doi-asserted-by":"publisher","first-page":"158","DOI":"10.1089\/ham.2023.0053","article-title":"Counseling patients with chronic obstructive pulmonary disease traveling to high altitude","volume":"24","author":"Bloch","year":"2023","journal-title":"High Alt. Med. Biol."},{"key":"B5","doi-asserted-by":"publisher","first-page":"145","DOI":"10.1186\/s40798-025-00933-7","article-title":"Effects of intermittent hypoxia protocols on physical performance in trained and untrained individuals: An umbrella review of systematic reviews and meta-analyses","volume":"11","author":"Boulares","year":"2025","journal-title":"Sports. Med. - Open"},{"key":"B6","doi-asserted-by":"publisher","first-page":"13","DOI":"10.3233\/jad-240711","article-title":"Effects of intermittent hypoxia protocols on cognitive performance and brain health in older adults across cognitive states: A systematic literature review","volume":"101","author":"Boulares","year":"2024","journal-title":"J. Alzheimers Dis."},{"key":"B7","doi-asserted-by":"publisher","first-page":"5757","DOI":"10.1113\/jp285230","article-title":"Mechanisms underlying the health benefits of intermittent hypoxia conditioning","volume":"602","author":"Burtscher","year":"2024","journal-title":"J. Physiol."},{"key":"B8","doi-asserted-by":"publisher","DOI":"10.1093\/jtm\/taag009","article-title":"Time requirements of pre-acclimatization at simulated altitude to prevent acute mountain sickness: A mini review","volume":"33","author":"Burtscher","year":"2026","journal-title":"J. Travel. Med."},{"key":"B9","doi-asserted-by":"publisher","DOI":"10.3389\/fphys.2025.1598275","article-title":"Health benefits of life at moderate altitude: does hypoxia matter","volume":"16","author":"Burtscher","year":"2025","journal-title":"Front. Physiol."},{"key":"B10","doi-asserted-by":"publisher","DOI":"10.1159\/000545228","article-title":"The impact of living at moderate altitude on healthy aging in Austria: Epidemiological findings and potential underlying mechanisms","volume":"71","author":"Burtscher","year":"2025","journal-title":"Gerontology"},{"key":"B11","doi-asserted-by":"publisher","DOI":"10.1093\/jtm\/taad011","article-title":"Flying to high-altitude destinations: is the risk of acute mountain sickness greater","volume":"30","author":"Burtscher","year":"2023","journal-title":"J. Travel. Med."},{"key":"B12","doi-asserted-by":"publisher","first-page":"446","DOI":"10.1089\/ham.2014.1039","article-title":"Symptom progression in acute mountain sickness during a 12-hour exposure to normobaric hypoxia equivalent to 4500\u2009m","volume":"15","author":"Burtscher","year":"2014","journal-title":"High Alt. Med. Biol."},{"key":"B13","doi-asserted-by":"publisher","first-page":"27","DOI":"10.1038\/s41392-025-02531-1","article-title":"Altitude hypoxia and hypoxemia: pathogenesis and management","volume":"11","author":"Cai","year":"2026","journal-title":"Signal Transduction. Targeted. Ther."},{"key":"B14","doi-asserted-by":"publisher","first-page":"123","DOI":"10.1089\/ham.2008.1099","article-title":"Air to muscle O2 delivery during exercise at altitude","volume":"10","author":"Calbet","year":"2009","journal-title":"High Alt. Med. Biol."},{"key":"B15","doi-asserted-by":"publisher","first-page":"715","DOI":"10.1113\/jphysiol.2002.029108","article-title":"Effect of blood haemoglobin concentration on V(O2,max) and cardiovascular function in lowlanders acclimatised to 5260 m","volume":"545","author":"Calbet","year":"2002","journal-title":"J. Physiol."},{"key":"B16","doi-asserted-by":"publisher","DOI":"10.3389\/fphys.2016.00675","article-title":"Physiological responses to two hypoxic conditioning strategies in healthy subjects","volume":"7","author":"Chacaroun","year":"2016","journal-title":"Front. Physiol."},{"key":"B17","doi-asserted-by":"publisher","first-page":"5449","DOI":"10.1113\/jp284550","article-title":"Expedition 5300: limits of human adaptations in the highest city in the world","volume":"602","author":"Champigneulle","year":"2024","journal-title":"J. Physiol."},{"key":"B18","doi-asserted-by":"publisher","first-page":"29","DOI":"10.24875\/ric.22000281","article-title":"Arterial Blood Gases in Normal Subjects at 2240 Meters Above Sea Level: Impact of Age, Gender, and Body Mass Index","volume":"75","author":"Cid-Ju\u00e1rez","year":"2023","journal-title":"Rev. Invest. Clin."},{"key":"B19","doi-asserted-by":"publisher","DOI":"10.1371\/journal.pone.0297007","article-title":"Effects of various living-low and training-high modes with distinct training prescriptions on sea-level performance: A network meta-analysis","volume":"19","author":"Feng","year":"2024","journal-title":"PloS One"},{"key":"B20","doi-asserted-by":"publisher","DOI":"10.1001\/jamanetworkopen.2023.18036","article-title":"Partial Pressure of Arterial Oxygen in Healthy Adults at High Altitudes: A Systematic Review and Meta-Analysis","volume":"6","author":"Forrer","year":"2023","journal-title":"JAMA Netw. Open"},{"key":"B21","doi-asserted-by":"publisher","first-page":"43","DOI":"10.1038\/s41572-024-00526-w","article-title":"Altitude illnesses","volume":"10","author":"Gatterer","year":"2024","journal-title":"Nat. Rev. Dis. Primers"},{"key":"B22","doi-asserted-by":"publisher","first-page":"107","DOI":"10.1016\/j.emc.2004.02.001","article-title":"High-altitude illness","volume":"345","author":"Hackett","year":"2001","journal-title":"N. Engl. J. Med."},{"key":"B23","doi-asserted-by":"publisher","first-page":"5461","DOI":"10.1113\/jphysiol.2012.238527","article-title":"Ageing and cardiorespiratory response to hypoxia","volume":"590","author":"Lhuissier","year":"2012","journal-title":"J. Physiol."},{"key":"B24","doi-asserted-by":"publisher","first-page":"2s","DOI":"10.1016\/j.wem.2023.05.013","article-title":"Wilderness medical society clinical practice guidelines for the prevention, diagnosis, and treatment of acute altitude illness: 2024 update","volume":"35","author":"Luks","year":"2024","journal-title":"Wilderness. Environ. Med."},{"key":"B25","doi-asserted-by":"publisher","DOI":"10.1113\/jp290022","article-title":"Reduced cardiovascular mortality at moderate altitude: a putative role of physical activity and body mass","author":"Mallet","year":"2025","journal-title":"J. Physiol"},{"key":"B26","doi-asserted-by":"publisher","DOI":"10.1186\/1471-2458-12-401","article-title":"Burden of disease resulting from chronic mountain sickness among young Chinese male immigrants in Tibet","volume":"12","author":"Pei","year":"2012","journal-title":"BMC Public Health"},{"key":"B27","doi-asserted-by":"publisher","first-page":"271","DOI":"10.1007\/s40279-023-01954-6","article-title":"Women at altitude: Sex-related physiological responses to exercise in hypoxia","volume":"54","author":"Raberin","year":"2024","journal-title":"Sports. Med."},{"key":"B28","doi-asserted-by":"publisher","DOI":"10.1126\/scitranslmed.adr4049","article-title":"Hypoxia as a medicine","volume":"17","author":"Rogers","year":"2025","journal-title":"Sci. Transl. Med."},{"key":"B29","doi-asserted-by":"publisher","DOI":"10.3389\/fnins.2022.941740","article-title":"Intermittent hypoxic training as an effective tool for increasing the adaptive potential, endurance and working capacity of the brain","volume":"16","author":"Rybnikova","year":"2022","journal-title":"Front. Neurosci."},{"key":"B30","doi-asserted-by":"publisher","DOI":"10.3390\/ijms24043670","article-title":"The oxygen cascade from atmosphere to mitochondria as a tool to understand the (mal)adaptation to hypoxia","volume":"24","author":"Samaja","year":"2023","journal-title":"Int. J. Mol. Sci."},{"key":"B31","doi-asserted-by":"publisher","DOI":"10.1016\/j.lana.2024.100887","article-title":"Micro- and macrovascular function in the highest city in the world: a cross sectional study","volume":"38","author":"Savina","year":"2024","journal-title":"Lancet Reg. Health Am."},{"key":"B32","doi-asserted-by":"publisher","first-page":"193","DOI":"10.1007\/s00421-007-0417-8","article-title":"Normo or hypobaric hypoxic tests: propositions for the determination of the individual susceptibility to altitude illnesses","volume":"100","author":"Savourey","year":"2007","journal-title":"Eur. J. Appl. Physiol."},{"key":"B33","doi-asserted-by":"publisher","first-page":"205","DOI":"10.1089\/15270290260131939","article-title":"Intermittent hypoxia research in the former soviet union and the commonwealth of independent states: history and review of the concept and selected applications","volume":"3","author":"Serebrovskaya","year":"2002","journal-title":"High Alt. Med. Biol."},{"key":"B34","doi-asserted-by":"publisher","first-page":"1708","DOI":"10.1177\/1535370216657614","article-title":"Intermittent hypoxia training as non-pharmacologic therapy for cardiovascular diseases: Practical analysis on methods and equipment","volume":"241","author":"Serebrovskaya","year":"2016","journal-title":"Exp. Biol. Med. (Maywood)."},{"key":"B35","doi-asserted-by":"publisher","first-page":"1387","DOI":"10.1113\/ep093029","article-title":"Early acclimatization to high altitude: Acid-base and fluid balance dynamics during the first 2 days at 3100 m","volume":"111","author":"Skalla","year":"2026","journal-title":"Exp. Physiol."},{"key":"B36","doi-asserted-by":"publisher","first-page":"575","DOI":"10.1152\/japplphysiol.00757.2021","article-title":"Acid-base balance at high altitude in lowlanders and indigenous highlanders","volume":"132","author":"Tymko","year":"2022","journal-title":"J. Appl. Physiol."},{"key":"B37","doi-asserted-by":"publisher","first-page":"25","DOI":"10.1093\/icb\/icj005","article-title":"Human responses to extreme altitudes","volume":"46","author":"West","year":"2006","journal-title":"Integr. Comp. Biol."},{"key":"B38","doi-asserted-by":"publisher","first-page":"80","DOI":"10.1186\/s12974-025-03652-8","article-title":"Mild chronic hypoxia and the brain: an ambiguous relationship","volume":"23","author":"Wsz\u0119dyby\u0142-Winklewska","year":"2026","journal-title":"J. Neuroinflamm."},{"key":"B39","doi-asserted-by":"publisher","first-page":"1551","DOI":"10.7150\/ijms.86622","article-title":"Intermittent hypoxia conditioning: A potential multi-organ protective therapeutic strategy","volume":"20","author":"Zhang","year":"2023","journal-title":"Int. J. Med. Sci."},{"key":"B40","doi-asserted-by":"publisher","first-page":"327","DOI":"10.1515\/reveh-2021-0172","article-title":"High Altitude Pulmonary Edema, High Altitude Cerebral Edema, and Acute Mountain Sickness: an enhanced opinion from the High Andes - La Paz, Bolivia 3,500 m","volume":"38","author":"Zubieta-Calleja","year":"2023","journal-title":"Rev. Environ. Health"}],"container-title":["Frontiers in Physiology"],"original-title":[],"link":[{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fphys.2026.1985822\/full","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,25]],"date-time":"2026-09-25T04:27:54Z","timestamp":1790310474000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fphys.2026.1985822\/full"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,25]]},"references-count":40,"alternative-id":["10.3389\/fphys.2026.1985822"],"URL":"https:\/\/doi.org\/10.3389\/fphys.2026.1985822","relation":{},"ISSN":["1664-042X"],"issn-type":[{"value":"1664-042X","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,25]]},"article-number":"1985822"}}