* cannot be randomised within twelve hours after present icu admission * chronic mechanical ventilation for any reason * use of home oxygen * previous treatment with bleomycin * organ transplant during current hospital admission * withdrawal from active therapy or brain death deemed imminent * fertile woman (\< 50 years of age) with positive urine human gonadotropin (hcg) or plasma-hcg * carbon monoxide poisoning * cyanide poisoning * methaemoglobinaemia * paraquat poisoning * any condition expected to involve the use of hyperbaric oxygen (hbo) * sickle cell disease * consent not obtainable according to national regulations * previously randomised into the hot-covid trial

* cannot be randomised within twelve hours after present icu admission * chronic mechanical ventilation for any reason * use of home oxygen * previous treatment with bleomycin * organ transplant during current hospital admission * withdrawal from active therapy or brain death deemed imminent * fertile woman (\< 50 years of age) with positive urine human gonadotropin (hcg) or plasma-hcg * carbon monoxide poisoning * cyanide poisoning * methaemoglobinaemia * paraquat poisoning * any condition expected to involve the use of hyperbaric oxygen (hbo) * sickle cell disease * consent not obtainable according to national regulations * previously randomised into the hot-covid trial

- cannot be randomised within twelve hours after present icu admission - chronic mechanical ventilation for any reason - use of home oxygen - previous treatment with bleomycin - organ transplant during current hospital admission - withdrawal from active therapy or brain death deemed imminent - fertile woman (< 50 years of age) with positive urine human gonadotropin (hcg) or plasma-hcg - carbon monoxide poisoning - cyanide poisoning - methaemoglobinaemia - paraquat poisoning - any condition expected to involve the use of hyperbaric oxygen (hbo) - sickle cell disease - consent not obtainable according to national regulations - previously randomised into the hot-covid trial

- cannot be randomised within twelve hours after present icu admission - chronic mechanical ventilation for any reason - use of home oxygen - previous treatment with bleomycin - organ transplant during current hospital admission - withdrawal from active therapy or brain death deemed imminent - fertile woman (< 50 years of age) with positive urine human gonadotropin (hcg) or plasma-hcg - carbon monoxide poisoning - cyanide poisoning - methaemoglobinaemia - paraquat poisoning - any condition expected to involve the use of hyperbaric oxygen (hbo) - sickle cell disease - consent not obtainable according to national regulations - previously randomised into the hot-covid trial