- 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