1) ARDS/pneumonia caused by other viral infections (positive for other virus)2) ARDS/pneumonia caused by other non-viral infections or trauma3) Known pregnancy or positive pregnancy test in women of childbearing age4) Patients with previous lung fibrosis more than 10%5) CT- or Spirometry-verified severe COPD with Emphysema6) Contraindication for HBO according to local guidelines7) Not likely to need ICU admission < 7 days of Screening (Subjective criteria that may exclude any patients that fullfill the other inclusion criteria but where the treating physician suspect a spontaneous recovery)8) Mental inability, reluctance or language difficulties that result in difficulty understanding the meaning of study participation

1) ARDS/pneumonia caused by other viral infections (positive for other virus)2) ARDS/pneumonia caused by other non-viral infections or trauma3) Known pregnancy or positive pregnancy test in women of childbearing age4) Patients with previous lung fibrosis more than 10%5) CT- or Spirometry-verified severe COPD with Emphysema6) Contraindication for HBO according to local guidelines7) Not likely to need ICU admission < 7 days of Screening (Subjective criteria that may exclude any patients that fullfill the other inclusion criteria but where the treating physician suspect a spontaneous recovery)8) Mental inability, reluctance or language difficulties that result in difficulty understanding the meaning of study participation