Inclusion Criteria:  Age >18 years admitted to the ICU with primary diagnosis of lung injury. The criteria for the diagnosis of lung injury include five principal elements: hypoxemia (blood oxygen saturation (PaO2) ≤92% when breathing ambient air, ratio of PaO2 to the fraction of inspired oxygen (FiO2) ≤300), diffuse pulmonary infiltrates on chest radiographs, decreased lung compliance, and the absence of congestive heart failure. Both males and females will be recruited in the proportion in which they are admitted to the ICU with lung injury. Arterial cannula in place as part of usual care for the measurement of blood gases. Patients may or may not be intubated and mechanically ventilated. Able to provide informed consent, or if unable to do so, a responsible person:  medical treatment decision maker is available (by telephone if necessary) who can be approached to seek consent.  Exclusion Criteria:  Other causes of lung infiltrates: pulmonary oedema, alveolar haemorrhage, adverse drug reactions, radiation injury and the idiopathic pneumonitis syndrome. Significant dysfunction of non-pulmonary organs in the opinion of the treating ICU consultant. Death is deemed imminent or inevitable or there is underlying disease with a life expectancy of less than 90 days. Previously enrolled in this study. Enrolled in another study. Usually receives home oxygen. Usually receives any type of assisted ventilation at home. e.g. continuous positive airway pressure for obstructive sleep apnoea. Pregnant or might be pregnant. Women aged 18 to 49 are excluded unless there is documented menopause, hysterectomy or surgical sterilisation, or a pregnancy test is negative. Objection from the treating clinician. Consent refused by the patient or substitute decision maker.