Inclusion Criteria:  Acute symptoms of respiratory failure PaO2/FiO2 ratio ≤ 200; PaCO2≤45mmHg; Absence of history of chronic respiratory failure or moderate to severe cardiac insufficiency (NYHA>2 or left ventricular ejection fraction<50%); Informed consent  Patients that have already received NIV, CPAP continuously for more than 24 hours before the screening visit will be excluded.  Other Exclusion Criteria:  Pregnancy; Exacerbation of asthma or chronic obstructive pulmonary disease; Hypercapnia (PaCO2>45 mmHg) with or without respiratory acidosis; More than 2 organ failures, including the lung. Documented pneumothorax; Clinical diagnosis of Cardiogenic pulmonary edema; Haemodynamic instability (Systolic blood pressure<90 mmHg or mean arterial pressure<65mmHg) and/or lactic acidosis (lactate>5 mmol/L) and/or clinically diagnosed Shock requiring administration of vasoactive agents (norepinephrine>0.1 mcg/Kg/min); Metabolic Acidosis (pH <7.30 with normal- or hypo-carbia); Chronic kidney failure requiring dialysis before ICU admission; Chronic hypoxemic respiratory failure requiring long-term oxygen therapy; Altered neurological status that requires immediate intubation and/or making the patient uncooperative; Urgent need for endotracheal intubation, according to the decision of the attending physician; Do not intubate order; Decision of withdrawal of life-sustaining therapy; Thoracic or abdominal surgery in the previous 7 days; Any condition that makes the patient very likely to require endotracheal intubation due to a reason different from respiratory failure; Recent head surgery or anatomy that prevent the application of helmet or HFNC to patient's face.