Heliox is a gas mixture (79% helium and 21% oxygen) which is commonly used in deep diving, and also for clinical purposes, particularly in the critical care setting. Due to its low density Heliox breathing reduces air flow resistance within the bronchial tree in patients with obstructive lung disease, and has beneficial effects in severe asthma attacks. Heliox may also reduce the work of breathing and improve pulmonary gas exchange efficiency. Lower density of Heliox compared with the air or oxygen regardless of the concentration of helium in the mixture; improves air flow through constricted ways by transforming turbulent flow into laminar flow. Benefits are seen quickly, usually within an hour of initiation of treatment.