In this issue of CHEST (see page 891), Carlos and Gustavo Rodrigo review studies that examine the clinical efficacy of heliox for the management of patients with asthma. To summarize, their meta-analysis concludes that the available data do not warrant the use of heliox for the treatment of most acute exacerbations of asthma. Heliox is a mixture of oxygen and helium that has a lower density and a higher viscosity than nitrogen-oxygen mixtures.1 These physical properties reduce the Reynolds number associated with its flow through hollow tubes (ie, the airways). Heliox should, therefore, promote the transformation of some areas of turbulent flow, as is thought to occur in the bronchioles of asthmatic patients, to laminar flow, thereby reducing the resistive-pressure work of breathing. Dynamic hyperinflation also contributes to increased elastic pressure work of breathing in patients with acute asthma. By promoting enhanced expiratory flow, heliox also should reduce the elastic-pressure work of breathing by allowing the passive exhalation to a lower end-expiratory volume. If this theory is applicable, heliox does not treat asthma. It merely reduces the inspiratory pressures that the patient (or a ventilator) is required to generate during tidal breathing at any given flow and tidal volume. So, heliox may temporize and give definitive treatments (bronchodilators and steroids) time to work. It also could improve the efficacy of definitive therapies if it better carried aerosolized medications to the target airways. So, heliox may impact outcomes in one of the following two ways: by reducing the work of breathing sufficiently to preclude the need for endotracheal intubation (and/or to reduce the sense of dyspnea in those with severe airflow obstruction); and as a carrier gas to improve the delivery of aerosols to the airways, thereby improving disposition outcomes.