Jon-Emile S. Kenny MD [@heart_lung] Of the countless things taught to me by Dr. Chitkara at the Palo Alto VA Health Care System, one that sticks is the difference between density-dependent and viscosity-dependent airflow. He often used the chronic bronchitic suffering through the viscous, humid New York City summers as a teaching example. The importance of gas density and viscosity is also frequently encountered when considering the properties and benefits of helium-oxygen gas mixtures [i.e. ‘Heliox’]. This brief review highlights the viscosity and density characteristics of gas within the context of the mechanical power applied to the lung; as such, roles for Heliox may extend beyond that of a rescue therapy for severe obstructive airways disease.