Heat-moisture exchangers or "artificial noses" are commonly used to humidify the inspired air in intubated or tracheostomized patients. These devices add resistance to breathing that may be significant in critically ill and weak patients, especially when the devices are humidified. Three of each six commonly used types (Portex, SH 150, SH 151, NCC, Engström, and PaII) were tested to define the pressure flow characteristics. Known flows were passed through these devices, and the pressure drops across them were measured. The devices were tested when they were dry and when they were humidified with 100% saturated water vapor for 8, 16, and 24 h. The amount of water added to the devices was measured. The pressure flow characteristics of the devices could be described by a power function: P = aVb, where P is pressure, V is flow, and a and b are constants determined by the characteristics of the devices. In addition, the resistance increased with the increasing weight of the devices from added moisture clogging the pores of the felt pads contained inside.