Described herein are passive nasal device having a resistance to exhalation that is greater than the resistance to inhalation. Also described are devices, methods and systems for sensing and measuring intranasal pressure when a subject is wearing a passive nasal respiratory device that is configured to inhibit exhalation more than inhalation. Also described are adapters for nasal devices and methods of using a nasal device adapter. Adapters may be used so that a passive nasal device may be applied indirectly in communication with a subject's nose; in some variations this may allow the passive nasal device to be re-used. Also described herein are nasal devices having a billowing airflow resistor that is configured to have a greater resistance to exhalation than to inhalation. The billowing airflow resistor typically includes a first layer that is adjacent to a second layer; the first layer is flexible and billows opens during inhalation so that the first layer remains separated from the second layer, but remains substantially parallel to the second layer. During exhalation, the first layer collapses back down against the second layer. Additional passive nasal devices, systems and methods of using them are also described.