A non-rebreather mask (NRB, non-rebreather, non-rebreather facemask, etc.) is a device used in medicine to assist in the delivery of oxygen therapy. An NRB requires that the patient can breathe unassisted, but unlike low-flow nasal cannulae, the NRB allows for the delivery of higher concentrations of oxygen. An ideal non-rebreather mask does not permit air from the surrounding environment to be inhaled, hence an event of a source gas failure (i.e. the oxygen cylinder being drained completely) is life threatening.