The non-rebreather mask covers both the nose and mouth of the patient and attaches with the use of an elastic cord around the patient's head.
The NRB has a 1 litre attached reservoir bag that connects to an external oxygen tank. Before an NRB is placed on the patient, the reservoir bag is inflated to greater than two-thirds full of oxygen, at a rate of 15 litres per minute (lpm).
Approximately one-third of the air from the reservoir is depleted as the patient inhales, and it is then replaced by the flow from the O2 supply. Exhaled air is directed through a one-way valve in the mask, which prevents the inhalation of room air and the re-inhalation of exhaled air.
The valve, along with a sufficient seal around the patient's nose and mouth, allows for the administration of high concentrations of oxygen, approximately 60% - 80% O2.