An ideal non-rebreather mask does not permit air from the surrounding environment to be inhaled, hence an event of a source gas failure (e.g., the oxygen tank being drained completely) is life-threatening.

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 liters per minute (lpm).

The patient must partially deflate the reservoir bag during inspiration or the high oxygen concentration will not be achieved, and the mask will provide only the flow rate setting on the flowmeter.

However, due to safety concerns regarding anti-suffocation protection in the event of a source gas failure (i.e. the oxygen cylinder being drained completely), one of the two one-way valves is normally removed, allowing inhalation of outside air to a significant degree.

Very little effort is required by most patients, to inspire at flow rates in excess of 50 LPM (easily seen in the pulmonary function lab with routine spirometric testing).