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 an attached reservoir bag, typically 1 liter, that connects to an external oxygen tank or Bulk Oxygen Supply system. 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). Approximately ¹⁄₃ of the air from the reservoir is depleted as the patient inhales, and it is then replaced by the flow from the O2 supply. If the bag becomes completely deflated, the patient will no longer have a source of air to breathe.