Oxygen is normally provided at atmospheric pressure (normobaric
oxygen (NBO)) using a variety of masks that can provide
inspired oxygen concentrations between 24%-90%. Masks with
reservoirs, tightly fitting continuous positive airway pressure
masks or mechanical ventilators can all deliver higher concentrations
of oxygen. Oxygen at pressures higher than 1 atmosphere
(hyperbaric oxygen HBO) requires a hyperbaric chamber, either
filled with 100% oxygen and them compressed to the required
pressure or a chamber filled with compressed air while the
occupant breathes 100% oxygen at the same ambient pressure