Air compressed chambers
Alternatively, the chamber can be compressed with air and oxygen is inhaled through a mask or a hood.
Oxygen is now absorbed almost exclusively through the lungs while the skin is exposed to excessive high concentrations of nitrogen.
The “oxygen therapy” does not start until the mask or hood is applied with a sufficient oxygen flow (>15 l/min).
Breathing compressed air inside the chamber exposes the patient to undesirably high concentrations of nitrogen effectively diluting the oxygen therapy (ref). This makes the air filled chambers less efficient than the oxygen chambers. The total therapy time is therefore usually longer and more sessions are needed to achieve results comparable to the oxygen chamber.
Air compressed chambers come in a variety of sizes and shapes with larger “multiplace” chambers being the hospital standard. They allow for specially trained health care professionals to attend patients inside the chamber.

As the air inside the chamber is compressed the environment becomes warm and humid with a potential risk of bacterial buildup and patient cross contamination. This mandates strict chamber cleaning procedures and careful screening of the patients.
Reducing the air pressure at the end of therapy has to be done carefully to minimize the risk of developing decompression sickness.