MSA Chemox since 1946

A rescue mask that has been in use for more than 75 years

The Chemox is a chemical-Oxygen , closed-circuit apparatus with a disposable KO₂ canister.  It was certified by the USBM in 1946 as a 1-hr breathing apparatus.  Because it was certified for less than a 2-hr duration, it was considered an auxiliary rescue apparatus.  It has seen extensive use from 1946 to the present as the primary U.S. Navy shipboard firefighting apparatus progressing from the A-1 model to the presently used A-4.  The early models had no chlorate candles and differed in the design of the breathing bag, relief valve, and the mechanism and procedures for loading.  The U.S. Navy calls the device the OBA, short for Oxygen breathing apparatus.  It is worn on the chest rather than the back. KO₂ not only absorbs CO₂ but also produces O₂ , so that a separate O₂ source is not needed.  The new KO₂ canisters contain sodium chlorate “oxygen candles,” which provide a quick source of O₂ , permitting the apparatus to be worn immediately. Otherwise, the KO₂ needs three or four exhaled breaths, which provide not only the H₂O and CO₂ required for the chemical reactions, but also sufficient initial volume in the breathing bag to permit self-contained use of the apparatus.  Since there is no O₂ cylinder and gauge to indicate quantity of O2 remaining, a mechanical timer is provided that informs the user how long the apparatus remains usable.  This allows for only a rough estimate of remaining duration, however, since KO₂ is strictly demand responsive.  KO₂ overproduces O₂ relative to human CO₂ production such that the breathing bag is always full and continually venting O₂ through the volume-activated overpressure valve.  Earlier models had a manually activated overpressure valve. The most modern model is the A-4 with an automatic relief valve and canisters containing chlorate candles. Exhaled air goes through the exhalation check valve and hose, through a passageway down the middle of the KO₂ canister, up through the chemical bed, and is channelled to the bottom of one breathing bag, sweeping over the inner surface of the bag to a connecting tube terminating at the top of the other breathing bag.  Upon inhalation, the air is drawn from the bottom of the second breathing bag, through the inhalation breathing hose and check valve, then back into the face mask.

The Appendix contains both the manual for the older type with a split window and the more modern types of A4 with an All Vision window. There are also systems on the market with a single hose to the canister where pendulum breathing is used. The canister has an overpressure valve. This rarer type has not been used on a large scale on naval vessels.