Their Chemox chemical rebreather, primarily designed for use in mines, has been modified for use on Mount Everest in 1952 and 1986. They are stated to be simple in construction and operation. It is essentially a canister of potassium superoxide connected to one-way flow valves to an air bag and thence to the user, as in Rebreathers whose absorbent releases oxygen hereinabove. There are no controls or operable valves of any kind. Breathing rate controls oxygen production. The main disadvantage is the 4-lbs canisters are good for only about 45-minutes of rapid climbing before another canister must be switched in. The canisters supply about 6 hours of sleeping oxygen.