1-54. There is one major disadvantage in using closed-circuit UBAs. Most UBAs are pure oxygen rebreathers, and the effects of oxygen toxicity place limitations on the divers. To preclude the effects of oxygen toxicity, combat divers limit the depth of the dive, adhere to time limitations, and reduce physical exertion. The factors pertaining to oxygen toxicity and the most efficient oxygen consumption establish the diver’s planning speed at about 1 knot when using a closed-circuit UBA.