This supplement applies to portable compressed air cylinders used in self-contained underwater breathing apparatus including apparatus for amphibious operations. The breathing gas shall be air. The dangers associated with cylinders are noted and practices and precautions to be observed during selection, handling, filling and maintenance are given for eliminating these dangers. Owners and users of cylinders should follow the recommendations given herein and should also ovserve manufacturers' or suppliers' instructions not inconsistent therewith for use and maintenance.