This supplement applies to portable gas cylinders used in self-contained breathing apparatus. The breathing apparatus may be air or a breathing gas mixture containing more than 22 per cent of oxygen. The dangers associated with cylinders are noted, and practices and precautions to be observed during selection, handling, filling and maintenance are given to assist in eliminating these dangers. Owners and users of cylinders should follow the recommendations given herein and should also observe manufacturers' or suppliers' instructions not inconsistent therewith for use and maintenance.