text: United States Every nitrox cylinder should also have a sticker stating whether or not the cylinder is oxygen clean and suitable for partial pressure blending. Any oxygen-clean cylinder may have any mix up to 100% oxygen inside. If by some accident an oxygen-clean cylinder is filled at a station that does not supply gas to oxygen-clean standards it is then considered contaminated and must be re-cleaned before a gas containing more than 40% oxygen may again be added. Cylinders marked as 'not oxygen clean' may only be filled with oxygen-enriched air mixtures from membrane or stick blending systems where the gas is mixed before being added to the cylinder, and to an oxygen fraction not exceeding 40% by volume.
  Hazards
  Nitrox can be a hazard to the blender and to the user, for different reasons.
  Fire and toxic cylinder contamination from oxygen reactions
  Partial pressure blending using pure oxygen decanted into the cylinder before topping up with air may involve very high oxygen fractions and oxygen partial pressures during the decanting process, which constitute a relatively high fire hazard. This procedure requires care and precautions by the operator, and decanting equipment and cylinders which are clean for oxygen service, but the equipment is relatively simple and inexpensive. Partial pressure blending using pure oxygen is often used to provide nitrox on live-aboard dive boats, but it is also used in some dive shops and clubs.
  Any gas which contains a significantly larger percentage of oxygen than air is a fire hazard, and such gases can react with hydrocarbons or lubricants and sealing materials inside the filling system to produce toxic gases, even if a fire is not apparent. Some organisations exempt equipment from oxygen-clean standards if the oxygen fraction is limited to 40% or less.