Editing Nitrox (section)

===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.<ref name=oxyhackers /> 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.<ref name="Rosales et al 2007" />

Among recreational training agencies, only ANDI subscribes to the guideline of requiring oxygen cleaning for equipment used with more than 23% oxygen fraction. The USCG, NOAA, U.S. Navy, OSHA, and the other recreational training agencies accept the limit as 40% as no accident or incident has been known to occur when this guideline has been properly applied. Tens of thousands of recreational divers are trained each year and the overwhelming majority of these divers are taught the "over 40% rule".<ref name=noaa/><ref name=dan/><ref name=padi/> Most nitrox fill stations which supply pre-mixed nitrox will fill cylinders with mixtures below 40% without certification of cleanliness for oxygen service.<ref name=dan/> Luxfer cylinders specify oxygen cleaning for all mixtures exceeding 23.5% oxygen.<ref name="Luxfer 23.5%" />

The following references for oxygen cleaning specifically cite the "over 40%" guideline that has been in widespread use since the 1960s, and consensus at the 1992 Enriched Air Workshop was to accept that guideline and continue the status quo.<ref name="dan"/>
* Code of Federal Regulations, Part 1910.430 (i) – Commercial Diving Operations
* OSHA Oxygen Specifications 1910.420 (1)
* NOAA Oxygen Specifications (appendix D)
* U.S. Navy Oxygen Specifications U.S. MIL-STD-777E (SH) Note K-6-4, Cat. K.6
* U.S. Coast Guard Oxygen Specifications Title 46: Shipping, revisions through 10-1-92. 197.452 Oxygen Cleaning 46 CFR 197.451

Much of the confusion appears to be a result of misapplying PVHO (pressure vessel for human occupancy) guidelines which prescribe a maximum ambient oxygen content of 25% when a human is sealed into a pressure vessel (chamber). The concern here is for a fire hazard to a living person who could be trapped in an oxygen-rich burning environment.<ref name="dan"/>

Of the three commonly applied methods of producing enriched air mixes – continuous blending, partial pressure blending, and membrane separation systems – only partial pressure blending would require the valve and cylinder components to be oxygen cleaned for mixtures with less than 40% oxygen. The other two methods ensure that the equipment is never subjected to greater than 40% oxygen content.

In a fire, the pressure in a gas cylinder [[Amontons' Law|rises in direct proportion to its absolute temperature]]. If the internal pressure exceeds the mechanical limitations of the cylinder and there are no means to safely vent the pressurized gas to the atmosphere, the vessel will fail mechanically.  If the vessel contents are ignitable or a contaminant is present this event may result in a "fireball".<ref name="DAN" />