http://www.advanceddiverforum.com/pdf/toburn.pdf Within the diving industry, systems with oxygen concentrations greater than 41% are treated as 100% oxygen regarding cleaning and oxygen service. Systems with 21% to 26% oxygen require no special cleaning. Between 40% and 25%, opinions and procedures vary widely. Most know that fires occur in 100% oxygen systems, but less known are fires in 46% oxygen systems, such as NASA’s Neutral Buoyancy Laboratory (NBL). So a big question centers around the 25% to 40% oxygen concentrations and potential combustion risk. Many materials that are not flammable in air are SEE ABOVE LINK TO COMPLETE EDITORIAL.