I read an
excellent article on wrong notions of safe oxygen level of 19.5%. One should find why oxygen levels are below 21% and only after getting satisfactory answer, should venture into confined space with proper PPE. I feel that, once oxygen levels are below 21%, one should go inside only with airline respirator / self contained breathing apparatus as suitable it is. We should remember that catridge/canister type respirators are useful only upto 2% of contaminant concentrations and above this, one should use fresh air supply system.