I frequently ask participants in my confined space courses, "What makes 19.5 percent so special? Why isn't the acceptable level of oxygen something else, like 19.3 percent, 19.8 percent or 20.2 percent?" The usual answer I get is, "Because OSHA says so." Contrary to popular belief, OSHA doesn't say that an oxygen level of 19.5 percent is "safe." Instead, 29 CFR 1910.146(b) defines a hazardous atmosphere as one "... that may expose employees to the ...

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