An atmosphere in which the oxygen content is below that needed for normal human function without impairment. For inert entry, the typical oxygen content (below 5 % in inert entry situations) is very much less than the oxygen-deficient definition often used (e.g. by OSHA) of “atmosphere containing less than 19.5 percent oxygen by volume” (the OSHA limit is appropriate for areas outside the “hot zone”). Inert entry requires much lower oxygen concentration percentages to provide a working environment minimizing fire hazards by maintaining the oxygen level low enough to prevent combustion.
Source: API Standards 2217A, Guidelines for Safe Work in Inert Confined Spaces in the Petroleum and Petrochemical Industries, Fourth Edition, July 2009. Global Standards