Confined Space Air Monitoring Calculator

This calculator checks four atmospheric readings against widely-used confined- space entry thresholds. Oxygen (%) is evaluated against OSHA's own 29 CFR 1910.146 boundaries for a permit-required confined space: below 19.5% is oxygen-deficient and above 23.5% is oxygen-enriched (a fire and explosion hazard), with the normal band in between. LEL (%) -- percent of the Lower Explosive Limit -- is flagged at 10%, OSHA's own definition of a hazardous atmosphere for flammable gas or vapor, with a stricter 25% commonly used as an evacuation trigger by many entry programs (some use 20% instead; there is no single fixed OSHA evacuation number).

H2S (ppm) and CO (ppm) are checked against commonly-used industrial- hygiene action levels below their published IDLH (Immediately Dangerous to Life or Health) values of 100 ppm and 1,200 ppm respectively -- these lower action levels vary somewhat by source (NIOSH, OSHA, and ACGIH publish different numbers for each gas), so treat them as conservative screening thresholds, not the only correct figures. Entry Permitted requires every reading to sit inside its safe band simultaneously; Immediate Evacuation Required triggers on oxygen deficiency or any gas crossing its IDLH-based danger level -- and on oxygen enrichment above 23.5% as well, since an enriched atmosphere is a get-out condition rather than merely a do-not-enter one: ordinary clothing and materials ignite readily and burn far faster than they do in normal air. This calculator is a screening aid, not a substitute for a competent person's real-time atmospheric monitoring and your facility's written confined-space entry program.