The four hazards that make a confined space permit-required

§1910.146(b) defines four hazard categories that upgrade a confined space to permit-required: (1) hazardous atmosphere; (2) engulfment material; (3) entrapment geometry — internally converging walls or a downward-sloping floor tapering to a smaller cross-section; and (4) any other recognised serious safety or health hazard.

Any single category is enough. Most real-world permit spaces trip more than one. See how the hazard picture feeds the permit lifecycle in our [confined space entry permits](/resources/confined-space-entry-permits) pillar.

Four sub-conditions each qualify as a hazardous atmosphere: flammable gas or vapour ≥10% LEL; airborne combustible dust at or above the LFL; oxygen concentration below 19.5% or above 23.5%; and any substance above its published dose or PEL that could cause death, incapacitation, or acute illness.

Note the catch-all — 'any other atmospheric condition immediately dangerous to life or health' (IDLH). If you don't know what a space contains, treat the atmosphere as hazardous until testing proves otherwise.

Engulfment is 'the surrounding and effective capture of a person by a liquid or finely divided (flowable) solid substance that can be aspirated to cause death by filling or plugging the respiratory system or that can exert enough force on the body to cause death by strangulation, constriction, or crushing.'

The classic case is a grain bin — but the same physics apply in sand hoppers, cement silos, wastewater basins, and any tank that periodically holds liquid. Bridging and rat-holing let apparently empty bins collapse under an entrant's weight without warning.

Any confined space with 'internally converging walls or a floor which slopes downward and tapers to a smaller cross-section' where an entrant could be trapped or asphyxiated is permit-required on geometry alone — no atmospheric hazard required.

Wet-wells, funnel-bottom hoppers, and inverted-cone silos are archetypal examples. The hazard is that gravity, once an entrant slides, does the rest.

This is the catch-all: unguarded mechanical parts, energised electrical, extreme heat or cold, noise capable of causing hearing damage, or any process hazard that could injure or kill. If you'd need a hazard-specific control beyond ventilation to make entry safe, the space is permit-required.

Getting this right up front avoids the trap of scoping permits too narrowly. Combine this list with the atmospheric-testing sequence and the rescue plan and you have the operational core of §1910.146 — see the full [confined space entry permits](/resources/confined-space-entry-permits) guide, then [book a demo](/book-demo) to see how the hazard categories drive template selection in the SafeGuard permit-to-work module.