3.1 The hazards that have nothing to do with the air
Most confined space training is really atmospheric training with a rescue chapter at the end. But two of the four permit space characteristics have nothing to do with the air, and the case files are full of workers killed in spaces where the meter read perfectly. A space can be full of clean, breathable air and still be lethal because of what is in it, what is above it, or what shape it is.
1 “Physical hazard” has a definition, and it is broader than you expect
General industry gathers these under characteristic 4 — “any other recognized serious safety or health hazard.” The construction standard does the work of listing them. 1926.1202: a physical hazard means an existing or potential hazard that can cause death or serious physical damage. Examples include, but are not limited to: explosives; mechanical, electrical, hydraulic and pneumatic energy; radiation; temperature extremes; engulfment; noise; and inwardly converging surfaces. And then a clause people miss entirely: physical hazard also includes chemicals that can cause death or serious physical damage through skin or eye contact rather than through inhalation.
That last clause matters because it decouples the chemical hazard from the gas meter. A tank with caustic residue on the walls can read clean on all four sensors and still burn a worker who kneels in it. There is no instrument for it; there is a safety data sheet and a decision about PPE.
2 Engulfment: bridges, funnels and a few feet of material
The definition at 1910.146(b) and 1926.1202 names two mechanisms: material that can be aspirated to cause death by filling or plugging the respiratory system, and material that can exert enough force on the body to cause death by strangulation, constriction, crushing or suffocation. Neither mechanism needs depth. A worker waist-deep in flowable material cannot lift their own legs out of it, and material that has closed over the chest prevents the ribs from expanding.
Flowable solids add a failure mode liquids do not have: bridging. Material arches over the void left by an outlet drawing from underneath, and the surface looks like a floor. Somebody stands on it to break up a blockage, the arch collapses, and they go with it into the space below. The same physics builds sidewall accumulations that can slab off and bury a person standing next to them.
OSHA Accident Search Summary Nr 143288.015 (01/30/2022) is the whole sequence in five lines: an employee was loosening powdered limestone inside silo 7 with a weight; he entered the silo to recover the tool; he walked on top of the powdered material inside the silo without anyone present and without proper PPE; he was engulfed as he sank into the material and was killed.
3 Inwardly converging walls and tapering floors
Characteristic 3 of the permit space definition covers a space with an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or by a floor which slopes downward and tapers to a smaller cross-section. Hoppers, bins, cyclones, silo cones, chutes, funnel-bottomed tanks and the discharge end of anything designed to make material flow.
The mechanism has nothing to do with the material and nothing to do with the air. A worker who slips down a cone wedges where it narrows, loses any purchase to climb back, and is compressed against a shape that tightens the further they slide. Asphyxiation follows from the position of the chest, in clean air, with a meter reading 20.9 percent.
These are also the hardest spaces to be rescued from, which is why Appendix F to 1910.146 asks about internal configuration and portal size before it asks about anything else. A rescuer entering a cone faces the same geometry that trapped the entrant, plus a patient. Part 4 covers what that implies for the rescue plan.
4 Energy: the space is small and the machine is in it
Agitators, mixers, augers, screw conveyors, rotating shafts, fans, dampers, rams, escalator drives, pumps: any of them inside or opening into a space is a recognized serious safety hazard on its own. What makes it worse in a confined space is geometry. There is nowhere to step back to, the guarding that protects an operator outside does not exist inside, and nobody outside can see the person.
OSHA Accident Search Summary Nr 142862.015 (01/19/2022): an employee was working in a confined space inside an escalator, installing lighting inside the pit at the top of the escalator. A coworker was at the bottom of the escalator conducting electrical testing. The escalator energized, pulled the employee in and crushed him. He was killed. Two workers, two ends of one machine, and no isolation between them.
The rule that governs it is not in 1910.146 at all. 29 CFR 1910.147(a)(3)(ii): when other standards in part 1910 require the use of lockout or tagout, they shall be used and supplemented by the procedural and training requirements of the lockout/tagout standard. So the isolation a permit space needs is a full 1910.147 energy control procedure — identified energy sources, isolation, lockout, stored-energy release, verification — not a switch turned off and a note on a clipboard. In construction, 1926.417 governs the tagging of de-energized circuits and 1926.702(j) covers concrete and masonry equipment.
5 The hazards nobody puts on the permit
| Hazard | What the standard requires |
|---|---|
| Falling into the space through an open portal, and objects falling onto the entrant | When an entrance cover is removed, the opening is promptly guarded by a railing, temporary cover or other temporary barrier — both to prevent an accidental fall through it and to protect the person inside from foreign objects — 1910.146(c)(5)(ii)(B); 1926.1203(e)(2)(ii) |
| Anything that makes it unsafe to remove the cover in the first place, such as internal pressure | Eliminated before the cover is removed — 1910.146(c)(5)(ii)(A); and stated as a general program duty for construction at 1926.1204(c)(8), which names high pressure as the example |
| Traffic, plant vehicles, pedestrians, anything outside that can reach the entrant | Pedestrian, vehicle or other barriers as necessary to protect entrants from external hazards — 1910.146(d)(3)(v), with the barriers and shields themselves required as equipment at (d)(4)(vi) |
| Climbing in and out | Equipment such as ladders needed for safe ingress and egress provided at no cost — 1910.146(d)(4)(vii); 1926.1204(d)(7) |
| Darkness | Lighting sufficient to see well enough to work safely and to exit the space quickly in an emergency — 1910.146(d)(4)(v). Construction adds the illumination minimums of 1926.56 and requires the lighting to be approved for the ignitable or combustible properties of what is present — 1926.1204(d)(5) |
| Noise | Named as a physical hazard at 1926.1202. In a steel space it also destroys the communication the attendant relies on — and communication equipment is itself required at 1910.146(d)(4)(iii) |
| Temperature extremes | Named at 1926.1202. A steam-jacketed vessel or a sun-heated tank imposes a heat load with no shade and no way to leave |
| Chemicals that injure through skin or eye contact | Explicitly a physical hazard at 1926.1202, invisible to a gas meter, addressed through PPE under 1910.146(d)(4)(iv) where engineering and work practice controls do not adequately protect |
Read the lighting row again. “And to exit the space quickly in an emergency” is a design requirement disguised as a comfort one: a single work light on the floor lets you do the job and leaves the ladder in the dark.
6 The construction wording that raises the bar
Two construction paragraphs go further than their general industry equivalents on physical hazards specifically, and they are worth knowing even if you work under 1910.146, because they describe what good practice looks like.
1926.1204(c)(3) requires isolating the permit space and the physical hazard(s) within the space. The general industry equivalent, 1910.146(d)(3)(iii), says only “isolating the permit space.” The construction wording is explicit that shutting the space off from the system is not enough if there is a machine still live inside it.
And 1926.1203(e)(1)(i) — the gateway to a ventilation-only entry — requires the employer to demonstrate that all physical hazards in the space are eliminated or isolated through engineering controls so that the only hazard posed is atmospheric. Not that there happen to be none: that they have been engineered away. If you cannot say what engineering control took the agitator out of play, you are running a full permit entry.
- 1926.1202 defines physical hazard to include explosives, mechanical, electrical, hydraulic and pneumatic energy, radiation, temperature extremes, engulfment, noise, inwardly converging surfaces — and chemicals that injure through skin or eye contact.
- Engulfment kills by aspiration or by force, needs no depth, and adds bridging: a flowable-solid surface that looks like a floor. Summary Nr 143288.015 — a worker entered a silo to recover a tool and sank into the limestone.
- Converging walls and tapering floors trap and asphyxiate in clean air, and they are the hardest spaces to be rescued from.
- Energy isolation for a permit space is a full 1910.147 procedure, because 1910.147(a)(3)(ii) requires lockout/tagout to be supplemented by that standard's procedures and training wherever another 1910 standard calls for it.
- An open portal must be promptly guarded against falls and falling objects, anything making it unsafe to remove the cover is eliminated first, and barriers protect entrants from traffic and other external hazards.
- Construction requires isolating the space and the physical hazards within it (1926.1204(c)(3)), and a ventilation-only entry there requires physical hazards eliminated or isolated through engineering controls (1926.1203(e)(1)(i)).
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.