1.2 The four characteristics that make it permit-required

1 Four characteristics, any one of them

1910.146(b) defines a permit-required confined space as a confined space that has one or more of the following characteristics. 1926.1202 repeats them for construction:

There is no scoring, no weighting and no threshold. One characteristic is the whole test. A perfectly ventilated, spotless, well-lit tank with a functioning agitator inside it is a permit space on characteristic 4 alone, because the agitator is a recognized serious safety hazard.

2 “Or has a potential to contain”

Characteristic 1 is not “the meter reads bad.” It is contains or has a potential to contain a hazardous atmosphere. That phrase does the heaviest lifting in the whole standard, because it makes the classification a property of the space rather than a property of this morning's reading.

A tank that has been emptied, washed and left open for a week still has the potential to contain a hazardous atmosphere: residue in the sludge line, product trapped under a baffle, a nitrogen blanket on the vessel next door with a common header, rust consuming oxygen off the steel, and whatever the crew is about to bring in with them — a solvent, a torch, an engine-driven pump. The potential does not disappear because the space smells clean.

3 Engulfment: it does not have to be liquid, and it does not have to be deep

The construction standard defines the word at 1926.1202: 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, crushing, or suffocation.”

Two mechanisms, then. The material can be breathed in — grain dust, flour, cement, sand, fly ash, sawdust — or it can simply squeeze. Both work in a few feet of material, and both work fast. Flowable solids also do something liquids do not: they bridge. Material arches over a void, the surface looks solid, somebody stands on it, and the bridge collapses under them into the space below.

4 Converging walls and tapering floors

Characteristic 3 is the only one that is purely about shape: a space that has 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 bottom of anything that was designed to make material flow toward an outlet.

The hazard here is not the material — it is the geometry. A worker who slides down a cone wedges at the narrow end, cannot get purchase to climb back, and is compressed by their own weight against a shape that gets tighter the further they go. Asphyxiation follows from the position, with nothing toxic anywhere in the space. This is why characteristic 3 stands on its own and does not need characteristic 1 or 2 to be present.

The same geometry is what makes rescue from these spaces so difficult, and it is why Appendix F to 1910.146 asks about internal configuration and portal size before it asks about anything else. Part 4 comes back to that.

5 “Any other recognized serious safety or health hazard”

Characteristic 4 is the catch-all, and it catches a great deal. General industry does not define it further, but the construction standard does the work for both: 1926.1202 defines a physical hazard as an existing or potential hazard that can cause death or serious physical damage, and gives examples — explosives; mechanical, electrical, hydraulic and pneumatic energy; radiation; temperature extremes; engulfment; noise; and inwardly converging surfaces. It adds one that surprises people: chemicals that can cause death or serious physical damage through skin or eye contact rather than through inhalation.

6 Non-permit confined space, and how it stops being one

The other half of the classification has a name. 1910.146(b): a non-permit confined space means a confined space that does not contain or, with respect to atmospheric hazards, have the potential to contain any hazard capable of causing death or serious physical harm. It is still a confined space — still awkward to get out of, still not built for people — but it carries no permit space characteristic.

That status is not permanent, and the standard says so. 1910.146(c)(6): when there are changes in the use or configuration of a non-permit confined space that might increase the hazards to entrants, the employer shall reevaluate that space and, if necessary, reclassify it as a permit-required confined space. Bringing a solvent in changes the use. Running a temporary gas line to it changes the use. Installing an agitator changes the configuration. So does parking a generator upwind of the only opening.

7 Why the classification is worth arguing about

Everything downstream depends on it. If the space is permit-required, the employer that sends people in needs a written permit space program (1910.146(c)(4), 1926.1203(d)), a permit for each entry, an attendant outside for the duration, atmospheric testing in a fixed order, designated and trained roles, and rescue arrangements it has actually evaluated. If the space is not permit-required, none of that machinery applies.

Which is precisely why the classification is where corners get cut, and why the incident abstracts so often read as though nobody had ever asked the question. The honest version of the test is short: name the characteristic that is not present, one at a time, and be able to say how you know. If you cannot do that for all four, you are looking at a permit space.