A confined space is anywhere big enough to work in, hard to get in and out of, and not made for people to stay in, like a manhole, tank, vault, sewer, or crawl space, and if it also has a serious hazard it becomes a "permit-required" space you cannot just climb into. These spaces kill workers who go in without testing the air, and they kill the coworkers who rush in to save them. OSHA's rules for construction are in 29 CFR 1926 Subpart AA. The core rule is simple: test the atmosphere first, control the space with a permit and an attendant, and never enter to rescue someone without proper gear. This is federal OSHA in 2026.
What counts as a confined space
A confined space has all three of these:
- Big enough to enter and do work.
- Limited or restricted ways in and out (a small hatch, a manhole, a narrow opening).
- Not designed for people to occupy continuously.
On a construction site that means manholes, sewers and storm drains, tanks, boilers, vaults, pits, silos, pipes, ductwork, and some crawl spaces. Plenty of everyday spaces qualify.
When it becomes "permit-required"
A confined space turns into a permit-required confined space (PRCS) when it also has one or more of these hazards:
- A hazardous atmosphere: too little oxygen, too much, flammable gas or vapor, or a toxic gas.
- Engulfment risk: loose material like grain, sand, or water that could bury or drown you.
- A configuration that could trap you, like inwardly sloping walls or a floor that tapers to a smaller cross-section.
- Any other recognized serious hazard, such as unguarded machinery, heat, or exposed live wires.
If any of those are present, you are in permit-required territory, and casual entry is off the table.
The air is the number-one killer
Most confined-space deaths come from the atmosphere, and you cannot see, smell, or feel the danger. A space can be short on oxygen or full of a toxic gas while looking completely normal.
- Test before entry with a calibrated gas meter, from outside the space, and keep monitoring while people are inside.
- OSHA treats an acceptable oxygen range as roughly 19.5 percent to 23.5 percent. Below that is oxygen-deficient and can drop you in seconds; above it is oxygen-enriched and a fire risk.
- Ventilate the space to bring it into a safe range, and never rely on "it's probably fine."
Never use pure oxygen to "freshen" a space. It creates an explosion hazard.
Roles, permit, and rescue
A permit-required entry runs on defined roles and paperwork:
- The entry supervisor authorizes the entry, signs the permit, and makes sure the conditions are safe before anyone goes in.
- The entrant does the work inside and gets out at the first sign of trouble.
- The attendant stays outside the whole time, keeps count of who is in, stays in contact, and never enters to rescue.
- The entry permit documents the space, the hazards, the test results, the controls, and who is doing what. No permit, no entry.
- Rescue has to be arranged before entry, with trained rescuers and retrieval equipment. Most confined-space deaths include would-be rescuers who ran in without protection and died too. If someone goes down inside, you call for the rescue plan, you do not dive in.
Common questions
What makes a space a confined space?
A confined space is large enough to enter and work in, has limited or restricted ways in and out, and is not designed for continuous occupancy. Manholes, tanks, vaults, sewers, pits, and some crawl spaces all qualify. On their own these are "confined spaces"; they become permit-required only when a serious hazard like a bad atmosphere or engulfment risk is also present.
What is a permit-required confined space?
It is a confined space that also has a serious hazard: a dangerous atmosphere, an engulfment risk, a shape that could trap you, or another recognized serious hazard. Once a space is permit-required, you cannot just enter. It takes atmospheric testing, a written entry permit, an attendant outside, and a rescue plan arranged before anyone goes in.
Do I have to test the air before entering a confined space?
Yes. Test the atmosphere with a calibrated gas meter from outside the space before entry, and keep monitoring while workers are inside. You are checking oxygen, flammable gases, and toxic gases, because none of them can be seen or smelled reliably. OSHA treats roughly 19.5 to 23.5 percent oxygen as the acceptable range; outside that, the space is not safe to enter until it is ventilated.
Why should you never rush in to rescue someone in a confined space?
Because most confined-space deaths include the rescuers, who go in without gear and are overcome by the same atmosphere that dropped the first person. A space with too little oxygen or a toxic gas can incapacitate you in seconds. Rescue has to be planned before entry, with trained rescuers and retrieval equipment. If someone goes down inside, call the rescue plan; do not enter after them unprotected.
The honest bit
- The confined-space rules here are federal OSHA under 29 CFR 1926 Subpart AA, in force since 2015 and current in 2026. Verify the current standard and the acceptable oxygen range at osha.gov.
- About 22 states run their own OSHA program and can require more. See Working in Your State.
- Confined-space entry is specialized, high-risk work that requires proper training, gas monitoring, and a rescue plan. This is general guidance, not training, and no substitute for a qualified confined-space program.
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Keep reading
Templates you might need
Sources
- 29 CFR 1926.1203 - General requirements (confined spaces in construction) · Permit-required confined space identification and entry duties in construction
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