OSHACONFINED SPACES

Permit-Required Confined Spaces
29 CFR 1910.146 Compliance

Q: What is a permit-required confined space?

A permit-required confined space (PRCS) under 29 CFR 1910.146 is one that is large enough for a worker to enter and perform assigned work, has limited means of entry or exit, is not designed for continuous occupancy, AND has one or more of: a hazardous atmosphere, a material that could engulf the entrant, internal configuration that could trap or asphyxiate, or any other recognized serious safety hazard. Vaults, silos, storage tanks, sewers, manholes, boilers, pits deeper than 4 feet, and HVAC plenums are the textbook examples.

Q: What are the correct steps for atmospheric testing?

Test in this exact order before every entry: (1) oxygen — must be between 19.5% and 23.5%, (2) combustibles — LEL must be below 10%, and (3) toxics — must be below the permissible exposure limit for any substance that could be present (typically CO and H₂S). Oxygen comes first because combustible-gas sensors require adequate oxygen to function; a low-oxygen reading invalidates any LEL reading you take after it. Bump-test and calibrate the meter per manufacturer instructions, and test at top, middle, and bottom of the space — gases stratify.

Q: Who is required at a confined space entry?

At minimum: an authorized entrant inside the space, an attendant stationed at the opening, and an entry supervisor who signs the permit. The attendant must maintain continuous communication, monitor conditions, summon rescue if needed, and never enter the space for rescue. A separately trained rescue team must be identified before entry begins — whether in-house (must practice annually per 1910.146(k)(2)) or an outside service that can respond within the timeframe dictated by the hazards.

Q: What is the difference between permit-required and non-permit confined spaces?

A non-permit confined space is a confined space that does not contain any actual or potential hazards capable of causing death or serious physical harm. A space starts as permit-required by default unless the employer can demonstrate, evaluate, and document that no PRCS hazards exist. Even a "non-permit" space can re-become permit-required if conditions change — new work, different materials, failure of controls — so reclassification is continuous, not one-time.

Q: Why are rescuers the most common fatalities in confined space incidents?

NIOSH data shows roughly 60% of confined space fatalities are would-be rescuers, typically coworkers who rush in without testing the atmosphere or wearing respiratory protection. The atmosphere that killed the original entrant is still there. OSHA 1910.146(k) requires employers to identify rescue methods before entry begins, provide SCBA or supplied air, retrieval systems (unless they create a greater hazard), and drill rescue operations at least annually using actual or representative spaces.

Q: What training is required for entrants, attendants, and supervisors?

OSHA 1910.146(g) requires training whenever duties change, whenever there is reason to believe duties are not being performed correctly, or whenever hazards change. Training must cover hazards of the specific space, use of equipment, modes of entry, communication signals, and emergency procedures. Retraining is required when deficiencies are observed. Records of training — name, trainer signature, date — must be kept.

Understanding confined space hazards, entry permits, atmospheric testing, and rescue requirements to prevent fatal entries

By Stanislav Samek, Samektra · 11 min read · Last updated April 23, 2026

Critical Safety Issue

Confined space incidents kill an average of 92 workers per year in the United States. Over 60% of confined space fatalities are would-be rescuers who entered without proper equipment or training. Understanding the difference between a confined space and a permit-required confined space — and following entry procedures — saves lives.

Confined Space vs. Permit-Required Confined Space

A confined space has three defining characteristics 1910.146(b): (1) it is large enough for an employee to bodily enter and perform work, (2) it has limited or restricted means of entry or exit (e.g., tanks, manholes, silos, vaults, pits), and (3) it is not designed for continuous occupancy.

A confined space becomes permit-required when it contains or has the potential to contain one or more of these additional hazards:

Hazardous Atmosphere

Oxygen-deficient (<19.5%), oxygen-enriched (>23.5%), flammable gases/vapors (>10% LEL), or toxic contaminants above the PEL

Engulfment Hazard

Material such as grain, sand, or water that could surround and suffocate an entrant

Converging Configuration

Inwardly converging walls or a floor that slopes downward and tapers to a smaller cross-section, trapping an entrant

Any Other Serious Hazard

Unguarded machinery, energized electrical conductors, extreme heat, or other conditions that could cause death or serious injury

Common examples of permit-required confined spaces include storage tanks, process vessels, sewers, boilers, underground vaults, and silos. Even a seemingly harmless pit or crawl space can become permit-required if a hazardous atmosphere develops.

Atmospheric Testing

Atmospheric testing is one of the most critical elements of confined space entry. Tests must be performed before entry and continuously during occupancy 1910.146(c)(5). The testing sequence matters — test in this exact order:

1stOxygen (O₂)19.5% – 23.5%If O₂ is outside this range, combustibility readings are unreliable and toxic gas sensors may malfunction

2ndCombustibility (LEL)< 10% LELFlammable atmospheres can ignite — do not enter if LEL exceeds 10%

3rdToxic gasesBelow PELTest for specific toxics expected (H₂S, CO, etc.) based on the space history and contents

Always test at multiple levels (top, middle, bottom) because gases stratify by density. Hydrogen sulfide (H₂S), for example, is heavier than air and accumulates at the bottom of a space, while methane rises to the top. Multi-gas monitors with remote sampling probes allow testing before any part of the body enters the space.

Entry Permit Requirements

The entry permit is a living document that authorizes and controls entry into a permit-required confined space. It must be completed before entry and posted at or near the entry point 1910.146(f). Required elements include:

Space to be entered and purpose of entry.
Date and authorized duration of entry.
Names of authorized entrants, attendants, and entry supervisor.
Hazards identified for the space.
Measures to isolate the space and eliminate or control hazards (lockout/tagout, ventilation, barriers).
Acceptable entry conditions (atmospheric test results with limits).
Results of initial and periodic atmospheric tests, with tester name and calibration date.
Rescue and emergency services — how to summon them and how they will reach the space.
Communication procedures between entrants and attendants.
Equipment provided (ventilation, PPE, lighting, communications, retrieval systems).
Entry supervisor signature authorizing entry.

Permits must be cancelled when entry is complete or conditions change. Cancelled permits are retained for at least one year for the annual program review.

Key Personnel Roles

Authorized Entrant

Knows the hazards, uses equipment properly, communicates with the attendant, exits immediately when ordered or when they recognize a dangerous condition

Attendant

Remains outside the space at all times, monitors entrants and conditions, maintains an accurate count, orders evacuation if needed, summons rescue, prevents unauthorized entry

Entry Supervisor

Authorizes entry by signing the permit, verifies all precautions are in place, terminates entry and cancels the permit when conditions change or work is complete

Critical Rule: The attendant must never enter the confined space to perform a rescue. Over 60% of confined space fatalities are rescuers. The attendant's job is to summon trained rescue services and, if equipped, perform non-entry rescue using retrieval systems.

Rescue Planning

Every permit-required confined space program must address rescue 1910.146(d)(9). Employers have three options:

Non-entry rescue — retrieval lines and mechanical devices (tripod/winch) pull the entrant out without anyone entering the space. This is the preferred method when space geometry allows it.
In-house rescue team — employer-trained rescue personnel who practice space entry at least annually and are equipped with SCBA, retrieval systems, and first aid/CPR capability.
Third-party rescue service — local fire department or private rescue company. The employer must verify that the service can respond in a timely manner, has the capability to rescue from the specific type of space, and has agreed to provide the service.

Entrants must wear a chest or full-body harness with a retrieval line attached to a mechanical device whenever entry is made into a permit space, unless the retrieval equipment would increase the overall risk of entry or would not contribute to rescue 1910.146(k)(3).

Reclassification to Non-Permit Space

A permit-required confined space may be reclassified as a non-permit confined space if the employer can demonstrate that all hazards have been permanently eliminated — not just controlled 1910.146(c)(7). For example, if a tank's only hazard was a toxic residue and the tank has been thoroughly cleaned, purged, and ventilated, it may qualify for reclassification. The employer must document the basis for the determination and certify that the space poses no actual or potential atmospheric hazard. If hazards cannot be permanently eliminated, the space remains permit-required even if hazards are temporarily controlled through ventilation or isolation.

Alternate Entry Procedures

When the only hazard is an actual or potential hazardous atmosphere that can be controlled with continuous forced-air ventilation alone, employers may use alternate entry procedures 1910.146(c)(5) instead of full permit entry. This allows entry without a complete permit system but still requires atmospheric monitoring, ventilation, and documentation. This is not the same as reclassification — the space is still permit-required, but simplified procedures apply.

Frequently Asked Questions

What is a permit-required confined space?

A permit-required confined space (PRCS) under 29 CFR 1910.146 is one that is large enough for a worker to enter and perform assigned work, has limited means of entry or exit, is not designed for continuous occupancy, AND has one or more of: a hazardous atmosphere, a material that could engulf the entrant, internal configuration that could trap or asphyxiate, or any other recognized serious safety hazard. Vaults, silos, storage tanks, sewers, manholes, boilers, pits deeper than 4 feet, and HVAC plenums are the textbook examples.

What are the correct steps for atmospheric testing?

Test in this exact order before every entry: (1) oxygen — must be between 19.5% and 23.5%, (2) combustibles — LEL must be below 10%, and (3) toxics — must be below the permissible exposure limit for any substance that could be present (typically CO and H₂S). Oxygen comes first because combustible-gas sensors require adequate oxygen to function; a low-oxygen reading invalidates any LEL reading you take after it. Bump-test and calibrate the meter per manufacturer instructions, and test at top, middle, and bottom of the space — gases stratify.

Who is required at a confined space entry?

At minimum: an authorized entrant inside the space, an attendant stationed at the opening, and an entry supervisor who signs the permit. The attendant must maintain continuous communication, monitor conditions, summon rescue if needed, and never enter the space for rescue. A separately trained rescue team must be identified before entry begins — whether in-house (must practice annually per 1910.146(k)(2)) or an outside service that can respond within the timeframe dictated by the hazards.

What is the difference between permit-required and non-permit confined spaces?

A non-permit confined space is a confined space that does not contain any actual or potential hazards capable of causing death or serious physical harm. A space starts as permit-required by default unless the employer can demonstrate, evaluate, and document that no PRCS hazards exist. Even a "non-permit" space can re-become permit-required if conditions change — new work, different materials, failure of controls — so reclassification is continuous, not one-time.

Why are rescuers the most common fatalities in confined space incidents?

NIOSH data shows roughly 60% of confined space fatalities are would-be rescuers, typically coworkers who rush in without testing the atmosphere or wearing respiratory protection. The atmosphere that killed the original entrant is still there. OSHA 1910.146(k) requires employers to identify rescue methods before entry begins, provide SCBA or supplied air, retrieval systems (unless they create a greater hazard), and drill rescue operations at least annually using actual or representative spaces.

What training is required for entrants, attendants, and supervisors?

OSHA 1910.146(g) requires training whenever duties change, whenever there is reason to believe duties are not being performed correctly, or whenever hazards change. Training must cover hazards of the specific space, use of equipment, modes of entry, communication signals, and emergency procedures. Retraining is required when deficiencies are observed. Records of training — name, trainer signature, date — must be kept.

References

1. OSHA 29 CFR 1910.146 — Permit-Required Confined Spaces (General Industry).

2. OSHA 29 CFR 1926 Subpart AA — Confined Spaces in Construction (2015).

3. NFPA 350: Guide for Safe Confined Space Entry and Work, 2022 Edition.

4. NIOSH: Confined Space — Worker Deaths in Confined Spaces.

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