Confined Space Glossary

A

Acceptable entry conditions

The conditions that must exist in a permit space to allow entry and to ensure employees can safely enter and work inside, such as oxygen 19.5% to 23.5% and flammable gas below 10% LEL. Listed on the entry permit.

Alternate procedures

A simplified entry method in 1910.146(c)(5) for permit spaces whose only hazard is an actual or potential hazardous atmosphere that continuous forced-air ventilation alone can control.

Attendant

An individual stationed outside one or more permit spaces who monitors the authorized entrants and performs the attendant duties in the permit program. See attendant duties.

Authorized entrant

An employee authorized by the employer to enter a permit space. See entrant duties.

B

Blanking or blinding

The absolute closure of a pipe, line or duct by fastening a solid plate that completely covers the bore and withstands the maximum pressure of the line with no leakage.

Bump test

A brief exposure of a gas monitor's sensors to a known concentration of gas to confirm they respond and the alarms work, without adjusting the instrument. See bump test vs. calibration.

C

Calibration

Adjusting a gas monitor's readings to match a known concentration of test gas.

Combustible dust

Dust that can burn or explode when suspended in air. At or above its LFL, roughly when it obscures vision at 5 feet or less, it creates a hazardous atmosphere.

Competent person (construction)

Under Subpart AA, someone capable of identifying existing and predictable hazards and authorized to take prompt corrective measures. A competent person must evaluate a construction worksite and identify confined spaces.

Confined space

A space large enough and so configured that an employee can bodily enter and perform work, with limited or restricted means of entry or exit, and not designed for continuous employee occupancy. See lesson 1.

Continuous monitoring

Atmospheric testing that runs throughout an entry, usually with a monitor worn by the entrant. Required in construction unless not commercially available or periodic monitoring is sufficient.

Controlling contractor

In construction, the employer with overall responsibility for construction at the worksite. Coordinates entry operations among employers. See Subpart AA.

D

Double block and bleed

Closing a line by closing and locking or tagging two in-line valves and opening and locking or tagging a drain or vent valve between them.

E

Early-warning system

In construction, a method such as an alarm or monitor that alerts entrants and attendants to a developing engulfment hazard, such as an upstream flow, in time to exit.

Emergency

Any occurrence, including a failure of hazard control or monitoring equipment, or an event inside or outside the space, that could endanger entrants.

Engulfment

The surrounding and effective capture of a person by a liquid or finely divided (flowable) solid that can be aspirated to cause death or can exert enough force to cause death by strangulation, constriction or crushing.

Enter (entry)

The action by which a person passes through an opening into a permit space. Entry occurs as soon as any part of the entrant's body breaks the plane of an opening into the space.

Entry employer

In construction, any employer who decides that an employee it directs will enter a permit space.

Entry permit

The written or printed document provided by the employer to allow and control entry into a permit space. See the permit template.

Entry rescue

Rescue in which rescuers enter the permit space to reach and remove the entrant.

Entry supervisor

The person responsible for determining whether acceptable entry conditions exist, authorizing entry, overseeing entry operations and terminating entry. See entry supervisor duties.

Explosion-proof

Equipment designed so that an explosion inside it won't ignite a surrounding flammable atmosphere.

H

Hazardous atmosphere

An atmosphere with flammable gas above 10% of its LFL, combustible dust at or above its LFL, oxygen below 19.5% or above 23.5%, a toxic substance above its PEL, or any other condition immediately dangerous to life or health.

Host employer

In construction, the employer that owns or manages the property where the work is done.

Hot work permit

The employer's written authorization for work that can create a source of ignition, such as welding, cutting, burning or heating. It is listed on the entry permit.

Hydrogen sulfide (H2S)

A toxic, flammable, heavier-than-air gas with a rotten-egg odor that deadens the sense of smell at high levels. See the H2S guide.

I

IDLH (immediately dangerous to life or health)

Any condition that poses an immediate or delayed threat to life, would cause irreversible adverse health effects, or would interfere with an individual's ability to escape unaided from a permit space.

Inerting

Displacing the atmosphere in a space with a noncombustible gas such as nitrogen so it can't support combustion. Inerting produces an IDLH oxygen-deficient atmosphere.

Intrinsically safe

Electrical equipment designed so that it can't release enough energy to ignite a flammable atmosphere.

Isolation

Removing a permit space from service and completely protecting it against the release of energy and material into it, by blanking, double block and bleed, line removal, lockout or tagout, and disconnecting linkages. See isolation.

L

LEL / LFL (lower explosive or flammable limit)

The lowest concentration of a gas or vapor in air that can burn. OSHA treats more than 10% of the LFL as a hazardous atmosphere.

Line breaking

Intentionally opening a pipe, line or duct that is or has been carrying flammable, corrosive or toxic material, an inert gas, or any fluid at a volume, pressure or temperature that could cause injury.

Lockout/tagout

Placing locks or tags on energy-isolating devices so equipment can't be operated until they're removed, under 29 CFR 1910.147.

M

Mechanical retrieval device

A winch or similar device used to lift an entrant out of a space. Required to be available for vertical permit spaces more than 5 feet deep.

N

Non-entry rescue

Rescue performed from outside the space, usually with a retrieval system, so rescuers don't enter.

Non-permit confined space

A confined space that doesn't contain, or with respect to atmospheric hazards have the potential to contain, any hazard capable of causing death or serious physical harm.

O

Oxygen-deficient atmosphere

An atmosphere containing less than 19.5% oxygen by volume.

Oxygen-enriched atmosphere

An atmosphere containing more than 23.5% oxygen by volume. A severe fire hazard.

P

Permit system

The employer's written procedure for preparing and issuing permits for entry and for returning the permit space to service after entry.

Permit-required confined space (permit space)

A confined space with one or more of: a hazardous atmosphere or the potential for one, engulfment material, a trapping internal configuration, or any other recognized serious safety or health hazard. See lesson 2.

Permit-required confined space program

The employer's overall written program for controlling, and protecting employees from, permit space hazards and for regulating entry. See the program guide.

Photoionization detector (PID)

A gas detector that measures many volatile organic compounds using ultraviolet light.

Prohibited condition

Any condition in a permit space that isn't allowed by the permit during the authorized entry period.

Purging

Ventilating a space before entry to remove or dilute contaminants. See ventilation.

R

Reclassification

Changing a permit space to a non-permit space under 1910.146(c)(7) when it has no atmospheric hazards and all hazards are eliminated without entry, documented in a written certification.

Rescue service

The personnel designated to rescue employees from permit spaces. See rescue.

Retrieval system

The equipment, including a retrieval line, chest or full-body harness, wristlets if appropriate, and a lifting device or anchor, used for non-entry rescue.

S

Self-retracting lifeline (SRL)

A fall-protection device with a line that retracts automatically. Some models include a retrieval winch for confined space rescue.

Stratification

Layering of gases by density inside a space, such as methane at the top and hydrogen sulfide at the bottom. Why testing must be done at several levels.

Supplied-air respirator

A respirator that supplies breathing air from a source outside the contaminated area, such as an SCBA or airline respirator. Required in IDLH atmospheres.

Suspension (of a permit)

In construction, temporarily stopping an entry under a permit for a temporary condition, then restoring acceptable entry conditions before re-entry.

T

Testing

Identifying and evaluating the hazards that entrants may be exposed to in a permit space, including the instruments and procedures used.

Tripod

A three-legged frame set over an opening to support a retrieval winch or SRL for vertical entries.

V

Ventilation (forced air)

Using blowers and ducting to supply fresh air to, or exhaust contaminants from, a space. See the purge calculator.

W

Wristlets

Retrieval straps worn on the wrists, allowed instead of a harness only if a harness is infeasible or creates a greater hazard.

Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.