The main OSHA standard for confined spaces in general industry is 29 CFR 1910.146, titled “Permit-Required Confined Spaces.” It sets out the rules employers must follow to identify confined spaces, evaluate their hazards, and protect workers who enter them. A separate standard, 29 CFR 1926 Subpart AA, covers confined spaces specifically in construction. Together, these regulations define what counts as a confined space, when a permit is required, and exactly what roles, equipment, and procedures must be in place before anyone goes in.
What Counts as a Confined Space
OSHA defines a confined space using three criteria. All three must be true for a space to qualify:
- Large enough to enter. A worker can physically get inside and perform work.
- Not designed for continuous occupancy. The space exists for storage, processing, or some other function, not as a regular workspace.
- Limited entry or exit. Getting in or out is restricted in some way, whether by size, location, or design.
Common examples include tanks, silos, storage bins, hoppers, vaults, pits, and sewers. A crawl space under a building or the inside of a large piece of equipment can also meet all three criteria. The key distinction is that these spaces weren’t built for people to work in continuously, and getting out quickly isn’t as simple as walking through a door.
When a Permit Is Required
Not every confined space needs a permit. A space becomes “permit-required” when it has one or more of these additional hazards:
- Hazardous atmosphere. The air contains or could contain dangerous levels of toxic gases, too little oxygen, or flammable vapors.
- Engulfment risk. The space holds material like grain, sand, or water that could bury or submerge a worker.
- Entrapment hazard. The interior shape could trap someone, such as walls that converge inward or a floor that slopes down to a narrow point.
- Any other serious hazard. This is a catch-all for dangers like exposed electrical components, moving mechanical parts, or extreme heat.
If even one of those conditions exists, the employer must treat the space as permit-required and follow the full set of OSHA requirements before allowing entry.
Atmospheric Hazards and Limits
Atmospheric testing is one of the most critical parts of confined space entry. Before anyone enters, the air inside must be checked for oxygen content, flammable gases, and toxic contaminants, in that order.
Normal air contains about 20.9% oxygen. OSHA considers an atmosphere hazardous when oxygen drops below 19.5% (oxygen-deficient) or rises above 23.5% (oxygen-enriched, which creates a fire risk). For flammable gases and vapors, concentrations at or above 10% of the lower explosive limit are considered hazardous. That threshold is intentionally conservative: it triggers protective action well before the air could actually ignite.
Monitoring doesn’t stop once workers are inside. The standard requires verifying that conditions remain acceptable for the entire duration of the entry.
The Written Permit Program
When employees will be entering permit-required spaces, the employer must develop and implement a written permit space program. This program must be available for workers and their representatives to review. At a minimum, the program must address:
- Measures to prevent unauthorized entry
- Identification and evaluation of hazards before entry
- Acceptable entry conditions (specific readings and thresholds that must be met)
- Isolation of the space from energy sources, piping, or other inputs
- Ventilating, purging, or flushing the space to control atmospheric hazards
- Barriers to protect entrants from traffic or other external dangers
- Continuous monitoring of conditions while workers are inside
Each entry requires a written permit that documents the space being entered, the hazards present, the conditions that were tested and met, and the names of authorized personnel. The permit is essentially a checklist that confirms every safety step has been completed before the first worker goes in.
Required Personnel and Their Roles
OSHA’s confined space standard assigns specific responsibilities to three roles: the authorized entrant, the attendant, and the entry supervisor. Each has distinct duties, and all three are legally required for permit-required entries.
The attendant stays outside the confined space at all times and serves as the primary safety link. They must keep a continuous, accurate count of who is inside, stay in communication with entrants, and monitor for signs of danger. The attendant has the authority to order an immediate evacuation if conditions change, if a worker shows signs of hazard exposure, or if something outside the space threatens the entrants. They also cannot perform any other duties that might interfere with their monitoring role.
The authorized entrant is the worker who actually enters the space. They must understand the hazards they could face, know how to use protective equipment, and communicate with the attendant throughout the entry. They also have the right to observe any atmospheric testing done before or during the entry.
The entry supervisor is responsible for authorizing the entry, verifying that all permit conditions have been met, and ensuring the team is properly equipped and trained. They sign off on the permit and can cancel it at any time if conditions deteriorate.
Rescue and Emergency Planning
One of the most important (and often overlooked) parts of the standard is rescue planning. Every permit-required entry must have a rescue plan in place before work begins, not as an afterthought.
Employers who designate a rescue service, whether internal or external, must evaluate that team’s ability to respond in a timely manner given the specific hazards involved. The rescue team must be equipped and proficient for the type of space they’d be entering, and they need access to the actual permit spaces so they can develop plans and practice. If a rescue team becomes unavailable, they’re required to notify the employer immediately.
For in-house rescue teams, OSHA requires practice rescue operations at least once every 12 months. These drills must involve removing dummies, manikins, or actual people from the permit space or a representative space that matches its opening size, layout, and accessibility. The only exception is if the team performed an actual rescue in the same or a similar space within the past year.
Employer Obligations Before Any Entry
Before confined space entry is even on the table, employers have baseline obligations under the standard. First, they must survey the workplace to determine whether any spaces meet the definition of a confined space and, if so, whether any are permit-required. If permit spaces exist, workers must be informed through danger signs or equally effective communication.
If the employer decides workers will never enter those spaces, the standard still requires effective measures to physically prevent entry, such as locks, covers, or barriers. Simply telling employees to stay out isn’t enough.
If workers will enter, everything described above kicks in: the written program, atmospheric testing, assigned roles, rescue planning, and permits for each entry. The standard also requires that all employees involved receive training before their first entry and whenever conditions change, such as new hazards being introduced or procedures being updated. Training must cover the specific hazards of the spaces they’ll encounter and the proper use of any equipment involved.
General Industry vs. Construction
The standard most people encounter, 29 CFR 1910.146, applies to general industry. It does not cover agriculture, construction, or shipyard employment, each of which has its own regulations. Construction work falls under 29 CFR 1926 Subpart AA, which was finalized in 2015 and mirrors many of the general industry requirements but adds provisions specific to construction environments, such as coordination between multiple employers on a single jobsite. Shipyard work is governed by 29 CFR 1915. If you work in one of these industries, the core concepts are similar, but the specific regulatory requirements differ in important ways.