Under federal workplace safety regulations, there are two types of confined spaces: non-permit required and permit-required. A separate framework from the National Institute for Occupational Safety and Health adds a three-tier severity ranking — Class A, Class B, and Class C — that grades hazard severity on top of the federal categories rather than replacing them. So depending on which system you’re working with, the answer is either two or three.
Both systems only apply once a space qualifies as “confined” in the first place, and the categories describe hazard level, not physical shape.
When a Space Counts as Confined at All
Federal regulations use a three-part test, and a space has to meet all three parts at once:
- It’s large enough for a worker to physically enter and do work inside. Reaching in with a hand or a tool doesn’t count.
- Entry or exit is restricted. Ladders, hatches, narrow openings, and portals that would slow an emergency escape all qualify.
- It wasn’t designed for continuous occupancy. The ventilation, lighting, and layout aren’t what you’d expect in a normal work area.
Storage tanks, silos, vaults, pits, manholes, tunnels, hoppers, and equipment housings are common examples.1Occupational Safety and Health Administration. Confined Spaces Once a space checks all three boxes, the employer has a legal duty to evaluate the hazards inside and classify it.2Occupational Safety and Health Administration. 29 CFR 1910.146 – Permit-Required Confined Spaces
Non-Permit Required Confined Spaces
A non-permit confined space meets all three physical criteria but contains no hazard capable of causing death or serious injury.2Occupational Safety and Health Administration. 29 CFR 1910.146 – Permit-Required Confined Spaces The air is safe, nothing inside could trap or engulf a worker, and no other recognized danger is present. A vented utility vault with free-flowing air and no machinery inside is a typical example.
The label isn’t permanent. If a new process introduces fumes nearby, or if materials get stored inside that could shift and engulf a worker, the space has to be re-evaluated. Treating a non-permit space as forever safe is a common and sometimes fatal mistake. Clear labeling and periodic re-inspection are the minimum.
Permit-Required Confined Spaces
A permit-required confined space has the same physical characteristics as any confined space but also contains at least one serious hazard. Under federal regulations, a space earns this designation if any of the following is true:2Occupational Safety and Health Administration. 29 CFR 1910.146 – Permit-Required Confined Spaces
- The space has a hazardous atmosphere. That means flammable gas, toxic vapors, oxygen levels below 19.5% or above 23.5%, or combustible dust at dangerous concentrations.
- The space contains material that could engulf a worker, such as grain, water, or sand.
- The space has entrapment geometry: walls that converge inward, or floors that slope down into a narrower section where a person could get stuck.
- Any other recognized serious hazard is present. This catch-all covers exposed electrical components, moving mechanical parts, extreme temperatures, and similar dangers.
Most of the regulatory weight in the confined space standard falls on this category. Employers must maintain a written program covering entry, air testing, assigned roles, and rescue planning before anyone goes inside.
Alternate Entry: A Middle Ground
Not every permit-required space demands the full permit program. Federal regulations allow a streamlined “alternate entry” option when two conditions are both met: the only hazard in the space is atmospheric, and continuous forced-air ventilation alone is enough to keep the space safe.2Occupational Safety and Health Administration. 29 CFR 1910.146 – Permit-Required Confined Spaces When both apply, the employer can skip several of the heavier permit-program requirements, including the full written permit and the attendant stationed at the opening.
This is not a free pass. The employer still has to test the air before entry (oxygen first, then combustibles, then toxics), keep ventilation running the entire time anyone is inside, document the supporting data, and make that documentation available to every worker who enters. If the air goes bad while someone is inside, the space must be evacuated immediately and re-evaluated. Many employers don’t realize this middle-ground option exists and end up either over-engineering their program or skipping protections because the full permit process feels like too much overhead for a simple job.
Moving Between the Two Categories
A permit-required space can be reclassified as non-permit, but only when every hazard has been completely eliminated, not merely controlled. If the hazards can be removed without entering the space, the employer documents the basis for reclassification in a signed certification that includes the date, the location, and the name of the person making the determination.2Occupational Safety and Health Administration. 29 CFR 1910.146 – Permit-Required Confined Spaces
If someone has to enter the space to eliminate the hazards, that initial entry must follow the full permit-required procedures. Only after entry confirms that all hazards are gone can the reclassification take effect. It lasts only as long as the non-hazardous conditions persist. The moment a hazard reappears, everyone must exit and the space reverts to permit-required status.
The NIOSH Severity Classes
The National Institute for Occupational Safety and Health uses a separate three-part system that ranks confined spaces by how severe their hazards are, rather than sorting them into permit or no-permit buckets:3National Institute for Occupational Safety and Health. Confined Spaces
- Class A spaces are immediately dangerous to life or health. The environment could kill or incapacitate a worker within minutes, from extreme oxygen depletion or high concentrations of toxic gas. These demand the highest level of protective equipment and a rescue team staged right outside the entry point.
- Class B spaces are dangerous but not IDLH. The hazards can injure or sicken a worker but aren’t expected to cause death or incapacitation on the timescale of minutes. Slippery surfaces, moderate chemical exposure, or heat stress fall here. Safety protocols are still significant.
- Class C spaces are potentially hazardous. Risks exist but are unlikely to threaten life or cause serious injury under normal conditions. Minor mechanical hazards or the need for slight ventilation adjustments are typical, and basic entry procedures still apply.
The NIOSH system isn’t a legal requirement the way the OSHA permit classification is. It’s a risk-management tool that helps safety managers decide how to allocate resources. A facility might have twenty permit-required spaces, but if only two are Class A, the rescue team and the best equipment belong near those two. The severity grading sits on top of the OSHA framework rather than replacing it.
Construction Sites Follow a Different Standard
The two-category framework above lives in the general industry standard. If the work is happening on a construction site, a separate rule applies: 29 CFR 1926, Subpart AA, finalized in 2015 specifically because construction confined spaces present hazards that change constantly as work progresses.4Occupational Safety and Health Administration. Confined Spaces in Construction A trench that wasn’t a confined space yesterday can become one today after excavation deepens it.
The biggest practical difference is coordination among multiple employers. Construction sites routinely have a host employer, a controlling contractor, and multiple subcontractors working in or near the same spaces. The construction standard requires controlling contractors and host employers to share information about confined spaces and their hazards with every entry employer before and after entry.5Occupational Safety and Health Administration. Confined Spaces in Construction – Frequently Asked Questions A generator running near an entrance can fill a space with carbon monoxide even though the crew inside had clean air when they started. The construction standard also includes an explicit employee participation provision that gives workers a more defined role in the permit-required program.