The OSHA standard on confined space has been in place for seven years and I still frequently get questions from clients and colleagues about what is or is not a confined space.

There are three definitions in the standard that we must use to determine whether a specific location is considered a confined space, and if so, which type of confined space it is.

To be covered by the standard, a space must first meet the confined space definition. A confined space has three characteristics:

  • Large enough to bodily enter,

  • Limited or restricted access, and

  • Not designed for continuous human occupancy.

What is a "confined space"?

A space must have all three of the following characteristics to be considered a confined space. If a location in question does not meet this definition, it is not covered by the confined space standard.

Size: The size of the space must be such that an individual could completely enter the space and work inside. This is sometimes confusing because the standard defines “entry” as having occurred anytime any part of the body crosses the plane of the opening. We must remember that the definition of entry is not an issue unless the area being evaluated is indeed a confined space according to the three criteria of the standard.

Access: The second criteria, limited or restricted access, may at times be difficult to assess. Many spaces will clearly be limited or restricted, such as manholes, bolted hatch covers, and very small openings. Many may easily be determined to not limit or restrict access, such as regular doors and access points with fixed stairs. The area most difficult to evaluate is the middle ground, when the degree of access is not obvious. When you make a final determination of this criteria, I believe errors should be made on the side of safety. If this characteristic is debatable, consider it limited or restricted.

One of the best ways to think about the opening is to visualize yourself inside the space when an emergency occurs. Ask yourself, “Would the size, shape, or configuration of the opening slow you down in your attempt to escape?” If it would, then the opening should be considered limited or restricted.

Occupancy: The third criteria — not designed for continuous human occupancy — is usually the easiest to evaluate. If the interior of the space during periods of normal use is not designed for an individual to continuously occupy, then it is not designed for continuous occupancy. The design element is a critical part of the definition. Whether the space is occupied is not important, the issue is whether it could be.

Defining a "permit-required confined space"

If any one of the following items is present in a location that meets the confined space definition, then it must be considered a permit-required confined space. If not, it is considered a non-permit confined space.

A permit-required confined space must have one or more of the following characteristics:

  • Hazardous atmosphere,

  • Potential for engulfment,

  • Internal configuration hazard, or

  • Other recognized serious safety or health hazard.

Hazardous atmospheres

The potential to contain a hazardous atmosphere as defined by OSHA should be an easy element of the definition to determine. But one of the problem areas with this component is the perception of “potential.” If we refer to the dictionary, potential means “capable of being but not yet in existence; latent.” In the broadest interpretation, almost all confined spaces have the potential for a hazardous atmosphere. To classify a space as not having a hazardous potential, we need to be certain beyond any reasonable doubt that an atmospheric hazard does not and will not exist in the space while we have entry personnel in the space.

Oxygen: Oxygen concentration is usually fairly straightforward. Normal air has about 21% oxygen. The range provided by OSHA (below 19.5% or above 23.5%) is wide and should be considered the extreme outer limits. Investigate any variance from normal atmospheric conditions.

Flammables: Flammables at or above 10% of their lower explosive limit (LEL, sometimes refered to as lower flammable limit (LFL)) are considered cause for concern. Any measureable quantity of flammable gases or vapors should be considered a problem and investigated.

Toxics: Toxic contaminants above their permissible exposure limit (PEL) are covered by the definition if they are capable of short-term harm or incapacitating the entrant. I usually use the threshold limit value (TLV) for these determinations, because they cover more chemicals and are more frequently updated. Any detectable concentration of toxic materials should be investigated thoroughly.

Dusts: The general rule that OSHA provides on dust does not provide adequate information to make safe choices concerning all combustible dusts and all conditions. Some dusts may not obscure your vision at five feet and still explode, others may obscure your vision at considerably less than five feet and not explode. Dust explosions and their prevention can be a complex process. If you have an area where combustible dust may be a problem, you will need to evaluate it thoroughly on a case-by-case basis. To make matters more challenging, there are no portable instruments available to help make this assessment.

Other Immediately Dangerous to Life and Health (IDLH) conditions could cover a multitude of potential hazards. This IDLH category is also designed as a catch-all.

Remember that many materials have multiple hazards. For example, a flammable solvent will also have a PEL that must be considered, and will typically be reached before 10% of the LEL.


Engulfment occurs when any material may be introduced to the space harming the entrant. For granular or flowable solid materials, engulfment can also occur if they are removed. For example, sand being released from the bottom of a hopper would create a funneling effect. Some flowable materials may also engulf an entrant simply due to their movement on the surface of the materials. An example of this is plastic pellets.


OSHA specifically mentions funnel-type configurations in its definition. This design is common in hoppers and some silos. Take a broad view of configuration issues when evaluating spaces, and look for any design that may compromise the safety of entrants.

Other serious hazards

Other recognized serious safety or health hazards is the fourth element of OSHA’s definition for permit-required confined space. This is very broad, performance-oriented regulatory language, open to considerable interpretation. It’s up to us to determine that no other problem exists that may harm the entrants.

Whether a hazard should have been recognized or not typically is evaluated in hindsight after an incident has occurred. With hindsight, the hazard will be obvious to everyone. This means we must be particularly vigilant in looking for hazards prior to entry.

“Serious” will often also be viewed through hindsight when the outcome of an incident is known. One example of this category is thermal extremes — with very hot conditions being the most common problem encountered in many industries.

Defining a "non-permit confined space"

A non-permit space is a location that meets the definition of confined space but does not have any of the elements that would make it a permit-required confined space. This means it has no hazards capable of causing death or serious physical harm.

Generally, in my experience, non-permit confined spaces are quite rare. The broad scope of the permit-required confined space definition covers the vast majority of confined spaces. We also want to err on the side of caution if a space is debatable.

One example of a non-permit confined space is typically a plenum. These areas usually meet the definition of confined spaces, but rarely have hazards that would make them permit-required confined spaces.

Please remember that compliance should be our second priority. Keeping our employees safe should always come first. If keeping employees safe requires going beyond the minimum requirements of OSHA, this is what you must do. This process often begins in confined space work with how we apply the definitions. If in doubt, consider the space permit-required.