Confined Space


Workers that assemble vessel structures and parts may face hazards from confined space.

Risk Description:

A confined space is any enclosed area not designed for continuous human occupancy that has a limited or restricted means of entry or exit. Further, existing ventilation is not sufficient to ensure that the space is free of a hazardous atmosphere, oxygen deficiency, or other potential hazards.

Working in confined spaces can present some conditions that can be immediately dangerous to life or health (IDLH).  These dangerous conditions can be engulfment, atmospheric, and/or other hazards.


Engulfment can happen when there is a liquid or a finely divided (flowable) solid substance in a confined space.  Engulfment occurs when there is an effective capture of a person by a liquid or flowable solid that can be aspirated to cause death by filling or plugging the respiratory system or that can exert enough force on the body to cause death by strangulation, constriction, or crushing.

Liquid may be hazardous because if it rises too fast, the occupant may not be able to get out in time.  As for flowable solids, there may be air pockets below the surface that could give way when an occupant steps on it.  If the material traps you below the surface, the occupant can suffocate due to the weight of the material or by the displacement of oxygen.  Flowable solids may spill into a confined space (i.e. working in trenches) and bury an occupant.  Solids, such as soil, can also be fluidized by water or vibration.  OSHA defines a trench as a permit-required confined space (more information below) if it is at least four feet deep.

Atmospheric Hazards

Oxygen deficiency and enrichment

Oxygen deficiency is the most common atmospheric hazard.  It is generally accepted that anything less than 19.5% oxygen is deemed to be an oxygen deficient environment.  Symptoms of oxygen deficiency can include: decreased coordination, impaired attention and thought process, abnormal fatigue, emotional upset, tunnel vision, nausea, vomiting, unconsciousness and death.  

One way of depleting oxygen in a confined space is by purging chemicals, gases, or vapors (usually with nitrogen) and not ventilating it.  Confined space work such as welding and burning can also deplete oxygen levels.  There are asphyxiating gases such as acetylene, argon, helium, hydrogen, propane and propylene that can displace oxygen.  Although rare, the biological production of methane from sewers and manholes can also potentially cause oxygen deficiency.  Finally, chemical reactions such as rusting, decaying, fermentation or even the drying of certain paints and cements can remove oxygen and cause a confined space to become oxygen deficient.

A confined space can also have elevated oxygen levels (i.e. a leaking welding torch).  Oxygen enrichment is generally defined as an atmosphere greater than 23.5% and can cause symptoms such as respiratory irritation, throat irritation, tracheal irritation, tingling of fingers and toes, visual impairment, hallucinations and other nervous system disorders.  

When the atmosphere is oxygen enriched, the risk of fire or explosion is increased.  Flammable materials ignite more easily and burn more violently in oxygen enriched atmospheres.

Flammable and Combustible Gases and Vapors

Confined spaces may include flammable gases and vapors in the atmosphere such as: methane, hydrogen, acetylene, propane and gasoline fumes.  Some possible sources of flammable content in confined spaces may stem from coatings on the internal surfaces, residue from incomplete cleaning, storage or processing.

Airborne Combustible Dusts

Any combustible material can burn rapidly when it is in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosive.  Coal dust from a pulverized coal-fired industrial boiler is an example of a combustible dust that is used to generate thermal heat.  Metal dusts, organic material (i.e. flour) and other materials that can be oxidized can also be combustible dusts.  Often, primary combustible dust explosions can dislodge settled combustible dust on horizontal surfaces and cause a much larger and more destructive secondary explosions.

Toxic Materials

There is also the possibility of exposure to toxins within confined spaces.  In terms of toxins, the differences between work in a confined space and work in an open area lie primarily in reduced air movement, and increased chances of excessive skin contact with agents affecting the skin.  The reduced air movement in most confined spaces results in increased airborne concentrations of volatile compounds which may out-gas from the inner surfaces of the space.

•    Methane- displaces oxygen and is flammable.  Methane occurs in natural gas and can be produced by decomposition of organic matter.  It can be used in the production of ammonia and other chemicals.
•    Hydrogen Sulfide- is a toxic and flammable gas.  It is a by-product of many industrial processes, sewers, and collected garbage.  It can be found where petroleum products are processed, stored and used.  It also occurs in coal, natural gas, oil and decaying organic matter.
•    Acetylene- displaces oxygen and is flammable.  It can be used for welding, cutting metals and in the production of vinyl chloride and other chemicals.
•    Carbon Monoxide- blocks the ability of the blood to carry oxygen.  It is a by-product of combustion.
•    Gasoline- is toxic and flammable.  Gasoline and related hydrocarbons are found in refinery work, and can also drift/leak into below ground confined spaces.
•    Solvents- are both flammable and toxic most of the time.  Common solvents include Stoddard solvent (mineral spirits), methylene chloride, toluene, methyl ethylene chloride (MEK) and thinners (for paint, varnish, lacquer).

Other Hazards

Some examples of other hazards in confined spaces may include: mechanical hazards from mixers and crushers; trench wall collapses caused by heavy equipment; bacteria and viruses in industrial sewers and waste water treatment facilities; falling objects into a top-hatch opening space; uncomfortable temperatures (heat can lead to fatigue and cold can affect musuclar coordination); or release of hazardous energy such as electricity when working in trenches or electrical vaults.

Level of Risk:

Compared to working in an open environment, working in confined spaces could be more dangerous given that there is restricted access and egress as well as diminished natural ventilation.  Because of these characteristics, work in confined spaces often increases risk of overexposure to atmospheric hazards.  Furthermore, work in confined spaces may be associated with physical hazards which are difficult to escape due to restricted egress.

Harris, MK.  Chapter 46: Confined Space Entry.  The Occupational Environment: Its Evaluation , Control, and Management 3rd Edition Volume 2.  2011.

Assessment Info:

Prior to entry into a confined space, it is important to determine if it is a "permit-required" one by OSHA definition:

  1. Does it have limited or restricted openings for entry and exit?
  2. Is it not meant for continuous human occupancy?
  3. Is it large enough and shaped so that a worker can get inside the space and perform work?

and at least one of the following:

  • Hazardous atmosphere (or potential)
  • Material that can engulf a person
  • A shape that can trap or engulf someone
  • Any other recognized serious safety or health hazard, including any other hazard that will prevent a worker from self-rescue.

Atmospheric Assessment

For a confined space with a hazardous atmosphere (or potential), an air quality assessment must be performed for the possible hazard(s).  At a minimum, oxygen levels, explosive gases and toxic gases (i.e. carbon monoxide and hydrogen sulfide) must be monitored.  A combustible gas detector, also known as an explosimeter, can be used to measure combustible gas levels in a confined space.

Regulations & Standards:

OSHA regulates confined spaces in non-construction settings under the General Industry Standard 29 CFR 1910.146.  Construction contractors must follow most of the requirements in this standard.  An employer must provide a workplace free of recognized hazards. The same level of protection given to workers in a general industry setting must also be given to worker at a construction site.

OSHA considers routine preventive maintenance in confined spaces to fall under the General Industry Standard.  Some work that is covered under the General Industry standard includes:

  • relining a furnace with refractory bricks
  • tuck pointing and individual brick replacement in a manhole
  • repainting to maintain a system or prevent its deterioration
  • relining of a sewer line using a sleeve pushed through a section of the existing section

OSHA considers new construction to fall under the Construction Standard which includes:

  • reconfiguring a confined space
  • installing new equipment

Here, OSHA regulates confined spaces in construction under the following standards:

1926.21(b)(6)(i) - Requires the employer to protect workers from recognized hazards in confined spaces.  This protection would be assured if a construction contractor followed all provisions of the General Industry Standard.

1926.21(b)(6)(1) - Requires training for employees who enter confined or enclosed spaces; requires employer to protect employees from recognized hazards in confined spaces.

1926.21(b)(6)(ii) - Defines the confined and enclosed places which require training.

1926.352(g) - Fire prevention methods associated with use of fuel gas and oxygen in confined spaces.

1926.353(b)(1) - Requirement for exhaust ventilation when welding, cutting, or heating in confined spaces.

1926.353(b) - Requires air line respirators and a standby person whenever the means of access is blocked by ventilation equipment.

An OSHA compliance officer cannot cite the construction contractor under the General Industry Standard.  However, if there is a recognized hazard that the contractor fails to eliminate or control, OSHA may cite the contractor under the Construction Standards or under the General Duty Clause.

Federal OSHA Standards are enforced by the U.S. Department of Labor in 26 states. There are currently 22 states and jurisdictions operating complete State plans (covering both the private sector and state and local government employees) and 5 - Connecticut, Illinois, New Jersey, New York and the Virgin Islands - which cover public employees only. If you are working in one of those states or jurisdictions you should ensure that you are complying with their requirements.