Confined space is a term from labor-safety regulations that refers to an area which is enclosed with limited access which make it dangerous. An example is the interior of a storage tank, which workers may enter for maintenance but which is not ordinarily a habitable space. Hazards in a confined space often include suffocation by unbreathable gases which may be present but not visible, or submersion in liquids or free-flowing granular solids (for example, grain bins).
Confined space accidents are of particular concern in occupational safety and health because often multiple casualties occur when untrained rescuers succumb to the same hazard as the initial victim. Confined space training outlines the skills and protocols for safe entry to confined spaces, and includes such precautions as lockout and tagout of any connecting piping, testing of breathable air quality, forced ventilation, observation of workers in the space, and a predetermined rescue plan with appropriate safety harness and other rescue equipment.
Although the definition of a confined space varies between jurisdictions, it is generally recognized as a space that:
- has limited or restricted means of entry or exit;
- is large enough for a person to enter to perform tasks;
- is not designed or configured for continuous occupancy; and
- has the potential for a significant hazard to be present.
A utility tunnel, the inside of a boiler (only accessible when the boiler is off), the inside of a fluid storage tank, a septic tank that has contained sewage, and a small underground electrical vault are all examples of confined spaces. Ships and other vessels commonly have confined spaces due to the need for compartmentalized watertight construction. The exact definition of a confined space varies depending on the type of industry. That is, confined spaces on a construction site are defined differently than confined spaces in a paper mill. Confined spaces that present special hazards to workers, including risks of toxic or asphyxiant gas accumulation, fires, falls, flooding, and entrapment may be classified as permit-required confined spaces depending on the nature and severity of the hazard.
Even normally habitable parts of a building such as corridors or offices may take on the characteristics of a confined space, during operations that alter normal ventilation and access. For example, a room may be wrapped in plastic sheeting for painting and any vapor emitted in the room may not be dispersed by blocked ventilation ducts.
In the U.S., entry into permit-required confined spaces must comply with regulations promulgated by the Occupational Safety and Health Administration (OSHA). These regulations include developing a written program, issuing entry permits, assigning attendant(s), designating entrants, and ensuring a means of rescue.
According to the OSHA, a permit-required confined space (permit space) has the three characteristics listed above (which define a confined space) and one or more of the following:
- Contains or has the potential to contain a hazardous atmosphere
- Contains a material that has the potential for engulfing the entrant
- Has an internal configuration that might cause an entrant to be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross section
- Contains any other recognized serious safety or health hazards.
In addition to the hazards posed by the design of the space, work activities can also pose serious safety hazards (heat, noise, vapors, etc.) that must be taken into account when identifying safety measures that must be taken.
Atmospheric hazards 
The most common hazard seen in confined spaces is that of atmospheric hazards. These affect air quality and present immediate hazards to health or life. Acceptable atmospheric conditions must be verified before entry, and must be monitored continuously while the space is occupied. The oxygen concentration, the presence of toxic gases, and flammable material are the three conditions that must be monitored.
Oxygen concentration is considered safe if it is between 19.5% and 23.5% of the total atmosphere. To protect against toxic gases, contaminants have permissible exposure limits (PELs), which are set by OSHA. Work also cannot continue if the concentration of a material reaches or exceeds 10% of its lower explosive limit.
Even if a tank or similar vessel initially is tested and found to contain breathable air, a hazard can develop during operations inside the tank if residues inside the tank can release toxic gas or vapor when disturbed or if accidentally ignited. Steel water tanks may have dangerously low oxygen concentration when the interior rusts.
Entry certification 
In many situations, certification of non-hazardous atmosphere by a trained or competent person is required before personnel may enter a confined space without the use of a respirator. In the United States Navy, that person is the designated shipboard gas-free engineer. Certification in civilian settings can be performed by an Entry Supervisor who, under OSHA regulations, is designated by the employer and ensures that the space is safe to enter and all hazards are controlled.
In the United States, agricultural and construction operations are exempted from regulations governing permit-required confined spaces (which is specific to general industry), but they are still required to identify and control confined space hazards.
Injuries and fatalities 
Injuries and fatalities involving confined spaces are frequent and often involve successive fatalities when would-be rescuers succumb to the same problem as the initial victim. Approximately 60% of fatalities involve would-be rescuers and more than 30% of fatalities occur in a space that has been tested and found to be safe to enter. One example was in 2006 at the decommissioned Sullivan Mine in British Columbia, Canada when one initial victim and then three rescuers all died.
Accidents in confined spaces present unique challenges and are often catastrophic, such as the Xcel Energy Cabin Creek Fire in 2007.
In 1999, North West OHS released a study of confined space fatalities based on reports from the Occupational Safety and Health Administration (OSHA), National Institute of Occupational Safety Health (NIOSH) and the Mines Safety and Health Administration (MHSA) with a breakdown of their causes. Researchers believe that the following numbers are only a fraction of the fatal confined space accidents that actually occurred as many locations are not initially identified as confined spaces, OSHA reports did not include non-hazardous confined space fatalities in their studies prior to 1982, NIOSH still do not include non-hazardous confined space fatalities in their studies and many American states do not note the presence of confined spaces in fatality reports submitted to OSHA.
- Fire and Explosion (OSHA 1982a): 50 confined space incidents from 1974 to 1979 with 76 fatalities.
The majority of incidents were caused by worker error or faulty equipment.
- Lockout-tagout (OSHA 1982b): 83 confined space incidents from 1974 to 1980 with 83 fatalities.
This category covers conveyor belts and machinery on the factory floor etc. that are not generally considered confined spaces, but which satisfy the criteria for a confined space.
- Grain Handling (OSHA 1983): 105 confined space incidents from 1977 to 1981 with 126 fatalities.
- Toxic and Asphyxiating Atmospheres (OSHA 1985): 122 confined space incidents from 1974 to 1982 with 173 fatalities.
- Welding and Cutting (OSHA 1988): 217 incidents from 1974 to 1985 with 262 fatalities.
OSHA reports of welding and cutting deaths do not record whether or not an incident has occurred in a confined space, it is estimated that 22% of the incidents were in a confined space.
- Shipbuilding & Repair (OSHA 1990): 151 incidents from 1974 to 1984 with 176 fatalities.
OSHA reports of shipbuilding deaths do not record whether or not an incident has occurred in a confined space, it is estimated that 36% of the incidents were in a confined space.
- Mining (MSHA Report 1988): 38 confined space incidents from 1980 to 1986 with 44 fatalities.
According to data collected by the U.S. Department of Labor, Bureau of Labor Statistics, Census of Fatal Occupational Injuries program, fatal injuries in confined spaces fluctuated from a low of 81 in 1998 to a high of 100 in 2000 during the five-year period, averaging 92 fatalities per year.
Where a system of entry permits is in place, a rescue plan is required. It will list the personnel and equipment required to be at the worksite before entry is allowed. Special equipment such as tripod hoists, harnesses, and others may be required to extricate a worker from a toxic environment, without unduly endangering rescue personnel.
See also 
Notes and references 
- BP International Ltd. Confined Space Entry, Institution of Chemical Engineers 2005 ISBN 978 0 85295 479 9 pp. 1-14
- "Permit-Required Confined Space Entry". Occupational Health & Safety Administration. Retrieved 2009-07-17.
- Entering enclosed spaces on a ship at Marine Insight
- "Confined Space Hazards". Confined Space Safety Training. Retrieved 2012-06-17.
- Trevor Kletz What Went Wrong:Case Histories of Process Plant Disasters, Fourth Edition, Elsevier, 1999 ISBN- 13: 978-0-884 15-920-9 Chapter 11 "Entry into Vessels"
- Confined space fatalities Ciaran MacCarron Edith Cowan University 2006
- "NIOSH - Confined Spaces". United States National Institute for Occupational Safety and Health. Retrieved 2007-10-14.
- United States Occupational Safety and Health Administration (OSHA) page on confined spaces
- California Occupational Safety and Health Administration (Cal/OSHA) Confined Space Guide: "Is It Safe To Enter A Confined Space?"
- Canadian Confined Space Rescue and Training Facilities
- National Institute for Occupational Safety and Health - Confined Space Page