What is a confined space?

Generally speaking, a confined space is an enclosed or partially enclosed space that:

  1. is not primarily designed or intended for human occupancy
  2. has a restricted entrance or exit by way of location, size or means
  3. Can represent a risk for the for the health and safety of anyone who enters, due to one or more of the following factors:
  • its design, construction, location or atmosphere
  • the materials or substances in it
  • work activities being carried out in it, or the
  • mechanical, process and safety hazards present

Confined spaces can be below or above ground. Confined spaces can be found in almost any workplace. A confined space, despite its name, is not necessarily small. Examples of confined spaces include silos, vats, hoppers, utility vaults, tanks, sewers, pipes, access shafts, truck or rail tank cars, aircraft wings, boilers, manholes, manure pits and storage bins. Ditches and trenches may also be a confined space when access or egress is limited.

Hazards In Confined Spaces

There are two primary hazards within confined spaces: atmospheric hazards and physical hazards.

Atmospheric refers to problems with the air in a space, while physical

refers to problems caused by equipment or by other dangerous conditions.

It is critical to identify all the hazards in a confined space and determine how they impact the health and safety of workers entering them.

Atmospheric Hazards

The inefficient or nonexistent ventilation of a confined space can cause the atmosphere in the space to become life threatening. Processes of biological activity, decomposition of natural materials, oxidation, percolation of vapors and structural leaks can cause the production and accumulation of toxic and/or flammable gases. Available oxygen levels may become seriously depleted or displaced through these processes. When the atmosphere becomes contaminated with harmful gases or depleted of oxygen, the exposed worker may not immediately feel the effects. A false feeling of euphoria or well being is a common side effect to such exposure. A number of these toxic gases have no odor or color detectable by the body’s senses. Many who die in confined space accidents simply slip into unconsciousness quietly, never realizing what is happening and never to reawaken.

One cardinal rule prevails in working in confined spaces: Never trust your senses. Only through using appropriate monitoring instruments capable of analyzing atmospheric gases can employees verify that the space is safe to enter.

The following section discusses the harmful effects of exposure to varying toxic gases and oxygen levels commonly found in confined space atmospheres.

Oxygen Depletion

Life ceases quickly without enough oxygen. Common sources of oxygen depletion in confined spaces include aerobic bacterial growth, oxidation/rusting of metals, combustion, and displacement by other gases. Oxygen comprises only a small percentage (20.9 percent) of the air we breathe. When levels of oxygen are reduced below 19.5 percent (the minimum acceptable level), serious health problems begin to occur very quickly. The following provides an overview of those effects at various oxygen levels:

  • 20.9-23.5 percent: Maximum permissible oxygen level. No effect.
  • 20.9 percent: Percentage of oxygen found in normal air. No effect.
  • 19.5 percent: Minimum permissible oxygen level. No effect.
  • 15-19 percent: Decreased ability to work strenuously. May impair coordination and may induce early symptoms with individuals that have coronary, pulmonary, or circulatory problems.
  • 12-15 percent: Respiration and pulse increase; impaired coordination, perception, and judgment occurs.
  • 10-12 percent: Respiration further increases in rate and depth; poor judgment and bluish lips occur.
  • 8-10 percent: Symptoms include mental failure, fainting, unconsciousness, an ash-colored-face, blue lips, nausea, and vomiting.
  • 6-8 percent: 8 minutes – 100 percent fatal; 6 minutes – 50 percent fatal; 4-5 minutes – recovery with treatment.
  • 4-6 percent: Coma in 40 seconds, convulsions, respiration ceases – death.

Oxygen within a space may be depleted through the displacement of other gasses. Some gases are heavier than air and move downward; others, being lighter than air travel upward, displacing the available oxygen as they fill the space.

Toxic Gases

There are many different types of toxic gases that can be found in confined spaces. Their sources and physical characteristics vary, but they all share one common thread – potential harm to individuals who enter a hazardous atmosphere in enclosed areas.

There are two categories of toxic gases: irritants and asphyxiates.


Many gases existing in low concentrations in the air are irritating to the body’s respiratory and nervous systems. When inhaled, they cause the mucous linings of the lungs and sinuses to swell, sometimes so severely that the respiratory tract closes, causing strangulation. Except under extreme conditions, the body normally recovers after exposure to toxic gases has been stopped. In higher concentrations, irritants can become asphyxiating gases.


An asphyxiate is any gas that, when present in a high enough concentration, causes displacement of oxygen in the body.

Carbon monoxide is one of the most common asphyxiates. Produced by the incomplete combustion of carbon fuels, carbon monoxide kills by chemically combining with the hemoglobin in red blood cells. This greatly reduces the ability of the blood to carry oxygen to body tissues and brain cells.

35 ppm:Permissible Exposure Limit during an eight-hour shift.
500 ppm:Slight headache.
1000 ppm:Confusion, nausea, discomfort.
2000 ppm:Tendency to stagger.
2500 ppm:Unconsciousness after a 30-minute exposure.
4000 ppm:Fatal in less than one hour.

Hydrogen sulfide is even more toxic than carbon monoxide. It is produced through the decay of organisms and natural materials.

This colorless gas has a characteristic rotten egg odor at first; however, within a short time the gas paralyzes the olfactory nerve, which controls the sense of smell. A worker may be lulled into a false sense of security because he/she no longer smells the substance, yet it is causing serious bodily harm (higher concentrations).

10 ppm:Permissible Exposure Limit during an eight-hour period.
50-100 ppm:Mild eye and respiratory irritation.
200-300 ppm:Marked increase in eye and lung irritation.
500-700 ppm:Unconsciousness or death after a 30-minute exposure.
1000 or more ppm:Death within minutes.

Flammable Gases

Many of the gases routinely found in confined spaces are flammable or combustible under the right combination of conditions. These gases include hydrogen sulfide, carbon monoxide, acetylene, and methane. When fuel, oxygen, and an ignition source are present at the same time and in the correct proportions, a serious explosion or fire can result. If a combustible gas and air are trapped in a confined space, only a source of ignition is needed to cause an explosion. Welding, sparking tools, smoking, or static electricity can easily provide an ignition source.

lowest concentration (air-fuel mixture) at which a gas can ignite is called its Lower Explosive Limit (LEL). Concentrations below this level are too lean to burn. The highest concentration that can be ignited is its Upper Explosive Limit (UEL). Above that concentration, the mixture is too rich to burn. A gas is flammable in concentrations between its LEL and UEL.

instruments analyze air samples and alarm at a predetermined concentration level, usually 10 percent of the LEL. The worker is then provided with advance warning concerning the potential hazard. Any concentration of flammable gas is reason for concern in a confined space.

Overly rich mixtures can collect in an area and reach combustible concentrations when fresh air is introduced, quickly changing its proportions to levels between the LEL and UEL. Confined space atmospheres containing an enriched oxygen level above 23.5 percent increase the flammability ranges of many gases as well as support violent reactions when ignition occurs. Oils and grease may unexpectedly burst into flames under such atmospheric conditions.

Physical Hazards

Employees working in confined spaces may face the following physical hazards:

  • The possibility of drowning or being trapped by flooding water while working in a storm sewer or wet-well.
  • Pipes, valves, and lines carrying harmful substances such as steam, natural gas, and electricity that can rupture while being worked on or activated if not locked out.
  • Loud noises reverberating from the use of hammers or hydraulic equipment.
  • Exposure to temperature extremes during work activities.
  • Slips and falls on wet or damaged walking or climbing surfaces.
  • Exposure to corrosive substances that could irritate unprotected skin.
  • Exposure to rodents, vermin, and other pests living in the area.
  • Poor or inadequate lighting.
  • Accidental activation of hazardous equipment while it is being repaired.

Moving equipment or parts and energized or pressured systems can be dangerous as well. Examples include shafts, belts, conveyors, mixers, rotors, and compressing devices. Entrapment hazards in a confined space include inwardly converging walls or floors that slope downward and taper to a smaller cross-section, such as air plenums. An engulfment hazard is any liquid or loose, finely divided solid material such as sand or grain that could bury, surround, suffocate, or drown an entrant.

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