Confined spaces present a number of hazards. The most common is atmospheric hazards. Atmospheric hazards include a lack of oxygen, an oxygen-enriched environment, explosive levels of vapors, and toxic air contaminants.
Oxygen
An oxygen-deficient atmosphere is one in which there is less than 19.5 percent oxygen available. Low oxygen levels in a confined space can be caused by a number of conditions, including chemical reactions in sewage or other decomposing organic matter (for example, plants and leaves) or oxidation of metals (i.e., rusting). An oxygen-deficient confined space should never be entered without a respirator that supplies air to the user.
An oxygen-enriched atmosphere is one in which there is more than 23.5 percent oxygen. An oxygen-enriched environment is dangerous because it will cause flammable and combustible materials, such as clothing and hair, to burn violently when ignited. For this reason, pure oxygen should never be used to ventilate a confined space; a combined space should be ventilated with ambient air.
Flammable atmospheres
Flammable atmospheres develop as the result of a buildup of flammable vapors in the confined space. Methane, a product of certain types of decomposition, is a common flammable vapor found in confined spaces. Even when a flammable vapor is present, it must be in a range of concentrations that will allow it to ignite in order for it to be a hazard. Below a certain chemical/oxygen mix, there will be no ignition. This is known as the lower explosive limit (LEL). Conversely, above a certain chemical/oxygen mix, there is too much of a chemical vapor and too little oxygen for ignition to occur; this is called the upper explosive limit (UEL). Between the LEL and UEL, there is a hazard of ignition if a source, such as a spark or flame, is present a fire or explosion can occur. According to OSHA, a confined space is unsafe if a flammable gas is present at a level above 10% of the LEL. Workers may not enter a confined space under this condition. It must be ventilated to safe levels prior to worker entry.
Toxic gases
Toxic gases and vapors come from a wide variety of sources. For example, hydrogen sulfide and carbon monoxide are produced by sewage and rotting organic materials. Other sources of toxic atmospheres include spills or dumping of chemicals in the confined space. If toxic substances exceed OSHA regulatory limits, then the confined space needs to be appropriately ventilated or the workers need to wear appropriate respiratory protection.
Required monitoring
Air monitoring must be performed before entering any permit-required confined space. At a minimum, oxygen and the LEL must be monitored.
Oxygen-Oxygen must be tested prior to testing for the LEL. Most combustible gas meters require sufficient oxygen be present in order to obtain a reliable LEL reading; they will not provide reliable LEL readings in an oxygen-deficient environment. As noted above, oxygen levels should be between 19.5 and 23.5 percent.
LEL-Once oxygen levels have been tested, testing needs to be completed for flammable gas and vapor levels. As noted, if readings indicate levels that are 10% or more of the LEL, the confined space may not be entered until it has been ventilated to reduce the LEL level.
Toxic atmospheres-If there are toxic chemicals that may be present in the space, such as carbon monoxide, hydrogen sulfide, and chlorine, testing needs to be done to ensure that the atmosphere is not hazardous. If contaminants exceed safe levels, either the space must be ventilated to reduce the level of the hazardous contaminant or workers must wear appropriate respiratory protection.
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