MRO Magazine

Beware the hidden dangers of confined spaces


June 14, 2001
By PEM Magazine

Most dangers are easy to spot in the plant or industrial environment. But, some dangers by their very nature are not readily visible. Recognition of hidden dangers is a matter of life and death. There are hidden dangers confronting the maintenance department; these mainly occur in confined space environments such as a vessel, boiler, machine pit, machine housing, vault or storage area, utility tunnel or deck. Confined spaces are defined by their small space with a restrictive entry/exit area: locations which are not designed for continued occupancy.

Confined spaces produce atmospheric hazards such as: oxygen deficiency, formation of combustible gas, and formation of toxic gas.

Oxygen deficiency is arguably the most common problem with confined spaces. Ambient breathable air contains 20.9 percent oxygen by volume, and even a small deviation in the level can cause serious concern for the maintainer. Too high a level of oxygen (23.5 percent and above) constitutes a serious fire risk, as oxygen enriched environments are literally explosive charges waiting for an ignition source. Too little oxygen poses an asphyxiation risk for the space occupant. The chart to the right depicts the human effects of various oxygen levels.

Oxygen levels below the 19.5 percent safe limit — minimum safe level as determined by the U.S. National Institute of Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Association (OSHA) — are mainly caused by oxygen displacement from other gases, and oxygen consumption. When maintenance activities such as cutting and welding are undertaken in a confined space, they will create gaseous fumes, which will displace valuable oxygen the maintainer is struggling to consume.


Metal oxidization (rusting) and organic decomposition are recognized as large consumers of available oxygen.

Combustible gases are formed when oxygen combines with a gas such as propane or methane, which results in a highly flammable atmospheric hazard. Similar to an oxygen-enriched atmosphere, the gas becomes a "bomb" awaiting only an ignition source such as a spark caused by a tool striking metal or open flame from a lit cigarette (a whole other safety issue iteself).

Toxic gases can kill very quickly. Two common toxic gases that are found in industrial confined spaces are carbon monoxide and hydrogen sulfide.

Carbon monoxide (CO) is a combustion by-product and is known as the "silent killer" because it is both odorless and tasteless. Hydrogen sulfide (H2S) is formed by the bacterial decomposition of organic plant and animal life and is only evident as a rotten egg smell in small concentrations. In highly dangerous large concentrations, no smell is evident. Both toxic gases build up in the maintainer’s bloodstream; carbon dioxide replaces oxygen in the bloodstream causing asphyxiation while the combustible hydrogen sulfide will paralyze the brain nerve centers that control the maintainer’s breathing response. Other dangerous toxic gases found in the industrial environment are chlorine, sulfur dioxide, and nitrogen dioxide.

Most often when confined space accidents occur more than one person is injured; the first maintainer experiences breathing problems and a second person tries to rescue the first person, only to experience the same problem themselves. A third person usually recognizes the magnitude of the problem and raises the alarm. In many cases the first two people lose their lives unnecessarily.

Overcoming confined space problems require the use of gas analyzers or "sniffers", as they are commonly known. Gas analyzers are atmospheric monitoring devices that can be sold to detect and monitor for ambient concentrations of single gases or multi gases. The units are usually battery operated and require calibration for continued effective use. Typically a maintainer would sample the air quality before entering the confined space and continue to monitor the air quality throughout the maintenance assignment. Before entering the confined space, the maintainer should always assess the oxygen level readings. For other gases, readings must be taken at different height levels within the confined area. Some gases, such as propane, are lighter than air and require a reading to be taken at the ceiling level of the room or vessel. Hydrogen sulfide is heavier than air and resides at the bottom of the areas, while carbon monoxide is slightly lighter than air and will usually at a mid-level height within the space or area. Any "above or below" normal atmospheric readings must be captured on the work order closing comments and reported to the safety officer. If levels are unsafe, don’t enter.

The other unspoken hidden danger in the plant maintenance environment is complacency. A complacent attitude toward work can result in disaster. Being safe is a direct result of knowledge, a good work ethic and common sense.

Ken Bannister is a principal management consultant with Engech Industries in Cambridge, Ontario. You can reach Ken at 519-662-4211 or by e-mail at