Stay On Your Toes
Monitor throughout all confined space operations because conditions may change.
- By Keshwar Anroedh
- Feb 01, 2009
Confined spaces represent a major health and safety risk for many employees. Recognizing and planning appropriately for confined space work can mean the difference between a job well done and disaster.
A confined space is defined as an area that is large enough for an employee to bodily enter and perform work, has limited or restricted means of entry or exit, and is not designed for continuous human occupancy. Some examples include storage tankers, sewers, boilers, manholes, ship voids, tunnels, pipelines, silos, trenches, pits, vats, and wells.
Careful planning and preparation of all personnel involved with the confined space entry should be completed before anyone enters work areas that may contain a variety of hazards. A hazardous atmosphere is one that exposes workers to the risk of death, incapacitation, injury, or acute illness; any immediately dangerous to life or health (IDLH) atmosphere that poses an immediate threat of loss of life; or an atmosphere that may result in irreversible or immediate, severe health effects, or may result in eye damage, irritation, or other conditions that could impair escape.
Atmospheric hazards are some of the more dangerous yet frequently unnoticeable hazards found in a confined space. While airborne dust or particle concentrations may be easy to spot with the naked eye, oxygen deficiency/enrichment conditions, as well as hazardous concentrations of vapors or gases, must be detected with reliable instrumentation.
Oxygen Deficiency and Enrichment
Normal ambient air contains an oxygen concentration of 20.8 percent by volume and a carbon dioxide concentration of 0.04 percent by volume. When a confined space’s oxygen level drops below 19.5 percent of the total atmosphere, the area is considered to be oxygen deficient. In oxygen- deficient atmospheres, life-supporting oxygen may be displaced by other gases, such as an increased level of carbon dioxide, which can result in an atmosphere that can be dangerous or fatal when inhaled. Oxygen deficiency also may be caused by rust, corrosion, fermentation, or other forms of oxidation that consume oxygen.
Oxygen deficiency’s impact can be gradual or sudden, depending on the overall oxygen concentration, the entrants’ activity levels within the confined space, and concentration levels of atmospheric gases present. Decreasing levels of atmospheric oxygen typically cause symptoms that include increased breathing rate, accelerated heart rate, impaired attention and coordination, poor muscular coordination, rapid fatigue, nausea, vomiting, unconsciousness, convulsions, and death within minutes.
When the oxygen concentration rises above 21 percent by volume, the atmosphere is considered to be oxygen enriched and is prone to instability. Rising oxygen levels significantly increase the likelihood and severity of a flash fire or explosion. OSHA’s oxygen enrichment alarm level is 23 percent.
Combustible and Toxic Gases
Four elements must be present for combustion to occur: fuel, oxygen, heat or a source of ignition, and a chemical chain reaction. These are known together as the fire tetrahedron (see diagram).
If any one element does not occur, neither will combustion. The percentage of combustible gas present in the air is important; for example, a manhole filled with fresh air may gradually fill with combustible gas because of a natural gas leak. The gas to- air ratio changes, and the sample passes through three ranges: lean, explosive, and rich. The lean range doesn’t provide enough gas in the air to burn. The rich range has too much gas and not enough air. The explosive range has just the right combination of gas and air to form an explosive mixture.
Toxic gas within a confined space is defined as an atmospheric concentration of any toxic contaminant above the OSHA permissible exposure limit (PEL). Toxic gas exposure within confined spaces may produce physiological effects that can vary according to the health or activity of exposed individuals, as well as because of present gas concentrations.
Carbon monoxide (CO) is a colorless, odorless gas often released by accident or improper maintenance, burner/flue adjustments in confined spaces, and by internal combustion engines. Called the “silent killer,” CO poisoning can occur suddenly. Physiological effects of exposure include heart pounding, dull headache, dizziness, eye flashes, ear ringing, nausea, rapid collapse, unconsciousness, and possibly death within minutes.
Hydrogen sulfide (H2S), a colorless gas, smells like rotten eggs, but the odor cannot be taken as warning because smell sensitivity disappears quickly after breathing just a small gas quantity.H2S is often found in sewers, sewage treatment facilities, and petrochemical operations; it is flammable and explosive in high concentrations. Sudden poisoning may cause unconsciousness, respiratory arrest, nausea, eye irritation, belching, coughing, headache, and lip blistering.
Determining Atmospheric Composition Hazards
To determine a confined space’s atmospheric composition, reliable instruments should be used to draw air samples through a weep hole or other small entry port leading into the confined space. If possible, do not open the confined space’s entry portal before completing this step. Sudden changes in confined space atmospheric composition could cause violent reactions or dilute the confined space contaminants, giving a false low initial reading of the gas concentration. When testing for acceptable entry conditions, always test oxygen content first, then flammable gases, then vapors, and potential toxic air contaminants last.
Comprehensive testing should be conducted in various locations within the work area because some gases are heavier than air and tend to collect at the bottom of confined spaces. Others are lighter and are usually in higher concentrations near the top of confined spaces. Still others are the same molecular weight as air and can be found in varying concentrations throughout confined spaces. Test samples should be drawn at the top, middle, and bottom of the space to pinpoint varying concentrations of gases or vapors.
Atmospheric testing results directly affect appropriate protective equipment selection for tasks to be performed within confined areas. Results also may dictate the duration of workers’ exposure or whether an entry should be made at all. Substance-specific detectors should be used whenever actual contaminants have been identified; assume that every confined space has an unknown, hazardous atmosphere. Under no circumstances should anyone ever enter or even stick his or her head into confined spaces for a quick look. Such an action constitutes confined space entry and can expose entrants to hazardous and possibly deadly atmospheres.
Beyond Atmospheric Hazards
Physical hazards should be assessed after confined space atmospheric hazards have been identified: grinding equipment, agitators, steam or steam fittings, mulching equipment, drive shafts, gears, and other moving parts can pose dangers in confined spaces. Hazards such as pipe fittings and uneven or wet surfaces also may pose slip, trip, and fall hazards.
Engulfment hazards frequently exist in areas where loose materials such as grains, crushed stone, flour, or sawdust are stored. Often housed in silos or other containment equipment, these materials can harbor air pockets that can collapse under an employee’s weight. Engulfment hazards either block the employee’s airway or compress his/her upper body to the point of suffocation.
Corrosive hazards typically concern chemicals such as acids, solvents, and cleaning solutions. Contact between these substances and skin, mucous membranes, or eyes can cause serious irritation or burns. Fumes given off by these materials also can irritate the respiratory system and can cause gastrointestinal distress.
Biological hazards such as molds, mildews, and spores frequently found in dark, damp spaces can irritate the respiratory system. Bacteria and viruses, found in applications such as sewage treatment, can threaten the body with a variety of diseases. In addition, bird and animal feces can present serious health hazards to humans.
Other hazards, such as poor visibility, inadequate lighting, and insecure footing, can cause significant safety hazards in confined spaces. Confined spaces may even harbor rodents, snakes, spiders, or insects. Finally, sudden changes in wind or weather and stirring up still water can contribute to unexpected variations in confined space environments.
Portable Gas Detection Instruments
Battery-powered, direct-reading portable gas detection instruments are practical devices for conducting confined space atmosphere spot checks after existing hazards have been assessed. These monitoring devices are classified into two groups: single-gas and multi-gas instruments. They typically monitor one or several atmospheric conditions, including oxygen deficiency or enrichment, combustible gas, and/or certain toxic gases.
Regular monitoring should be performed during all confined space operations because a contaminant’s level of combustibility or toxicity might increase, even if readings initially appear to be low or nonexistent. In addition, oxygen deficiency can occur unexpectedly.
For full compliance with the Occupational Safety and Health Administration (OSHA) standard governing confined spaces, 29 CFR 1910.146, it is necessary to rely on the expertise of safety and health professionals, such as industrial hygienists. For more complete information, please refer to the following publications:
1. Permit-Required Confined Spaces, Final Rule; OSHA, 29 CFR Part 1910.146; Federal Register 66038 (Dec. 1, 1998).
2. A Guide to Safety in Confined Spaces, DHHS (NIOSH Publication Number 87- 113), July 1987.
3. Working in Confined Spaces, (NIOSH Publication Number 80-106), December 1979.
4.ALERT: Request for Assistance in Preventing Occupational Fatalities in Confined Spaces (NIOSH Publication Number 86- 100), January 1986.
5. Safety Requirements for Confined Spaces,American National Standards Institute, ANSI/ASSE Z117.1-2003.
This article originally appeared in the February 2009 issue of Occupational Health & Safety.