Hazardous Atmospheres in Confined Spaces
Entrants are in the most danger because of the many, often invisible, hazards associated with permit space environments. In order to properly identify the hazards of confined spaces, each space needs to be individually surveyed. By assessing the specific space, as well as the work to be performed, appropriate measures can be taken to avoid tragic incidents.
In general, the hazards of confined spaces are categorized as either atmospheric or physical. Atmospheric is usually the most lethal because dangerous atmospheres are not always detectable through the senses. Also, the atmosphere in a confined space may be extremely hazardous because of the lack of natural air movement. This characteristic of confined spaces can result in:
§ Flammable/combustible atmospheres, and/or
§ Toxic atmospheres.
Normal air has an oxygen content of 20.8 percent. An atmosphere is considered “oxygen-deficient” when there is less than 19.5 percent available oxygen. Any atmosphere with less than 19.5 percent oxygen should not be entered without an approved self-contained breathing apparatus (SCBA).
When the oxygen level drops below 17 percent, an entrant may experience rapid breathing and an accelerated heartbeat. As the oxygen content decreases, other physical effects become evident including poor muscle coordination, rapid fatigue, and intermittent respiration, nausea, and an inability to perform tasks. At concentrations less than six percent, there is rapid loss of consciousness, and death occurs in minutes.
Oxygen deficiency occurs from chemical or biological reactions which displace or consume oxygen from the space. Oxygen consumption takes place during:
§ Combustion of flammable substances as in welding, cutting, or brazing; and
§ Bacterial action, such as in the fermentation process.
Oxygen deficiency can result from bacterial action in excavations and manholes which are near garbage dumps, landfills, or swampy areas. Slow chemical reactions such as in the formation of rust on the exposed surface of metal tanks, vats, and ship holds will also consume oxygen in a confined atmosphere.
A simple asphyxiating atmosphere contains an inert gas (or gases) which does not produce any ill effects on the body. However, in sufficient quantity, an inert gas will displace oxygen and may result in an atmosphere unable to support normal breathing.
If 100 percent nitrogen — a non-toxic, colorless, odorless gas — is used to displace the oxygen in a confined space, it would cause immediate collapse and death to an entrant if the confined space is not adequately ventilated before worker entry. Other examples of simple asphyxiants which have claimed lives in confined spaces include carbon dioxide, argon, and helium.
A flammable atmosphere generally results from the vaporization of flammable liquids, by-products of chemical reactions, enriched oxygen atmospheres, or concentrations of combustible dusts. For combustion to occur, three elements — oxygen, fuel, and heat — must be present in the atmosphere. In the right amounts, these elements create an unrestricted chemical reaction which produces a fire. If one of these elements is missing, or is not present in the appropriate amount, combustion will not occur.
§ Fuel — a flammable gas, vapor, or dust
§ Oxygen — to support combustion
§ Source of ignition — a sparking, heat, pressure, shock, or impact
The proper mixture of fuel and oxygen varies from gas to gas. The flammability range for each gas is measured in terms of the lower flammability limit (LFL) and the upper flammability limit (UFL). Ten percent of flammable gas and vapor lower exposure limits is generally considered a safe level.
Example: The explosive range for methane is between five percent and 15 percent in air. Concentrations below five percent methane are below the explosive range (lean), and concentrations above 15 percent are too rich to support combustion. If a confined space contains 27 percent methane and forced ventilation is started, the introduction of air into the confined space may dilute the methane in air, taking it into the explosive range.
An oxygen-enriched atmosphere (above 23.5 percent) will cause flammable materials, such as clothing and hair, to burn violently when ignited. Never use pure oxygen to ventilate a confined space — always ventilate with normal air.
Most substances (liquids, vapors, gases, mists, solid materials, and dusts) should be considered hazardous in a confined space. Toxic gases may be present in a confined space due to the:
§ Product stored in the space
The manufacturing process uses toxic gases and the product may be absorbed into the walls of the space and give off toxic gases. Also, there may be biological or chemical processes occurring in the product stored in the confined space.
Examples: Hydrogen chloride and vinyl chloride monomerin are used in producing polyvinyl chloride. The product can be absorbed into the walls and give off toxic gases when removed; or when cleaning out the residue of a stored product, toxic gases can be given off.
Removing sludge from a tank or sump — decomposing oganic material can give off deadly hydrogen sulfide gas.
§ Work being performed in the space
The operation being performed in the confined space can release a toxic gas. Toxic gases may be created when acids are used for cleaning the interior of a confined space.
Examples: Hydrochloric acid reacts chemically with iron sulfide to produce hydrogen sulfide which is heavier than air and will settle out at the bottom of a confined space. Hydrogen sulfide is extremely toxic and exposure can cause paralysis of the olfactory system (making the victim unable to smell the gas), loss of reasoning, respiratory failure, unconsciousness, and death.
A welding, cutting, or brazing operation can cause the release of nitrogen, ozone, and carbon monoxide. Painting, scraping, sanding, degreasing can also cause the release of toxic gases.
Cleaning solvents used in many industries for cleaning/degreasing produce vapors which are very toxic in a confined space. Solvent vapors cause unconsciousness by depressing the central nervous system. Some chlorinated hydrocarbon solvents, such as chloroform, have been used as anesthetic agents.
In addition, certain chlorinated or fluorinated hydrocarbon solvents are toxic to the heart and have been associated with sudden death in confined spaces. Methylene chloride can be toxic in confined spaces both because of its solvent properties and also because it is metabolized in the body to carbon monoxide.
§ Areas adjacent to the space
Toxicants produced by work in the area of a confined space can enter and accumulate in the space. Gases that are heavier than air may migrate across a work area and accumulate in the lowest level, such as a maintenance pit.
Some toxic gases such as phosgene or carbon monoxide are particularly insidious because of their poor warning properties. Toxic gases which have been reported to cause death in workers in confined spaces include carbon monoxide, hydrogen cyanide, hydrogen sulfide, arsine, chlorine, oxides of nitrogen, and ammonia.
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