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Some materials — hydrogen fluoride gas and cadmium vapor, for example—may produce immediate transient effects that, even if severe, may pass without medical attention, but are followed by sudden, possibly fatal collapse 12 to 72 hours after exposure. The victim “feels normal” from recovery from transient effects until collapse. Such materials in hazardous quantities are considered to be “immediately” dangerous to life or health.
OSHA regulations require that any confined space entry, including emergency response, be preceded by testing of the internal atmosphere with a calibrated direct-reading instrument for oxygen content, for flammable gases and vapors, and for potential toxic air contaminants, in that order. Monitoring should be continuous while anyone is in the confined space. Investigations conducted by the National Institutes for Occupational Safety and Health (NIOSH) indicate that workers usually do not recognize that they are working in a confined space and that they may encounter unforeseen hazards. Testing and evaluation of the atmosphere are typically not initiated prior to entry and monitoring is not performed during the confined space work procedures, which results in the need for rescue.
Oxygen content of a confined space includes both oxygen-deficient and oxygen-enriched atmospheres. Oxygen deficiency results from chemical or biological reactions that displace or consume available oxygen within a confined space. Biological action can cause oxygen depletion in excavations and manholes located near garbage dumps, landfills or swampy areas. Chemical reactions that occur slowly as in the rusting of metals on the surface of tanks, vats and ship holds can deplete oxygen in a confined space.
Consumption of oxygen can also occur during welding, cutting or brazing operations. As a result of displacement of the oxygen in a confined space, asphyxiation can occur to anyone entering the space without self-contained breathing apparatus (SCBA). This is also known as “simple asphyxiation.”
Ambient air usually has an oxygen content of around 21%. Once the oxygen level drops below 17%, the initial signs of hypoxia start to occur as breathing becomes faster and heartbeat increases. Hypoxia occurs when the depletion of oxygen reaches the cellular level and causes damage. At oxygen concentrations of 14% to 15%, breathing is increased further, rapid heartbeat, lack of muscular coordination, fatigue and irregular respiration occurs. When the oxygen level reaches 6% to 10%, nausea, vomiting, inability to perform and unconsciousness occur. When levels fall below 6%, rapid unconsciousness occurs followed by death within minutes.
Displacement of oxygen may occur from a chemical with additional hazards beyond asphyxiation, including toxicity and flammability. These additional hazards can further complicate the entry of employees or rescue personnel. Oxygen-enriched atmospheres can also be dangerous because they can increase the risk of combustion. Levels greater than 25% are considered oxygen-enriched atmospheres. The bottom line is that before anyone enters a confined space, the space must be monitored to determine whether there is enough oxygen to work safely.
Response personnel should wear SCBA whenever they enter a confined space to perform a rescue or an investigation. Attempting to enter a confined space with the idea that you will hold your breath to make a rescue or that you will don your SCBA once you enter the space can be a fatal mistake and should never be attempted. In most cases, entry into confined spaces occurs through a small or restricted opening that requires additional effort to accomplish.
Once in the space, mobility is often reduced by impediments such as piping or baffles. These actions will take time and reduce your ability to hold your breath, even though you think you can based on past efforts to hold your breath for a time under less physical or stressful circumstances. If the size of the opening to the confined space does not allow entry with the SCBA tank on your back, then don the mask and start air flow outside the space, have the tank passed to you when you enter, then place it on your back once inside, if possible. If available, hose-supplied SCBA are easier to use in a confined space and increase the amount of air available.