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For the most part, confined space entry is treated as a physical threat to the worker. Protocols are established to ensure the safe entry into confined and suspected hazardous environments. Although definitions for confined space vary, they essentially agree that it is a close area with limited entry and a possibility of exposing the worker to a toxic atmosphere (29CFR 1926.21(b)(6)). A confined space may be necessary to create a hazard--such as oxygen deficiency in a tank or tunnel. In other instances, it simply serves to conceal a common hazard--such as caustic materials. The confined space need not be small. Large storage vats are considered a confined space be cause they have limited entry and exit and limited ventilation.
CHEMISTRY
Chemicals with a vapor density greater than air find the hospitable characteristics of a confined space an excellent location to form hazardous concentrations. These heavier-than-air chemical vapors are particularly treacherous because concentration levels are heaviest at the lowest point and gradually diminish as the height increases. Unfortunately, this poses two problems. First, personnel entry into a confined space at grade level generally exposes the worker to higher concentrations of toxic material. Entry without respiratory protection may result in collapse. The worker then becomes enveloped in even more concentrated levels of contamination when falling to the ground. The second problem arises when atmosphere testing is conducted prior to entry but measurements are not taken at the lowest level of the confined space. Incomplete readings may be taken if a probe (dropped from the top of the tank) does not reach the bottom.
Chemicals that have a vapor density less than or equal to air have a tendency to mix in a homogeneous fashion, although readings will always be higher at the source. Hazardous chemical changes may take place in a confined space that has remained closed for some time and has limited air exchange rate. Certain corrosive materials will deplete the oxygen; when the space is opened, it may be quite some time before sufficient oxygen is introduced to exceed the minimum requirement of 19.5 percent. Additional chemical exchanges may take place where oxygen is displaced and/or combined with other chemicals creating a non-breathable atmosphere.
Entry into this class of hostile environment demands maximum respiratory protection....