In some cases, neither RfDs nor RfCs are available, and regulators must use another source of information to derive ACLs. Occupational limits, usually in the form of threshold limit values (TLVs) and permissible exposure limits (PELs), are often used to establish ACLs. Both establish allowable concentrations and times that a worker can be exposed to a pollutant in the work place. TLVs and PELs are particularly useful in establishing acute exposure ACLs.
The American Conference of Governmental Industrial Hygienists (ACGIH) develops TLVs. Three types of TLVs are the time-weighted average (TLV-TWA), the short-term exposure limit (TLV-STEL), and the ceiling limit (TLV-C). The TLV-TWA is the time-weighted average concentration for a normal eight-hour work day and forty-hour work week to which almost all workers can be repeatedly exposed without adverse effects. TLV-STELs are fifteen-minute time-weighted average concentrations that should not be exceeded during the normal eight-hour work day, even if the TLV-TWA is met. TLV-Cs are concentrations that should never be exceeded.
PELs are established by the U.S. Occupational Safety and Health Administration (OSHA) and are defined in much the same way as the TLVs. OSHA adopted the ACGIH's TLVs when federal occupational standards were originally published in 1974. Since that time, many of the values have been revised and published as PELs.
These occupational levels were developed for relatively healthy workers exposed only eight hours a day, forty hours a week. They do not apply to the general population, which includes the young, the old, and the sick and which is exposed twenty-four hours a day, seven days a week. However, using safety factors, regulators can use occupational levels as a basis for extrapolation to community levels. Different regulatory agencies use different safety factors.
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