3.2.1. OCCUPATIONAL EXPOSURE LIMITS — VARIOUS SOURCES AND ORGANIZATIONS
Several considerations apply to the selection of appropriate occupational exposure limits; they include both a maximum concentration of chemical in air and a defined exposure duration. The range of available limits include: (1) 8- to 10-hour time-weighted average (TWA) limits, (2) ceiling values, which are concentrations that should not be exceeded at any time during an 8-hour workday, and (3) short-term exposure limits (STELs), which are generally 15-minute exposure limits that should not be exceeded during the course of a workday. The ceiling and STEL values are assigned to substances that exert toxic effects over a short period of time.
Chemicals may have one or more of these values. For example, OSHA has assigned carbon disulfide both a ceiling value and a TWA. In this case, neither the ceiling value nor the TWA should be exceeded. A worker may experience multiple peak exposures during the work shift; however, none of these peaks may exceed the ceiling value. In addition, the average of these peaks and other total exposures over the entire work shift may not exceed the TWA value.
The STEL, ceiling, and TWA values are concentrations to which workers may be safely exposed daily, throughout their entire working life (up to 40 years). They are designed to protect healthy adults. It is, however, important to note that not all workers will be protected from adverse health effects even though their exposures are maintained below these levels. Some may experience adverse health effects because of personal susceptibility, a preexisting medical condition, and/or hypersensitivity (allergy). The occupational criteria are not intended for application to community exposure or the general public.
The primary sources of occupational exposure criteria for the workplace are: (1) NIOSH Recommended Exposure Limits (RELs); (2) the American Conference of Governmental Industrial Hygienists’ (ACGIH) Threshold Limit Values (TLVs); and, (3) OSHA’s PELs, which include TWA, ceiling and STEL values.94, 95, 96 The OSHA PELs are legally enforceable exposure limits, whereas the NIOSH RELs and ACGIH TLVs and BEIs are recommended guidelines.
Within an occupational setting, Occupational Exposure Banding (OEB) may be useful when other occupational exposure limits are not available or applicable. Additional information is available at the NIOSH Occupational Exposure Banding website.97
Additionally, the CDC has recommended exposure limits for workers to protect against potential exposure to the chemical warfare agents GA, GB, VX, H, and HD.98, 99 These exposure limits are intended for use among workers involved in chemical weapons disposal. Similar to other occupational exposure guidelines, these worker population limits for chemical warfare agents are described in terms of 8-hour TWAs and STEL values and are applicable to long-term, routine work in dismantling chemical weapons.
3.2.2. IMMEDIATELY DANGEROUS TO LIFE OR HEALTH (IDLH) VALUES – NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH (NIOSH)100
IDLH values are published by NIOSH,101 which defines an IDLH condition as a situation “that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment.” Furthermore, the stated purpose of establishing an IDLH values is to “ensure that the worker can escape from a given contaminated environment in the event of failure of the respiratory protection equipment.” A situation resulting in airborne concentrations at or near the IDLH value should be considered a non-routine event, and exposure duration should not exceed 30 minutes. However, the 30-minute period was not meant to imply that workers should stay in the work environment any longer than necessary following the failure of respiratory protection equipment. All available precautions should be taken to ensure that workers exit the environment immediately if exposures are at or near concentrations equivalent to IDLH values. NIOSH recently updated their methodology for deriving IDLH values and is in the process of establishing new values for hundreds of chemical agents.
The NIOSH respirator selection logic uses an IDLH value as one of several respirator selection criteria. Under the NIOSH respirator decision logic, respirators with the highest protection factor would be selected for emergency situations, firefighting, exposure to carcinogens, entry into oxygen-deficient atmospheres, entry into atmospheres that contain a substance at a concentration greater than 2,000 times the NIOSH REL or OSHA PEL, and for entry into potential IDLH conditions. These respirators with the highest protection factor include either a self-contained breathing apparatus (SCBA) that has a full face piece and is operated in a pressure-demand or other positive-pressure mode, or a supplied-air respirator that has a full face piece in combination with an auxiliary SCBA, both operated in a pressure-demand or other positive-pressure mode.