PRINCIPLES OF TOXICOLOGY - Biology East Borneo

526 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDSthe “action level.” “Action levels” are typically one-half the PEL, and certain actions are required todetect potential adverse health effects, such as periodic air monitoring and medical surveillanceprograms. Because exposures can vary greatly from one day to another with minor changes inprocesses, work practices, and environmental conditions, the “action level” concept is intended toidentify, with 95 percent confidence, processes that are likely to exceed the occupational exposure limiton 5 percent or more of the work days. An underlying assumption of the “action level” is that exposuresbelow the occupational exposure limit cause minimal or acceptable risk to the worker. As exposuresincrease above the action level, the number of days the exposure limit will likely be exceeded increases,and the probability of adverse health outcomes increases. An additional advantage of the “action level”is that workers more susceptible to the effects of a chemical are more likely to be identified throughthe required medical surveillance programs.Between 1968 and 1988, ACGIH lowered many of the TLVs ® and many practicing industrialhygienists referenced the TLV ® s rather than the PELs (1968 TLVs ® ) to provide better protection toworkers. The National Institute of Occupational Safety and Health (NIOSH), an agency created by theOSHAct to recommend standards to OSHA, among other responsibilities, developed many recommendedexposure levels (RELs), which were also increasingly referenced by practicing industrialhygienists. But these were largely ignored by OSHA in the rulemaking process, in part because theywere based strictly on health criteria, while OSHA was required to include feasibility considerationsin their standards. NIOSH has recently stated their intent to develop RELs that do include feasibility.In 1989, OSHA again adopted many of the ACGIH TLVs ® , this time from 1987/88, as well asseveral NIOSH RELs. Many industrial hygienists agreed with updating the PELs because of theadditional protection it afforded workers. Unfortunately, this represented a reaffirmation that theexposure limits are “safe” for all workers rather than merely guidelines. As a result of legal challengesfrom both organized labor and industry groups, the updated PELs were struck down by the 11th CircuitCourt of Appeals in 1992, and after several continuances, OSHA accepted the decision in 1993, andthe PELs reverted to those adopted in 1971. The dilemma of setting new exposure limits in a timely,but scientifically rigorous, fashion within the established regulatory framework is a difficult one. Oneoption to speed up the process is to periodically establish a prioritized group of chemicals for whichnew health effects data has become available and propose new exposure limits for each compound inthe group. Other alternatives may be possible if OSHA reform efforts are successful.Another potential problem associated with adoption of TLVs ® is their use by other governmentalagencies as a basis for environmental exposure limits. The ACGIH considers this use generallyinappropriate because of differences in the exposed populations (adults in the occupational settingversus all age groups in the general environment), exposure patterns (8 h/day, 40 h/week, 40-year workinglifetime vs. 24 h/day for 70+ years), and exposure routes (inhalation versus ingestion and inhalation).Nevertheless, occupational limits are often used by environmental agencies because no otherstandards are available. The environmental exposure limits, usually referred to as acceptableambient-air concentration guidelines or standards, usually reduce the OEL by an uncertainty factor. Theuncertainty factor is based on the averaging time, usually 1, 8, or 24 h or 1 year; the duration of exposure;and the type of adverse health outcomes. Although several formulas are used, a typical one is:Uncertainty factor = (safety factor/TLV ® ) × (hours per day/8) × (days per week/5)The safety factor is usually 10, 100, or 1000, depending on the health effects. However, Williams etal. compared environmental exposure limits developed by this method with some developed from “scratch,”and found little differences. While the scratch method may be the best, as a means of providinginterim protection, the use of TLVs ® as a basis for environmental exposure limits may be reasonable.Another group has proposed using the USEPA Integrated Risk Information System (IRIS) (database)to establish air contaminant standards, referred to as workplace allowable air concentrations(WACs), which are based exclusively on systemic and carcinogenic effects of chronic and subchronicexposure. The resulting exposure limits are generally lower (frequently by several orders of magnitude)than the TLVs ® and PELs, which are often based on prevention of acute irritant effects. WACs are