PRINCIPLES OF TOXICOLOGY

526 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDS
the “action level.” “Action levels” are typically one-half the PEL, and certain actions are required to
detect potential adverse health effects, such as periodic air monitoring and medical surveillance
programs. Because exposures can vary greatly from one day to another with minor changes in
processes, work practices, and environmental conditions, the “action level” concept is intended to
identify, with 95 percent confidence, processes that are likely to exceed the occupational exposure limit
on 5 percent or more of the work days. An underlying assumption of the “action level” is that exposures
below the occupational exposure limit cause minimal or acceptable risk to the worker. As exposures
increase 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 through
the required medical surveillance programs.
Between 1968 and 1988, ACGIH lowered many of the TLVs ® and many practicing industrial
hygienists referenced the TLV ® s rather than the PELs (1968 TLVs ® ) to provide better protection to
workers. The National Institute of Occupational Safety and Health (NIOSH), an agency created by the
OSHAct to recommend standards to OSHA, among other responsibilities, developed many recommended
exposure levels (RELs), which were also increasingly referenced by practicing industrial
hygienists. But these were largely ignored by OSHA in the rulemaking process, in part because they
were based strictly on health criteria, while OSHA was required to include feasibility considerations
in 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 as
several NIOSH RELs. Many industrial hygienists agreed with updating the PELs because of the
additional protection it afforded workers. Unfortunately, this represented a reaffirmation that the
exposure limits are “safe” for all workers rather than merely guidelines. As a result of legal challenges
from both organized labor and industry groups, the updated PELs were struck down by the 11th Circuit
Court of Appeals in 1992, and after several continuances, OSHA accepted the decision in 1993, and
the 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. One
option to speed up the process is to periodically establish a prioritized group of chemicals for which
new health effects data has become available and propose new exposure limits for each compound in
the 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 governmental
agencies as a basis for environmental exposure limits. The ACGIH considers this use generally
inappropriate because of differences in the exposed populations (adults in the occupational setting
versus all age groups in the general environment), exposure patterns (8 h/day, 40 h/week, 40-year working
lifetime 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 other
standards are available. The environmental exposure limits, usually referred to as acceptable
ambient-air concentration guidelines or standards, usually reduce the OEL by an uncertainty factor. The
uncertainty 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 et
al. 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 providing
interim 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 subchronic
exposure. 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