PRINCIPLES OF TOXICOLOGY

22.2 EXPOSURE LIMITS 525
The TLVs ® are designed to provide guidance to industrial hygienists in determining the potential
for harm associated with air contaminant exposures. They must never be viewed as a sharp dividing
line between “safe” and “unsafe” conditions, and they certainly do not preclude the appearance of
adverse health effects in individuals with greater sensitivity, either through genetic or environmentallyinduced
mechanisms.
For chemicals, the TLVs ® address exposure primarily from the inhalation route and take several
forms. The TLV ® -TWA is an eight hour time-weighted average (TWA) exposure to which the ACGIH
TLV ® Committee believes most workers may be exposed for 8-h workdays, 40 hours per week for a
working lifetime without becoming ill. Some chemicals also have a short-term exposure limit
(TLV ® -STEL), which is a 15-min TWA exposure limit to prevent acute effects such as irritation,
chronic or irreversible tissue damage, or narcosis, which could impair judgement or performance. A
ceiling limit (TLV ® -C) is a concentration above which employees should not be exceeded at any time.
Some compounds are considered to be carcinogens based on epidemiology studies, toxicology
studies, or case histories. The qualitative differences in research results lead the ACGIH to place a
carcinogen into one of five categories: (1) Confirmed Human Carcinogen, (2) Suspected Human
Carcinogen, (3) Confirmed Animal Carcinogen, (4) Not Classifiable as a Human Carcinogen, and (5)
Not Suspected as a Human Carcinogen. In some cases the carcinogen has an exposure limit assigned,
while in others no exposure limit is assigned. For carcinogens with an exposure limit, this does not
imply that the compound has a threshold, but exposures at or below the exposure limit should not result
in a measurable incidence of cancer or mortality. Exposures to suspected carcinogens should be
controlled to a level as low as reasonably achievable below the listed TLV ® . For confirmed human
carcinogens without an exposure limit, engineering controls should be installed, and workers should
wear personal protective equipment that will eliminate exposure to the greatest extent possible.
The workplace has gone through many changes in the 40 years since the TLVs ® were first adopted.
For example, the number of women in the workplace has increased drastically. Although men and
women frequently exhibit little difference in the quantitative and qualitative responses to chemical
exposures, their responses to some chemicals differ. There are also more older workers in the workforce
today whose sensitivities to chemicals may be increased because of age-related physiological changes.
Also significant, the presence of women in the workplace has brought another part of our population,
the fetus, into the workplace. The sensitivities and responses of the fetus may be much different than
the sensitivities of the mother or other adult workers. As more information on these effects becomes
available, the TLVs ® for substances with teratogenic potential are likely to incorporate these effects
and may be substantially reduced. In the interim, programs that are intended to minimize reproductive
effects are likely to be implemented. The message from the Supreme Court in a legal case, the “Johnson
Controls” case, in which the company policy was to exclude fertile women of child-bearing age from
certain jobs where workers were exposed to lead was that these programs will be difficult to administer
in ways that do not discriminate.
The Occupational Safety and Health Administration adopted the 1968 TLVs ® as legally enforceable
permissible exposure limits (PELs). When these were adopted, many people not familiar with the TLV ®
concept mistakenly interpreted these levels as “safe” for all workers since the intent of the OSHAct
is “to assure as far as possible every working man and woman in the nation safe and healthful working
conditions.” That was never the intent of ACGIH, and the most recent version of the ACGIH TLV ®
booklet (1998) states
These limits are not fine lines between safe and dangerous concentration nor are they a relative
index of toxicity. They should not be used by anyone untrained in the discipline of industrial
hygiene.
Before 1988, OSHA was able to promulgate only a handful of new or changed standards for air
contaminants. In part, this was caused by the detailed and costly (but democratic) process prescribed
for adoption of new standards, which is designed to ensure that all groups have a say in how the standard
is formulated. One important concept included in many of the standards promulgated by OSHA was