제  목 : Q＆A Today!:Uncertainty Factors in the Development and Application
  일  자 : 1997년 12월
  제공처 : INTERNET

   Q＆A Today!: Uncertainty Factors in the Development and Application of TLVs
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Q:

    I have read that "safety factors" are involved in the development of TLVs.
    How are these safety factors derived and how big are they?

A:

    The derivation of the safety factors used in developing TLVs and the size
    of these factors varies from agent to agent. In general, the size of the
    safety factor is not definable because of the large variability in
    susceptibility to hazardous substances present in the population. This
    issue is briefly addressed in the "Introduction to the Chemical Substances"
    and the "Introduction to the Physical Agents" sections in the 1997 TLVs
    and BEIs  Booklet (pages 9 and 76, respectively) as well as in more detail
    in most Documentations for individual TLVs. Occupational hygienists
    should consult the TLV Documentations for the specific agents to answer
    this question on a case-by-case basis.

    Some occupational hygienists inadvertently misuse TLVs by assuming
    they are exact values that can be considered absolutely safe levels of
    exposure to hazardous agents. TLVs are not precisely defined values and
    should not be considered as absolutely safe levels of exposure for all
    individuals. Every TLV has a degree of imprecision partly because of
    individual susceptibility. Indeed, a degree of imprecision is considered in
    the development of every TLV. Each of the TLV Introductions states that
    most TLVs "...have built-in safety factors to guard against adverse health
    effects to moderate deviations from normal environments ...." Further,
    regarding the level of safety provided by a TLV, "... the safety factors are
    not of such a magnitude as to take care of gross deviations." TLVs are
    intended to protect "nearly all workers." Not even in cases where a large
    safety factor is used when setting the TLV can it be assured that the TLV
    will protect all workers. Allowing even small exposure excursions above a
    TLV may increase the size of the unprotected group of workers.

    The safety factors built into TLVs have no set size or value, and no
    formula is used to derive them. Instead, "safety factor" is really an
    expression of the concept of allowing for the uncertainty involved in
    developing a TLV. Varying degrees of uncertainty are unavoidable in
    developing TLVs, depending on the relative weakness and imprecision of
    the data on which they are based. This variability is considered on a
    case-by-case basis by the members of the TLV Committees for Chemical
    Substances and Physical Agents. In each case, the Committee members
    make a decision based on their collective judgement, after reviewing the
    available, pertinent literature. The basis for assessing the magnitude of
    uncertainty for each TLV is included in ACGIH's Documentation of the
    TLVs and BEIs.

Q:

    Appendix C to the Chemical Substances TLVs suggests that TLVs "...
    should be reduced by a suitable factor ..." under certain circumstances of
    exposure to several agents. To correctly apply TLVs in these cases, what
    is meant by the term "suitable factor" and how should it be derived?

A:

    Appendix C goes on to explain how to derive this factor by providing
    guidance for adjusting chemical substance TLVs when a worker is
    exposed to two or more substances. Taken in context, Appendix C is
    simply intended to provide the occupational hygienist with some guidance
    about what may be necessary to protect workers exposed to several
    chemical substances in a workday. So, "suitable factor" isn't meant to be
    a term in and of itself. Rather, this intends simply to remind the
    occupational hygienist that under certain circumstances the TLV should
    be effectively reduced by a "factor" that is "suitable" to the environment.

    Just as members of the TLV Committees often must allow for uncertainty
    in developing TLVs for chemical substances and physical agents, the
    occupational hygienist also needs to consider adjusting TLVs under
    circumstances for uncertainty, such as when exposures occur in a
    mixture of chemical substances, physical agents, or both. In this case,
    the occupational hygienist determines and applies the allowance for
    uncertainty as members of the TLV Committees do when recommending
    TLVs. The occupational hygienist may also need to know if chemical
    substances in the mixture may interact regarding potential toxicity. The
    occupational hygienist should consult the TLV documentation on these
    chemical substances as they may provide information pertaining to
    synergy, potentiation, or inhibition in the case of chemical mixtures. It is
    not possible to empirically derive one or a range of values to be used in
    lowering a TLV because of the almost infinite variety of conditions of
    exposures possible. Conditions of exposure, along with other workplace
    factors, must be taken into consideration by the occupational hygienist in
    assessing the need for lowering a TLV in these instances. The
    occupational hygienist must be familiar with the information in the TLV
    Introduction sections, the relevant appendices, and especially the
    Documentations of the TLVs to determine which conditions of exposure
    are relevant in a given work situation.