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Elements of Industrial Toxicology 1308 TABLE 42-B THRESHOLD LIMIT VALUES FOR 1968 Adopted at the 30th Annual Meeting of the American Conference of Governmental Industrial Hygienists, St Louis, Missouri, May 13> 1968 Copyright 1968, by Amen can Conference of Governmental Indus trial Hygienists Reprinted with Permis sion The Threshold Limit Values refer to air borne concentrations of substances and rep resent conditions tinder which it is be lieved that neatly all workers may be repeatedly exposed, day alter day, without adverse effect Because of wide variation in individual susceptibility, exposure ol an occasional individual at or even below the threshold limit may not prevent discomfort, aggravation ol a pre-existing condition, or occupational illness Clinical tests are becoming available that pennit detection of those individuals who will hyperreact upon exposure to certain industrial substances Being predictive in character, the> may he applied as screening tests in the prcplaccnient job examination Bequests lor furthei details of these tests should he directed to the Chairman of the Committee Threshold limits should he used as guides in the control ol health hazards and should not be regarded as fine lines between safe and dangerous concentra tions Exceptions ate the substances given in Appendix A and certain of the substances given a "CV listing 1 he values not given a "C" listing refer to tune-weighted average concentrations for a conventional 7 or 8 hour workday Tune-weighted average concentrations permit excursions above the limit, provided they are compensated by equivalent ex cursions below the limit during the work day The degree of permissible excursion is pegged to the threshold limit value of the particular substance as given in table in Appendix C under "Test TLV Factor " Hence, it is not considered appropriate to interpret air concentration values as ex ceeding time-weighted average limits, if such values he within the permissible ex cursions (See section on "Legislative G>de ") The amount by which these concentra tions may be exceeded for short periods without injury to health depends upon a number of factors such as the nature of the contaminant, whether very high concentra tions even lor short periods produce acute poisoning, whether the effects are cumu lative, the frequency with which high con centrations occur, and the duration of such periods All must be taken into considera tion in arriving at a decision as to whether a hazardous situation exists Enlightened industrial hygiene practice inclines toward controlling exposures below the limit rather than maintenance at the limit Threshold limits are based on the best available mfoimation from industrial ex perience, from experimental human and animal studies, and when possible, from a combination of the three The basis on which the values arc established may differ from substance to substance, protection against impairment of health may be the guiding factor from some, whereas reason able freedom from lintation, naicosis, nuisance or other forms of stress may dom inate the basis lor others The Committee holds to the opinion that limits based on physical irritation should be considered no less binding than those based on physi cal impairment, growing bodies of evi dence indicate that physical untation may promote and accelerate physical impair ment These limits are intended for use in the field of industrial hygiene and should be interpreted and applied only by persons trained in this field They are not intended for use. or for modification for use, (1) as a relative index ol toxicity, by making a ratio of two limits, (2) in the evaluation or control of community air pollution or air pollution nuisances, (3) in estimating the toxic potential of continuous uninterrupted exposures, (4) as proof or disproof of an existing disease or physical condition, or (5) foi adoption by countries whose work ing conditions differ from those in the United States ol America and where sub stances and processes differ Documentation of Threshold Limit Val ues A separate companion piece to the TLV's is issued by ACCIH under this title This publication gives the pertinent scien tific information and data with reference