Document DGGyOxq21701a843kdzKRdLV5

Q THRESHOLD LIMIT VALUES FOR 1966 Adopted at the ZBth Annual Meeting ol the Ameiican Conference ol Governmental Industrial Hygienists, . Pittsburgh, Pennsylvania, May 16-17, 1966. The Threshold Limit Values refer to air-borne concentrations of substances and represent con ditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect. Because of wide variation in individual susceptibility, exposure of an occasional individual at or even below the threshold limit may not prevent discomfort, ag gravation of a pre-existing condition, or occupa tional illness. Threshold limits should be used as guides in the control of health hazards and should not be regarded as fine lines between safe and dan gerous concentrations. Exceptions are the sub stances given in Appendix A and certain of the substances given a "C" listing. The values not given a "C" listing refer to time-weighted aver age concentrations for a normal workday. The amount by which these concentrations 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 concentrations even for short periods produce acute poisoning, whether the effects are cumula tive, the frequency with which high concentra tions occur, and the duration of such periods. All must be taken into consideration in arriving at a decision as to whether a hazardous situa tion exists. Enlightened industrial hygiene prac tice inclines toward controlling exposures below the limit rather than maintenance at the limit. Threshold limits are based on the best avail able information from industrial experience, from experimental human and animal studies, and when possible, from a combination of the three. The basis on which the values are established may differ from substance to substance; protec tion against impairment of health may be the. guiding factor for some, whereas reasonable free dom from irritation, narcosis, nuisance or other forms of stress may dominate the basis for others. The Committee holds to the opinion that limits based on physical irritation should be considered no less binding than those based on physical impairment; growing bodies of evidence indicate that physical irritation may promote and accelerate physical impairment. Documentation of Threshold Limit Values. A separate companion piece to the TLVs is is sued by ACGIH under this title. This publication gives the pertinent scientific information and da ta with reference to literature sources, that were used to base each limit. Each documentation al so contains a statement defining the type of response against which the limit is safeguarding the worker. For a better understanding of the TLVs it is essential that the Documentation be consulted when the TLVs are being used. Ceiling vs Time-Weighted Average Limits. Although the time-weighted average concentra tion provides the most satisfactory, practical way of monitoring air-borne agents for compli ance with the limits, there are certain substan ces for which it is inappropriate. In the latter group are substances which are predominantly fast acting and whose threshold limit is more appropriately based on this particular response. Substances with this type of response are best controlled by a ceiling "C,# limit that should not be exceeded. It is implicit in these defini tions that the manner of sampling to determine compliance with the limits for each group must differ, a single brief sample, that is applicable to a *<C" limit, is not appropriate to the time* weighted limit: here, a sufficient number of sam ples are needed to permit a time-weighted average concentration throughout a complete cycle of operations or throughout the work shift* Whereas the ceiling limit places a definite boundary which concentrations should not be permitted to exceed, the time-weighted average limit requires an explicit limit to the excursions that are permissible above the listed values. The magnitude of these excursions may be peg ged to the magnitude of the threshold limit by an appropriate factor shown in Appendix C. It should be noted that the same factors are used by the Committee in making a judgment whether to include or exclude a substance for a "C" listing. "Shin** Notation. Listed substances followed by the designation "Skin** refer to the potential contribution to the over-all exposure by the cu taneous route including mucous membranes and eye, either by air-bome, or more particularly, by direct contact with the substance. Vehicles can alter skin absorption. This attention-calling de signation is intended to suggest appropriate measures for the prevention of cutaneous absorp tion so that the threshold limit is not invalidat ed. Mixtures. Special consideration should he given also to the application of these values in assessing the health hazards which may be as sociated with exposure to mixtures of two or more substances. A brief discussion of basic considerations involved in developing threshold limit values for mixtures, and methods for their development, amplified by specific examples are given in Appendix B. "Inert"* or Nuisance Particulates. A number of dusts or particulates that occur in the working environment ordinarily produce no specific ef fects upon prolonged inhalation. Some insoluble substances are classed as inert (c.g. iron and steel dusts, cement, silicon carbide, titanium dioxide, cellulose); others may be solu ble (starch, soluble oils, calcium carbonate) but are of such a low order of activity that in con centrations ordinarily encountered do not cause physiologic impairment; still others may be rap idly eliminated or destroyed by the body (veg etable oils, glycerine, sucrose). In the case of the insoluble substances, there may be sot.ie ac cumulation in the respiratory passages. In the case of the soluble substances, this accumula tion will ordinarily be temporary but may inter fere to some extent with respiratory processes. Hence, it is desirable to control the concentra tions of such particulates tn the air breathed by any individual, in keeping with good industrial hygiene practice. A threshold limit of I5mg/m'*, or 50 mppcf, whichever is less, is recommended for substan- AT4