Document nkenbDyg0Kan8pGr58EMQYd21

FILE NAME: Kentile (KEN) DATE: 1968 DOC#: KEN005 DOCUMENT DESCRIPTION: Supplemental Documentation of Threshold Limit Values 1963 SUPPLEMENTAL DOCUMENTATION OF THRESHOLD LIMIT VALUES ASBESTOS - ALL FORMS C 5 mppcf (2 mppc.f, time-weighted average), or 12 fibers/ml >5yu injengxh Asbestos is a generic term that applies to a number of mineral silicates, incombu. tile in air and separable into filaments. The most widely used in industry is chrysotile, a .. ignesium silicate from serpentine. Other typos include amosite (an iron magnesium Bilicate) crocidolite (a sodium iron silicate), tremolite (a calcium magnesium silicate) and anthophyllite (also an iron magnesium silicate). That exposure to asbestos is associated with development of a potentially disabling pneumoconiosis in man has been amply demonstrated by industrial experienced- 2, 3, 4. 5,6, ` ). A threshold limit of 5 mppcf was recommended by Dreessen et al. (), in 1938 after studying 541 employees in 4 asbestos textile plants where massive exposures to chrysotile occurred. Only 3 doubtful cases of pneumoconiosis were found at the time of the study in those exposed to dust concentrations under 5 mppcf, whereas numerous well-marked cases were found above 5 mppcf. Counts were from impinger-collected samples in ethyl alcohol and distilled water. Both fibrous and nonfibrous particles were counted, but the latter greatly predominated. Although chemical analyses of collected samples of airborne dust corresponded to those of settled dust, it is believed that dust counts of particulates by im pinger can be expected to give only an indirect measure of the risk of asbestosis because im pinger sampling collects particulates other than asbestos. A conference on the biologic effects of asbestos(10) in 1965 called attention to the very real probability that the 5 mppcf limit recommended by Dreessen is inadequate to give complete working-lift-time protection against all forms of asbestos. Medical data on which the limits had been based were inadequate; more than half of the asbestos workers studied were under 30 years of age and thus provided an insufficient exposure time for asbestosis to develop. Of the 105 workers exposed to < 5 mppcf, 82 had workedC 5 years: 101, < 10 years; only 4 had > 10 years exposure. Seven of 36 workers exposed to 5 - 9.9 years had asbestosis; 3 of 50 workers exposed to 10 - 19.9 mppcf for < 5 years had asbestosis(). Moreover it was a "point-in-time" study; many of the ill were missing and the dead uncounted, hence not con sidered in the over-all evaluation of the limit. The asbestos conference(^) further called attention to the rising worldwide rate of increase in lung cancer among asbestos workers, particularly bronchial cancers, and also noted with alarm a wide-spread incidence of pleural and diffuse mesotheliomas, tumors heretofore e'e rs looked and only recently (1959) associated with exposure to asbestos. The epidemiology of the early cases was especially revealing in pointing out the scope of asbestos exposures; many o f the m e s o th e lio m a c a s e s had no in d u s tr ia l a s s o c ia tio n , but were "neighborhood" cases or cases traced to users of asbestos. Exposures to asbestos extend far beyond those of miners and millers of asbestos or textile weavers, and include carpenters (sawing asbestos board), insulators and pipe laggers, dockers handling asbestos cargo, brake-lining workers, rubber compounders and several others. The Committee on Hygienic Standards of the British Occupational Hygiene Society from an evaluation of medical evidence from Great Britain and data from the U. S. Public Health Service Study(^) has recommended the following criteria for limiting exposure to chrysotile asbestos assuming a 1% risk of contracting asbestos disease during a 50-year working exposure^11). Dust Category Chrysotile Concentration*1) ____ Fibers/m l^)______ Negligible Low Medium High 0.5 0.5 - 2.0 >2-10 > 10 a) Averaged during a 3-month period k) Greater than 5yu in length From these criteria, a limit of 100 fibers/ml-years was proposed, hence a limit of 2 fibers/ml averaged over a 50-year period. In arriving at an asbeBtos limit based on American experience and considerations applicable to the American industrial scene, the following predominated. Recent evaluation of the health experience in asbestos plants indicated that the 5 mppcf limit was not sufficiently low to protect workers exposed for 30 years, a period rarely exceeded in America. Balzer and Cooper(*^ supported this view in a report of asbestos is occurring among insulati a workers from levels that were deemed highly unlikely to have exceeded a time-weighted average of 5 mppcf. A retrospective investigation of the association between lung cancer in asbestos workers and smoking by Selikoff et al. (^ ) revealed that asbestos neoplasias were strongly associated with smokers. In accordance with these findings, the 5 mppcf limit was made a ceiling value below which all concentrations should fluctuate. This is equivalent to a time-weighted average value of slightly less than 2 mppcf, as shown by A yer(^ ) from the following determined for asbestos where 5% of the samples only exceed the 5 mppcf limit: % Samples Exceeding 5 mppcf Average Concentration mppcf 1 1. 3 2 1.5 5 1.9 10 2.4 25 4. 5 Inasmuch aB the count by impinger procedure often consists primarily of extraneous particulates (textile plants) an alternate fiber-count may be made by the membrane filter method using phase-contrast illumination at 430 X m agn ification ^), por the purpose of determining significant asbestos exposure, the membrane-filter method in which only fibers greater than 5 micra (u) in length are counted, is recommended. On this basis, 6 fibers/milliliter greater than 5 u had been found to be equivalent to 1 mppcf. Hence for a time-weighted average limit by count of asbestos particles of 2 mppcf, the corresponding limit based on fi bers of specified size is 12 fibers/ml. This limit is intended to reduce to an insignificant risk, the occurrence of asbestos disease among those exposed for 30 years to all forms of asbestos. There is still asbestos disease appearing in industry. It is not known to what extent thiB represents a reservoir effect of paBt exposures, or if the 5 mppcf level is still contributing to the disease incidence. For this reason the 5 mppcf iB recommended as a ceiling value (2 mppcf as a time-weighted avernge)until a more definite value can be established based on medical findings. References .1234567890 1. Merewether, E. R. A.: J. Ind. Hyg., 12: 198, 239 (1930). PneumoconiosiB Abstracts, 1926-1938, Vol. 1 p. 128. ' 2. Wood, W. B., Gloyne, S. R.: Lancet, Dec. 22, 1934, pp. 1383-1385. 3. Fulton, W. B ., Dooley, A., Matthews, J. L. Houtz. R. L .: Penn. Dept, of Labor and Ind. Bull. 42 (1935). 4. Lanza, A. J. , McConnell, W. J . , Fehnel, J. W.: Pub. Health Rep. 50, 1 (1935).- 5. Donnelly, J .: J. Ind. Hyg. &Tox. 18, 222 (1936). 6. Dreessen, W. C., DallaValle, J. W., Edwards, T. I., Miller, J. W ., Sayers, R. R.: Pub. Health Bull. No. 24, Washington, D. C. 1938. 7. ' Lynch, M.: Arch. Ind. Health H., 185 (1955). 8. Smith, K. W.: Arch. Ind. Health 12, 198 (1955). 9. Cartier, P.: Arch. Ind. Health 11, 204 (1955). 10. Biologic Effects of Asbestos, H. E. Whipple, ed. Ann. N. Y. Acad. Sciences 132, Art. 1, 1965, 766 pp. 11. Report of Committee on Hygiene Standards, Brit. Occ. Hlth. Soc. "Hygiene Standards for Chrysotile Asbestos Dust" Dec. 1967. 12. Balzer, J. L ., Cooper, W. C., Ind. Hyg. News Report Dec. 1967. 13. Sellkoff, 1. V., Hammond, E C. . Churg, V., J. Am. Med. Assn. 204, 104 (1968). 14. Ayer, H. .: Memorandum to Committee Member Jan. 1968. 15. Ayer, H. E. , Lynch, J. R. , Fanney, J. H.: Ann. N. Y. Acad. Sciences 132, An. 1, p. 274 (1965). CRISTOBALITE* * Use one-half the value obtained from cither the count or mass formula for qu/riz. Cristobalite, one of the three major crystalline forms of silicon dioxide is stable at high temperatures and is formed when quartz or amorphous silica is heated, sb in the calcining of diatomaceous earth or in the silica brick industry. G ardner^ reported on experiments in which rabbits were injected intravenously and guinea pigs intraperitoneally with various mineral dusts. Responses to cristobalite were more se vere than from quartz, and the fibrosis that followed was diffuse rather than nodular. King, Mohanty, Harrison, and Nagelschmidt(2) had similar results in rats given intratracheal in jections of criBtobalite, fused silica, quartz and tridymite. ' The present recommended threshold limit is based on studies in the diatomite industry(3 >4) and by analogy with the threshold limit for quartz. Recent experimental studies(5) in ani mals with diatomaceous earth of 61% cristobalite content at levels of 2, 5 and 50 mppcf have presented strong evidence foi .he need to review' the present limit. Although no frank fi brosis of the lung developed from any level of exposure in 2. 5 years, the lifetime of one of the exposed species (rat), there was considerable cellular infiltration in the lung and hvalinized fibrotic nodules developed in the pulmonary lymph nodes in 1 animal species (dog) at the 5 mppcf level with scattered nodules at the 2 mppcf level. Long industrial medical experience indicates that the limit of 5 mppcf does not possess the requisite safety factor; occasional cases of pneomoconiosis continue to occur after cristo . balite exposures had been reduced to levels approximating 5 mppcf(6). Accordingly, a limit approximating one-half that for quartz of the corresponding percentage in the dust is recommended as calculated either from the count or mass formula. References 1. Gardner, L. U. : Am. Inst. Mining Si Metall. Engrs. Tech. Pub. No. 929 (1938). 2. King, E. J. , Mohanty, G. P ., Harrison, C. V., Nagelschmidt, G.; Brit. J. Ind. Med. 10. 9 (1953). 3. Smart, R. H. , Anderson, W. M. : Ind. Med. & Surg. 21, 509 (1952;.