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Health effects that have been associated with silica exposure include silicosis, silico-tuberculosis, cor pulmonale, auto-immune diseases, nephritis, emphysema.
Ann. occup. Hyg., Vol. 41, Supplement 1, pp. 448-453, 1997 © 1997 British Occupational Hygiene Society Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0003^878/97 $17.00 + 0.00 Inhaled Particles Vlll

PII: S0003-^878(96)00095-6

RESPIRATORY HEALTH EFFECTS FROM AMBIENT SILICA EXPOSURE: A BENCHMARK DOSE ANALYSIS J. S. Gift* and D. F. Goldsmith! * United States Environmental Protection Agency, Research Triangle Park, North Carolina; and •(•California Public Health Institute, 2001 Addison St., Suite 210, Berkeley, CA 94704-1103, U.S.A.

The U.S. Environmental Protection Agency (EPA) has established National Ambient Air Quality Standards (NAAQS) of 150 u,g m~3 (24 h average) and 50 u.g m~3 (annual arithmetic mean) for particles < 10 urn in aerodynamic diameter (PM10).* State health officials and some members of the public have expressed concern over whether this standard is adequate with respect to silica, a major component in some locations of airborne particulate matter. Quartz is the primary crystalline polymorph of silica and the predominant form found in the environment. This paper summarises what is known about ambient quartz levels in the United States (U.S.) and describes a benchmark concentration (BMC) analysis of the exposure-response relationship for fibrotic lung disease associated with chronic, low-level silica exposures. One study was found that measured ambient silica levels in the U.S. Davis et al. (1984) report average and upper-bound quartz fractions of 5 and 10%, respectively, in particulate samples of less than 15 urn dae taken from 22 United States metropolitan areas. In a mining environment, the silica fraction within airborne particle samples of less than 10 um dae did not differ markedly from those reported by Davis et al. (1984) for the larger particle size range (Verma et al., 1994). Thus, 10% is a considered reasonable upper-bound estimate of the quartz fraction within PM10 samplers. Davis etal. (1984) reported average and upper-bound quartz levels of 3 and 8 |ig m~3, respectively. As shown in Table 1, quartz levels estimated from 7 year average PM10 levels (1987-1993) and quartz fractions reported by Davis et al. (1984) for 17 cities are slightly lower, but in general agreement with measured levels. Due to uncertainties in estimating quartz fractions in the 1987-1993 PM10 samples, the direct measurements are preferred as realistic and conservative estimates of ambient U.S. silica levels. METHODS

A BMC (or a benchmark dose) analysis is defined as the use of a mathematical model to determine an inhaled concentration (or oral dose) and its lower * EPA has also recently proposed the addition of two new PH2 5 standards at 15 ugm , annual mean, and 50 ugm~3, 24-h avegate (U.S. Federal Register, 1996). 448

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INTRODUCTION

Respiratory health effects from ambient silica exposure

449

Table 1. Comparison of estimates using 7 year average PM 10 levels and measured quartz levels

29 32 28 33 32 52 28 21 49 49 33 34 29 84 63 31 39

1.7 1.8 0.8 1.4 1.5 1.6 1.5 1.1 3.6 3.6 2.7 0.3 0.7 2.4 5.0 0.9 1.2

4.2 8.0 2.4 2.6 2.9 2.3 3.0 2.4 5.1 4.3 3.8 1.4 1.3 3.0 4.5 2.0 1.1

39

1.9

3.2

7 year avg. PM 10 b (Hgm 3 )

Akron, OH Boston, MA Buffalo, NY Cincinnati, OH Dallas, TX El Paso, TX Hartford, CT Honolulu, HI Kansas City, KS Kansas City, MO Minneapolis, MN Portland, OR RTP, NC Riverside, CA St. Louis, MO San Jose, CA Seattle, WA

6.0 5.7 2.9 4.1 4.7 3.0 5.5 5.2 7.4 7.4 8.2 1.0 2.5 2.8 7.9 3.0 3.1

Averages

4.7

Site

Estimated 7 year avg. quartz c

a

Davis et al. (1984); quartz as percent of combined coarse and fine dust ( < 15 urn d a e ). U.S. Environmental Protection Agency (1996). ^Estimate of 7 year annual quartz level — 7 year annual PM 10 level x quartz weight percent. d Davis et al. (1984); combined coarse and fine quartz captured in a dichotomous sampler designed to eliminate particles > 15 urn aerodynamic diameter. b

confidence bound (e.g. 95% confidence limit) that is associated with a predefined effect level. General methods for the performance of a BMC analyses are described in U.S.EPA (1994). The log-logistic model used for the purposes of this analysis is described in Appendix B of U.S.EPA (1996). The following is a discussion of how data for use in the BMC analysis were chosen. Selection of noncancer endpoint Health effects that have been associated with silica exposure include silicosis, silico-tuberculosis, cor pulmonale, auto-immune diseases, nephritis, emphysema and airflow abnormalities. Silico-tuberculosis and cor pulmonale are secondary effects of silicosis. Emphysema and airflow abnormalities are generally considered nonspecific pulmonary effects caused by dust exposures. Auto-immune diseases and nephritis from silica exposures have been documented (Sanchez-Roman et al., 1993; Steenland and Goldsmith, 1995), but the mechanisms involved and the relationship between exposure and effect are not well understood. Pulmonary fibrosis (as silicosis) determined by radiography or autopsy is the endpoint of choice for the benchmark analysis. It characterises a specific, primary health effect from silica exposure, and good occupational exposure-response data exist at exposure levels not far from estimated ambient concentrations (see Table 2).

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(ng m " 3 )

1980 Measured quartz levels'1 (M£ m" 3 )

Quartz percentage of TDM a (weight %)

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J. S. Gift and D. F. Goldsmith Table 2. Key occupational studies

Reference

Study type3

Study population

Definition of silicosis

% Silica (quartz)

LRC

2235 South African miners; started after 1938 and worked > 10 years; followed to 1991.

313 cases of silicosis (ILO category > 1/1)

30

Muir et al. (1989a,b), Muir (1991), Verma et al. (1989)

LRC

2109 Canadian miners; started 1940-1959; followed to 1982 or end of exposure.

32 cases of silicosis (ILO category > 1/1)

6.0-8.4

Ng and Chan (1994)

XRC

338 Hong Kong granite workers; 132 past workers (1967-1985) and 206 current workers (1985); only most recent X-rays examined.

36 radiographical abnormalities, rounded opacities (ILO category > 1/1)

27

Rice et al. (1986)

CC

U.S. (North Carolina) dusty trades workers diagnosed with silicosis, 1935-1980.

216 cases of silicosis; 672 controls

1-50

Steenland and Brown (1995)

LRC

3330 South Dakota gold miners who worked at least 1 year underground between 1940 and 1965; followed to 1990.

170 cases of silicosis (ILO category > 1/1)

13

a CC = case control, L = longitudinal, RC = retrospective cohort, X = cross-sectional, Q = quartz.

Evaluation of key studies Only three of the studies described in Table 2 attempted to define exposureresponse relationships among silica-exposed miners at environmentally relevant levels: Hnizdo and Sluis-Cremer (1993), Muir et al. (1989b) and Steenland and Brown (1995). As can be seen from Fig. 1, the results of Muir et al. (1989b) are not consistent with data from other studies, possibly due to the lack of follow-up beyond retirement. While a recent comparison of X-ray and autopsy diagnosis (Hnizdo et al., 1993) suggests that X-rays diagnosis used by both Hnizdo and Sluis-Cremer (1993) and Muir et al. (1989b) may not be the most sensitive diagnostic method for determining a pulmonary fibrotic effect, Hnizdo and Sluis-Cremer's research was continuous and longitudinal, involving multiple X-ray examinations both before and beyond retirement. Further, autopsy results for the nearly complete cohort were available to Hnizdo and Sluis-Cremer (1993) to allow for more accurate interpretation of radiographic results, while death certificates were available for only half of the cohort from the Steenland and Brown (1995) study. Thus, Hnizdo and Sluis-Cremer (1993) study is considered the most appropriate for use in derivation of a BMC. RESULTS AND DISCUSSION

Results of our log-logistic regression analysis are presented in Table 3. Several factors which limit our confidence in this analysis are discussed here. First,

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Hnizdo and Sluis-Cremer (1993)

Respiratory health effects from ambient silica exposure

451

Hnizdo and Sluis-Cremer, 1993 - - Muiretal., 1989b X Steenland and Brown, 1995b • Ng and Chan eta I., 1994 Muir et al. (1989b) assessment ofThenauttetal, 1974

1 2 3 Cumulative Silica Exposure (mg/m3 x years)

4

Fig. 1. Cumulative risk curves estimated for South African gold miners (—) (Hnizdo and Sluis-Cremer, 1993) and Canadian hardrock miners (—) (Muir et al., 1989b); cumulative silica risk points estimated for South Dakota gold miners (X) (Steenland and Brown, 1995), Hong Kong granite workers (•) (Ng and Chan, 1994) and Vermont granite miners (•) (Muir et al., 1989b; Theriault et al., 1974).

radiographs are not a particularly reliable or sensitive diagnostic tool. Hnizdo and Sluis-Cremer (1993) found that 57% of cases were diagnosed an average of 7.4 years after work in mines. While they attempted to follow workers beyond employment, participation by retired workers was voluntary. Also, the best of radiographers did not diagnose silicosis in 61% of 326 cases categorized at autopsy as slight to marked silicosis (Hnizdo et al., 1993). Available exposure data are limited. All three studies had to estimate early exposures without high quality monitoring information. Further, peak exposures may be better predictor than cumulative silica exposure (Checkoway and Rice, 1992). In addition, differences Table 3. Risk estimates from application of log-logistic model to data of Hnizdo and Sluis-Cremer (1993)ab Risk for CSE of:

CSE for Risk of:

Best-fit estimates Lower bounds on CSEs Upper bounds on risks a

r%

5%

10%

0.6

1.0

1.6

1. 39 1. 31

1.97 1.90

2.30 2.24

0.019%

0.21%









0.032%

0.30%

1.9% — 2 .4%

See USEPA 1996, Appendix B for a discussion of methods used for estimating these risk levels. CSE = Cumulative silica exposure. CSEs are in mg m 3-years. Bounds are 95% lower bounds or 95% upper bounds, as appropriate.

b

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0

452

J. S. Gift and D. F. Goldsmith

between occupational and ambient exposure scenarios generally suggest that the mining environment may over estimate risk from ambient silica exposure. Silica in mining environment tends to be of smaller, more respirable size (Verma et al., 1994; Davis et al., 1984). More toxic, freshly fractured silica is more prevalent in occupational setting (Shoemaker et al., 1995). A higher fraction of silica has been reported in mining dust (e.g. 30% by Hnizdo et al., 1993) vs ambient upper-bound estimate of 10% (Davis et al., 1984). Further, miners may have used preventative measures such as masks and aluminium treatment (Muir et al., 1989b) not readily available to the general population. Finally, workers may be less susceptible to silicosis than the general population, including children, elderly and people with existing respiratory disease such as tuberculosis and pneumonia.

Using ambient silica measurements from Davis et al. (1984), average and high cumulative silica exposures (CSEs) of 0.6 and 1.6 mg silica m~3 years were estimated for 22 U.S. cities (U.S.EPA, 1996). Using a high estimate of 10% for silica fraction in PM10, 1 mg silica m~3 years is the highest expected from continuous lifetime exposure at or below the annual PM10 NAAQS of 50 um~3. The 95% upperbound confidence limits on the risk estimated for CSEs of 0.6,1 and 1.6 mg silica m~3 years were 0.032, 0.3 and 2.4%, respectively. Thus, even using conservative assumptions for example ambient environment not markedly different from mining environment with respect to duration of exposures, peak exposures, particle character and size distribution silica content in PM]0 at 10%; cumulative risk when PM10 levels are maintained at or below the 50 are estimated to be close to 0%. This assessment may not be relevant for people with other respiratory ailments and for dusty environments containing more than 10% silica. Disclaimer.—The views expressed in this paper are those of the author(s) and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. The U.S. Government has the right to retain a non exclusive royalty-free license in and to any copyright covering this article. REFERENCES Checkoway, H. and Rice, C. H. (1992) Time-weighted averages, peaks, and other indices of exposure in occupational epidemiology. Am. J. ind. Med. 21, 25-33. Davis, B. L., Johnson, L. R., Stevens, R. K., Courtney, W. J. and Safriet, D. W. (1984) The quartz content and elemental composition of aerosols from selected sites of the EPA inhalable particulate network. Atmos. Environ. 18, 771-782. Hnizdo, E., Murray, J., Sluis-Cremer, G. K. and Thomas, R. G. (1993) Correlation between radiological & pathological diagnosis of silicosos: an autopsy population based study. Am. J. ind. Med. 24, 427-445. Hnizdo, E. and Sluis-Cremer, G. K. (1993) Risk of silicosis in a cohort of white South African gold miners. Am. J. ind. Med. 24, 447-457. Muir, D. C. F. (1991) Correction in cumulative risk in silicosis exposure assessment. Am. J. ind. Med. 19, 555. Muir, D. C. F., Shannon, H. S., Julian, J. A., Verma, D. K., Sebestyen, A. and Bernholz, C. D. (1989a) Silica exposure and silicosis among Ontario hardrock miners: I. methodology. Am. J. ind. Med. 16, 5-11. Muir, D. C. F., Julian, J. A., Shannon, H. S., Verma, D. K., Sebestyen, A. and Bernholz, C. D.

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CONCLUSIONS

Respiratory health effects from ambient silica exposure

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(1989b) Silica exposure and silicosis among Ontario hardrock miners: III. analysis and risk estimates. Am. J. ind. Med. 16, 29-43. Ng, T. P. and Chan, S. L. (1994) Quantitative relations between silica exposure and development of radiological small opacities in granite workers. In Inhaled particles VII: Proceedings of an International Symposium (Edited by J. Dodgson and R. I. McCallum), September 1991, Edinburgh, U.K. Ann. occup. Hyg. 38 (Suppl. 1), 857-863. Rice, C. H., Harris, R. L., Jr, Checkoway, H. and Symons, M. J. (1986) Dose-response relationships for silicosis from a case-control study of N. Carolina dusty trades workers. In Silica, Silicosis and Cancer: Controversy in Occupational Medicine (Edited by D. F. Goldsmith, D. M. Winn and C. M. Shy), pp. 77-86. Praeger Publishers, New York. Sanchez-Roman, J., Wichmann, I., Salaberri, J., Varela, J. M. and Nunez-Roldan, A. (1993) Multiple clinical and biological autoimmune manifestations in 50 workers after occupational exposure to silica. Ann. Rheum. Dis. 52, 534-538. Shoemaker, D. A., Petty, J. R., Ramsey, D. M., McLaurin, J. L., Khan, A., Teass, A. W., Castranova, V., Pailes, W. H., Dalai, N. S., Miles, P. R., Bowman, L., Leonard, S., Shumaker, J., Vallyathan, V. and Pack, D. (1995) Particle activity and in vivo pulmonary response to freshly milled and aged alpha-quartz. Scan J. Work Environ. Health 21 (suppl. 2), 15-18. Steenland, K. and Brown, D. P. (1995) Mortality study of goldminers exposed to silica and nonasbestiform amphibole minerals: an update. Am. J. ind. Med. 27, 217-229. Steenland, K. and Goldsmith, D. F. (1995) Silica exposure and autoimmune diseases. Am. J. ind. Med. 28, 603-8. Theriault, G. P., Peters, J. M. and Jonson, W. M. (1974) Pulmonary function and roentgenographic changes in granite dust exposure. Arch. Environ. Health 28, 23-27. U.S. EPA. (1994) Methods for derivation of inhalation reference concentrations and application of inhalation dosimetry [draft final]. Research Triangle Park, NC, Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office, EPA/600/8-88/066F. U.S. EPA. (1996) Ambient levels and noncancer health effects of inhaled crystalline and amorphous silica [draft final]. Research Triangle Park, NC, Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office, report no. EPA/600/R-95/115. U.S. Federal Register (1996) National Ambient Air Quality Standards for Particulate Matter; Proposed Role. F.R. (December 13) 61 (241), 65638-65714. Verma, D. K., Sebestyen, A., Julian, J. A., Muir, D. C. F., Schmidt, H., Bernholz, C. D. and Shannon, H. S. (1989) Silica exposure and silicosis among Ontario hardrock miners: II. exposure estimates. Am. J. ind. Med. 16, 13-28. Verma, D. K., Sebestyen, A., Julian, J. A., Muir, D. C. F., Shaw, D. S. and MacDougall, R. (1994) Particle size distribution of an aerosol and its sub-fractions. Ann. occup. Hyg. 38, 45-58.