Silica (crystalline, respirable) - OEHHA

FINAL February 2005
The last line of Appendix I of Page-Shipp and Harris (1972) gives a mean value for stopers of
1.57 (mg/m 3 )-hours respirable dust mass after acid treatment. Since the average work shift for
stopers was 7.8 hours (Page-Shipp and Harris, 1972, Table III, last row), the average exposure
level was 0.20 mg/m 3 . If 54% of this were quartz, the quartz level would be 0.11 mg/m 3 . Table
II of Hnizdo and Sluis-Cremer (1993) lists 0.37 mg/m 3 respirable dust for stopers. Thirty % of
0.37 mg/m 3 equals 0.11 mg/m 3 , the same value reported by Page-Shipp and Harris. In Table 4 of
Gibbs and Du Toit (2002) stopers are also reported to be exposed to 0.37 mg/m 3 respirable dust.
If 54% were quartz, as Gibbs and Du Toit contend, the quartz level would be 0.2 mg/m 3 . For 6
of the 9 categories of workers comprising 83% of the samples taken the silica levels correspond
more closely to values used by Hnizdo and Sluis-Cremer than to those suggested by Gibbs and
Du Toit.
Several more recent analyses of quartz content of South African mining rock have been reported
(Table 21). Kielblock et al. (1997) give the overall silica content of the dust as 15% for the late
1980s to early 1990s. Dr. Eva Hnizdo (personal communication, 2003), now with the U.S.
National Institute of Occupational Safety and Health (NIOSH), provided a summary of various
other estimates that have been made. “Past surveys indicate that the amount of airborne
respirable dust in SA gold mines in 1980's and in 1970's was on average around 0.4 mg/m 3 with
average quartz concentration of 0.08 mg/m 3 ” (about 20%). In a Ph.D. thesis submitted by the
late R.E.G. Rendall (1999) on dust in the air of gold mines, the silica percentage averaged 22%
during the period from 1964 to 1988. In summary,
(1) Notwithstanding some apparent contradictions in the various accounts, the silica
concentrations in air proposed by Hnizdo and Sluis-Cremer, based on the Corner House
Laboratory data, are a reasonable contemporary estimate of the exposures experienced by
the workers examined in the study by Hnizdo and Sluis-Cremer (1993).
(2) Other, more recent estimates of percent silica in the mine dust were lower than the value
of 30% used by Hnizdo and Sluis-Cremer (1993). Newer studies, which using more
sophisticated methods to measure silica in the dust, indicate lower silica concentrations in
the various occupational settings. Since dust levels in the mines were fairly constant for
decades and quantification of silica was improving, 30% is more likely to be an
overestimate than an underestimate of silica levels.
(3) Analysis of the data of Page-Shipp and Harris (1972) by OEHHA staff indicated that
Hnizdo and Sluis-Cremer (1993) used the correct silica content, despite an erroneous
statement in a footnote to Table II of their paper 1 .
1 Dr. Eva Hnizdo reviewed this analysis of the silica content of the dust and agrees with the
assessment. (“I am very pleased that you studied carefully all the reports and came to the
conclusion that our study was after all reasonably correct. Based on the Churchyard study and
the measurements data I have seen in SA during the 1990s, I am also convinced that our results
are reasonable estimates of the exposure of the cohort.” (Hnizdo, personal communication
October 2004)
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