Trends in silicosis prevalence and the healthy worker effect among gold miners in South Africa: a prevalence study with follow up of employment status

Table 3 Comparison of prevalence of silicosis by duration: 2000 study [12, 13] versus current study (ILO grade ≥1/1)

2000 study (≥37 years of age)				Current study (≥40 years of age)^a
Duration years^b	N %	Cases	Silicosis prevalence % (95 % CI)^c	Duration years^b	N %	Silicosis prevalence (reader I) % (n-cases) (95 % CI)^c	Silicosis prevalence (combined reading) %
≤20	179	14	7.8 (4.3, 12.8)	≤20	48	12.5 (6)(4.7, 25.2)	8.3
21+	331	79	23.9 (19.4, 28.8)	21+	182	20.3 (37)(14.7, 26.9)	14.4
Crude	510^d	93	18.2 (15.0, 21.9)	Overall	230	18.7 (43)(13.9, 24.3)	13.0
Adjusted^e			20.5

CI confidence interval. All figures rounded to one decimal
^aFor comparability, restricted to participants in this study from the same shaft and ≥40 years of age (N = 230)
^bDuration of exposure (2000 study), duration since first employment (current study)
^c95 % confidence intervals using binomial exact standard errors
^dNumbers used to derive Fig. 1 in 2000 study obtained from authors
^eStandardised to duration distribution of current study and the same shaft

ISSN: 1471-2458