Table 5

Incidence of seropositive rheumatoid arthritis in the cohort, grouped by yearly average exposure to airborne silica dust

Mean silica (mg/m³) per year*	n	Person-years	Observed	Expected	SIR	95% CI
	2187	23 689	18	10.6	1.70	1.01 to 2.69
0.012–0.023	546	6267	2	1.667	1.20	0.15 to 4.32
0.024–0.035	547	6144	1	2.35	0.43	0.01 to 2.37
0.036–0.047	547	5868	5	2.70	1.86	0.60 to 4.33
0.048+	547	5410	10	3.90	2.59	1.24 to 4.76

*Average annual silica exposure (mg/m³), divided into quartiles.
SIR, standardised incidence ratio.