Table 1

Distribution of hours worked by quartz concentration, % quartz, and dust concentration

	Profusion category						All
	0 (n=389)		1 (n=111)		2+ (n=47)
	Mean	%	Mean	%	Mean	%	Mean	%
Results expressed as the mean numbers of thousand hours per subject within each profusion category.
Exposure history time
Pre-1954	9.58		14.69		12.00		10.83
Post-1954	4.36		4.46		5.02		4.44

Attendance record time (post-1954)
	25.76	100	30.94	100	34.61	100	27.57	100
By quartz conc.
0–0.01	5.15	20	2.80	9.0	2.37	6.8	4.43	16
0.01–0.1	14.51	56	17.84	58	18.68	54	15.55	56
0.1–0.3	3.60	14	5.95	19	8.07	23	4.46	16
0.3–1.0	2.37	9.2	4.08	13	4.63	13	2.91	11
>1.0	0.13	0.5	0.27	0.9	0.87	2.5	0.22	0.8

By % quartz
0–5	16.66	65	17.34	56	18.76	54	16.98	62
5–10	5.49	21	8.55	28	9.47	27	6.45	23
10–15	2.67	10	3.33	11	3.28	9.5	2.86	10
15–20	0.61	2.4	0.93	3.0	1.51	4.4	0.75	2.7
>20	0.33	1.3	0.78	2.5	1.60	4.6	0.53	1.9

By dust conc. (mg.m⁻³)
0–1	12.51	49	11.36	37	10.92	32	12.14	44
1–2	7.65	30	9.47	31	9.12	26	8.15	30
2–3	3.52	14	5.90	19	7.40	21	4.34	16
3–5	1.86	7.2	3.58	12	5.83	17	2.55	9.3
>5	0.22	0.8	0.64	2.1	1.34	3.9	0.40	1.4

Missing concentrations	0.24		0.27		0.40		0.26