Table 3.

Hazard Ratios for Cumulative Silica Exposure Associated With Lung Cancer Death in the Absence of Silicosis, China, 1960–2003

Exposure Lag Period	No.	Continuous				Quartilesa, mg/m3-years
		1 mg/m3-years		1 ln(mg/m3- years + 1)		0.01 to <1.12			1.12 to <2.91			2.91 to <6.22			≥6.22
		HR	95% CI	HR	95% CI	No.	HR	95% CI	No.	HR	95% CI	No.	HR	95% CI	No.	HR	95% CI
No lag
Lung cancer deaths	427					89			75			70			65
Subjects at risk	28,721					7,451			4,757			3,300			2,823
Cumulative silica exposureb		1.02	0.99, 1.05	1.17	1.03, 1.34		1.02	0.77, 1.35		1.26	0.93, 1.70		1.40	1.02, 1.93		1.50	1.07, 2.13
15-Year lag
Lung cancer deaths	427					92			77			62			59
Subjects at risk	28,721					7,797			4,124			3,272			2,574
Cumulative silica exposureb		1.02	0.99, 1.06	1.18	1.03, 1.35		0.99	0.75, 1.29		1.42	1.06, 1.92		1.28	0.92, 1.78		1.65	1.16, 2.34
25-Year lag
Lung cancer deaths	427					102			63			65			41
Subjects at risk	28,721					8,325			3,478			3,140			1,970
Cumulative silica exposureb		1.03	1.00, 1.07	1.25	1.08, 1.44c		1.12	0.86, 1.46		1.41	1.03, 1.93		1.58	1.14, 2.19		1.70	1.15, 2.52

Abbreviations: CI, confidence interval; HR, hazard ratio.

^aCutpoints of cumulative silica exposure were defined by quartiles of cumulative silica exposure among lung cancer cases who were ever exposed to silica.

^bCumulative silica exposure was included in all models as a time-dependent variable. All Cox proportional hazards models were conducted by using risk sets with only observations that were not diagnosed as silicosis before the index age. All models were adjusted for sex, facility, year of birth (continuous, time-dependent) and smoking amount (categories of 0, 0.01 to <26.76, 26.76 to <34.93, 34.93 to <44.68, and ≥44.68 pack-years, time-dependent). Subjects without crystalline silica exposure were considered the reference category.

^cModel with lowest Akaike’s information criterion statistic.