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Table 4 Estimated HRs for mortality, associated with combined exposure to silica and smoking (n = 7,665), 1960–2012

From: Combined effect of silica dust exposure and cigarette smoking on total and cause-specific mortality in iron miners: a cohort study

Cause of death (ICD-10)

Hazard ratio and 95% confidence interval

Interaction effect for additivity (RERI)b

Interaction factor on multiplicative scaleb

Unexposed workers

Exposed workers

Malignant neoplasm (C00-C97)

  Never smokers

1.000 (reference)

1.271 (0.923, 1.748)

0.306 (− 0.306, 0.835)

0.999 (0.995, 1.004)

  Ever smokers

1.602 (1.212, 2.119)

2.196 (1.679, 2.871)

 Lung cancer (C33-C34)

  Never smokers

1.000 (reference)

1.460 (0.698, 3.051)

1.893 (0.628, 3.441)

0.999 (0.993, 1.005)

  Ever smokers

3.840 (2.079, 7.094)

6.586 (3.616, 11.992)

Cardiovascular disease (I00-I52, I70-I99)

  Never smokers

1.000 (reference)

1.445 (0.979, 2.132)

−0.237 (− 1.062, 0.381)

1.002 (0.997, 1.007)

  Ever smokers

1.190 (0.828, 1.711)

1.709 (1.214, 2.405)

 Ischaemic heart disease (I20-I25)

  Never smokers

1.000 (reference)

1.487 (0.900, 2.455)

−0.144 (− 1.196, 0.592)

1.005 (0.998, 1.013)

  Ever smokers

1.049 (0.651, 1.690)

1.690 (1.083, 2.635)

Disease of the respiratory system (J00-J99)

  Never smokers

1.000 (reference)

3.488 (2.095, 5.808)

0.136 (− 1.784, 1.633)

1.002 (0.998, 1.006)

  Ever smokers

1.679 (0.996, 2.828)

5.052 (3.144, 8.116)

 Pneumoconiosis (J60-J65)a

  Never smokers

1.000 (reference)

3.319 (0.757, 14.560)

6.457 (0.725, 39.114)

1.003 (0.996, 1.009)

  Ever smokers

1.973 (0.429, 9.067)

8.658 (2.110, 35.530)

 Chronic lower respiratory disease (J40-J47)

  Never smokers

1.000 (reference)

3.200 (1.566, 6.536)

−1.170 (− 4.236, 0.488)

1.000 (0.993, 1.008)

  Ever smokers

2.142 (1.057, 4.341)

3.725 (1.901, 7.301)

All diseases (A00-Y98)

  Never smokers

1.000 (reference)

1.139 (0.989, 1.313)

0.158 (−0.136, 0.421)

1.002 (1.000, 1.004)

  Ever smokers

0.977 (0.858, 1.113)

1.344 (1.189, 1.520)

  1. HRs were calculated by using Cox proportional hazards models, which included the durations of follow-up as time variables and were adjusted for gender, year at hire (five categories: 1955 or earlier, 1956–1960, 1961–1965, 1966–1970, 1971 or later), age at hire (continuous)
  2. The dichotomous analyses were conducted on the basis of silica exposure (unexposed or exposed) and smoking status (never of ever), and the reference category was defined as the non-silica-exposed and non-smoking level, except pneumoconiosis
  3. aFor pneumoconiosis, the dichotomous analysis was conducted on the basis of silica exposure (lower or higher level divided by 0.65328 mg/m3-y) and smoking status (never of ever), and the reference category was defined as the lower-silica-exposed and non-smoking level
  4. bInteraction effects were not statistically significant, unless 0 was included into the confidence intervals of relative excess risk due to interaction (RERI), or 1 was included into the confidence intervals of interaction factor on multiplicative scales