Table 1 Baseline characteristics of the cohort (n = 4315) by lung function categories

Subjects No. (%)

4315 (100.0)

2129 (49.3)

473 (11.0)

1148 (26.6)

565 (13.1)

Age

 18–34

36 (0.8)

28 (1.3)

2 (0.4)

1 (0.1)

5 (0.9)

< 0.001

 35–49

1215 (28.2)

814 (38.2)

146 (30.9)

183 (15.9)

72 (12.7)

 50–64

2140 (49.6)

1013 (47.6)

232 (49.0)

607 (52.9)

288 (51.0)

 ≥ 65

924 (21.4)

274 (12.9)

93 (19.7)

357 (31.1)

200 (35.4)

Sex

 Male

4287 (99.4)

2118 (99.5)

467 (98.7)

1145 (99.7)

557 (98.6)

0.01

 Female

28 (0.6)

11 (0.5)

6 (1.3)

3 (0.3)

8 (1.4)

BMI categories

 Underweight (< 18.5)

371 (8.6)

87 (4.1)

73 (15.4)

83 (7.2)

128 (22.7)

< 0.001

 Normal (18.5–22.9)

2132 (49.4)

1016 (47.7)

227 (48.0)

623 (54.3)

266 (47.1)

 Overweight (23-24.9)

942 (21.8)

530 (24.9)

80 (16.9)

251 (21.9)

81 (14.3)

 Obese (≥ 25)

870 (20.2)

496 (23.3)

93 (19.7)

191 (16.6)

90 (15.9)

Smoking status

 Never smoker

493 (11.4)

288 (13.5)

71 (15.0)

78 (6.8)

56 (9.9)

< 0.001

 Current smoker

1965 (45.5)

1018 (47.8)

181 (38.3)

569 (49.6)

197 (34.9)

 Former smoker

1857 (43.0)

823 (38.7)

221 (46.7)

501 (43.6)

312 (55.2)

Pack-years

 Never smoker

493 (11.5)

288 (13.6)

71 (15.0)

78 (6.8)

56 (10.0)

< 0.001

 Below 20

1455 (33.9)

820 (38.7)

151 (31.9)

338 (29.6)

146 (26.0)

 20 to 39

1408 (32.8)

675 (31.8)

158 (33.4)

405 (35.5)

170 (30.2)

 40 or above

942 (21.9)

338 (15.9)

93 (19.7)

321 (28.1)

190 (33.8)

History of tuberculosis

 Yes

1951 (45.2)

812 (38.1)

254 (53.7)

560 (48.8)

325 (57.5)

< 0.001

 No

2364 (54.8)

1317 (61.9)

219 (46.3)

588 (51.2)

240 (42.5)

Shape of small nodules

 Round

2873 (67.3)

1483 (70.3)

290 (62.2)

802 (70.5)

298 (54.0)

< 0.001

 Irregular

1393 (32.7)

628 (29.7)

176 (37.8)

335 (29.5)

254 (46.0)

Size of small nodules

 Category p or s

1796 (42.1)

984 (46.6)

152 (32.6)

485 (42.7)

175 (31.7)

< 0.001

 Category q or t

2092 (49.0)

1022 (48.4)

256 (54.9)

531 (46.7)

283 (51.3)

 Category r or u

378 (8.9)

105 (5.0)

58 (12.4)

121 (10.6)

94 (17.0)

Profusion of small nodules

 Category 1 (1/0, 1/1, 1/2)

2384 (55.9)

1284 (60.9)

162 (34.7)

674 (59.4)

264 (48.0)

< 0.001

 Category 2 (2/1, 2/2, 2/3)

1528 (35.9)

719 (34.1)

217 (46.5)

382 (33.7)

210 (38.2)

 Category 3 (3/2, 3/3, 3/+)

350 (8.2)

107 (5.1)

88 (18.8)

79 (7.0)

76 (13.8)

Progressive massive fibrosis

 No (small opacities only)

3516 (82.2)

1853 (87.7)

352 (75.1)

900 (78.9)

411 (73.8)

< 0.001

 Yes (with large opacity)

763 (17.8)

259 (12.3)

117 (24.9)

241 (21.1)

146 (26.2)

Respiratory symptoms

 Cough

2903 (67.3)

1386 (65.1)

303 (64.3)

806 (70.3)

408 (72.2)

< 0.001

 Dyspnoea

3366 (78.0)

1536 (72.1)

387 (82.0)

931 (81.2)

512 (90.6)

< 0.001

 Sputum

2540 (58.9)

1176 (55.2)

252 (53.4)

734 (64.0)

378 (66.9)

< 0.001

 Chest pain

1444 (33.5)

807 (37.9)

134 (28.4)

359 (31.3)

144 (25.5)

< 0.001

 Wheeze

738 (17.1)

253 (11.9)

58 (12.3)

287 (25.0)

140 (24.8)

< 0.001

 Haemoptysis

416 (9.7)

179 (8.4)

43 (9.1)

125 (10.9)

69 (12.2)

0.02

Duration of silica exposure (year)

23.5 (11.3)

22.5 (10.9)

22.9 (11.5)

25.5 (11.2)

23.8 (11.9)

< 0.001

Cumulative silica exposure (mg/m³·year)

1.80 (0.86)

1.74 (0.84)

1.76 (0.89)

1.93 (0.85)

1.80 (0.91)

< 0.001