Methods All employees (n = 3,924) aged 20–55 years in 24 Norwegian smelters and related workplaces were invited to participate in a longitudinal respiratory study. The study was conducted between 1996 and 2003. They were followed annually (16,570 observations) with spirometry and a respiratory questionnaire (Kongerud et al. 1989); details are explained elsewhere (Johnsen et al. 2008c; Soyseth et al. 2007). In smelters producing FeSi, Si-metal,

FeMn, SiMn, FeCr or SiC, measures of dust exposure using personal samplers were available. Therefore, the current study was limited to these smelters (n = 18). Accordingly, the number of employees was 3,084 and they underwent 12,996 examinations. The age distribution is shown in Table 1. Table 1 The prevalence of respiratory symptoms during the follow-up

  Examination no. Symptom, n (%) 1 2 3 4 5 6 Dyspnoea 708 (23.0) 605 (21.4) 475 (19.3) 398 (19.3) 301 (18.2) 151 BMN673 (16.8)  Unknown 47 (1.5) 74 (2.6) 76 (3.1) 69 (3.3) 15 (0.9) 4 (0.4) Wheezing 598 (19.4) 500 (17.7) 443 (18.0) 341 (16.5) 273 (16.5) 142 (15.8)  Unknown 55 (1.8) 76 (2.7) 76 (3.1) 69 (3.3) 15 (0.9) 4 (0.4) Cough C646 clinical trial without a cold 772 (25.0) 655 (23.2) 488 (19.9) 422 (20.4) 308 (18.6) 158 (17.6)  Unknown 76 (2.5) 101 (3.6) 101 (4.1) 82 (4.0) 26 (1.6) 8 (0.9) Cough >3 months last year 267 (8.7) 271 (9.6) 224 (9.1) 181 (8.8) 137 (8.3) 66 (7.3)  Unknown 82 (2.7) 106 (3.8) 103 (4.2) 85 (4.1) 29 (1.8) 8 (0.9) Phlegm when coughing 648 (21.0) 566 (20.0) 484 (19.7) 388 (18.8) 297 (18.0) 168 (18.7)  Unknown 139 (4.5) 144 (5.1) 126 (5.1) 97 (4.7) 40 (2.4) 13 (1.5) Dropouts  N 149 158 192 80 5 0  Symptom score, mean 1.24 1.16 1.04 0.98 0.95 0.90 On the respiratory questionnaire, the subjects were asked to report their symptoms during the last year. Symptom score was constructed as the sum of a confirmative answer (score = 1

if ‘yes’, 0 if ‘no’, otherwise ‘missing’) to the following questions: dyspnoea, wheezing, cough without a cold, daily cough for 3 months or longer and phlegm. Hence, in each subject, the symptom score was an integer between 0 and 5. The symptom score could vary within each individual during the follow-up. In case of URMC-099 missing value(s), the corresponding record was excluded. In total, 1,496 (12%) of the records (n = 12,996) were excluded from the analyses due to missing values. Allergy was considered to be present if the employee had a history Thymidine kinase of either hay fever or atopic eczema. Information about job category and smoking habits during the previous year was obtained from the questionnaire. Occupational exposure was assessed using a qualitative job classification and a quantitative job-exposure matrix (JEM). The qualitative job classification was constructed as follows: Employees working full time in the production line during the last year were classified as line operators, whereas employees who never worked in the production line during the last year were classified as non-exposed.