Longitudinal Changes in Prevalence of Respiratory Symptoms Among Canadian Grain Elevator Workers: Goodness of Fit

Longitudinal Changes in Prevalence of Respiratory Symptoms Among Canadian Grain Elevator Workers: Goodness of FitInitially four multivariable marginal models were fitted using different within-subject covariance structures for each of the respiratory symptoms and the within-subject covariance structure that provided minimum SEs for most of the covariates was selected in order to fit the final model (shown in Appendix). The exchangeable within-subject covariance structure was selected for model building because it provided the minimum standard errors for most of the covariates. Significant predictors were determined by using the Wald statistic.

Tables 2, 3 display the demographic description of the study population. The means and SEs for the demographic variables (age, height, and weight) and years in the grain industry are presented in Table 2. Grain workers in the Central region were the youngest in all the cycles. Age increased approximately 3 years only for the St. Lawrence and the Mountain regions between cycle 3 and cycle 4 and for the Central region between cycle 2 and cycle 3. For other regions, between any two consecutive cycles, the mean age either increased or decreased by > 3 years, indicating that workers were entering and leaving the industry. St. Lawrence workers were shorter in height and lighter in weight compared to the grain workers in other regions. Table 3 shows the maximum proportion of current smokers (60.8% at cycle 1) were in the St. Lawrence region, which declined to 26% by cycle 5.

When stratified by cycle, the prevalence of chronic respiratory symptoms reveals a possible trend over time with a maximum prevalence in cycle 2 and minimum in cycle 5, except for chronic wheeze, which had minimum prevalence in cycle 4 (Table 4). The prevalence of chronic respiratory symptoms in five cycles declined after dust control. In 1987, the occupational exposure limit of 10 mg/m on existing facilities was recommended and new facilities or remodeled facilities were required to achieve an exposure limit of 5 mg/m. Seventy percent of grain terminals/elevators had achieved dust control by 1986. It was reported that approximately 80% of all dust level measurements made during the grain dust medical surveillance program until 1986 were < 10 mg/m, and 60% of all dust level measurements were < 5 mg/m. Based on this information, the results in Table 5 are presented based on before and after dust control. The most common reported symptom was chronic cough (27.8%) among St. Lawrence grain workers followed by chronic sputum (20.9%) before dust control.

Table 2—Descriptive Statistics of Demographic Variables, Years in the Industry, and Lung Function Values Stratified by Region and Cycle

Cycles Regions!
Atlantic St. Lawrence Great Lakes Central Mountain Total
1
Age, yr 44.3 ± 1.49(70) 42.2 ± 0.50 (555) 36.1 ± 0.85 (260) 33.6 ± 0.21 (4,081) 35.4 ± 0.51 (734) 34.9 ± 0.18 (5,702)
Height, cm 174.5 ± 0.75 (70) 170.2 ± 0.30 (554) 175.0 ± 0.44 (260) 174.5 ± 0.10 (4,083) 177.7 ± 0.26 (734) 174.5 ± 0.09 (5,702)
Weight, kg 80.2 ± 1.62 (69) 76.8 ± 0.54 (555) 79.6 ± 0.80 (258) 81.4 ± 0.21 (4,083) 79.5 ± 0.43 (734) 80.6 ± 0.17 (5,682)
Time in industry, yr 12.2 ± 1.42 (46) 13.7 ± 0.50 (443) 11.0 ± 1.18 (85) 8.7 ± 0.24 (1,892) 5.6 ± 0.27 (572) 9.0 ± 10.05 (3,038)
2
Age, yr 40.3 ± 1.30(96) 42.4 ± 0.50 (570) 41.1 ± 0.95 (206) 34.8 ± 0.22 (3,644) 34.3 ± 0.41 (972) 35.9 ± 0.18 (5,488)
Height, cm 175.7 ± 0.76 (96) 171.6 ± 0.28(571) 175.8 ± 0.45 (207) 175.2 ± 0.11 (3,644) 176.9 ± 0.24 (973) 175.1 ± 0.09 (5,491)
Weight, kg 82.7 ± 1.66(95) 76.7 ± 0.51 (571) 80.7 ± 0.89 (206) 82.4 ± 0.23 (3,644) 79.0 ± 0.36 (973) 81.1 ± 0.18 (5,385)
Time in industry, yr 17.3 ± 1.99(43) 14.3 ± 0.45 (534) 14.4 ± 0.90 (171) 10.9 ± 0.17 (3,477) 9.8 ± 0.33 (774) 11.2 ± 0.14 (4,999)
3
Age, yr 52.6 ± 1.61 (15) 42.2 ± 0.74 (258) 41.0 ± 1.04 (119) 37.2 ± 0.25 (2,466) 35.4 ± 0.44 (855) 37.3 ± 0.21 (3,713)
Height, cm 171.0 ± 2.98(15) 170.9 ± 0.45 (258) 175.4 ± 0.62 (119) 174.8 ± 0.13 (2,466) 177.4 ± 0.23 (855) 175.1 ± 0.11 (3,713)
Weight, kg 81.8 ± 4.15 (15) 77.9 ± 0.82 (256) 83.1 ± 1.31 (118) 83.4 ± 0.27 (2,463) 80.5 ± 0.39 (855) 82.3 ± 0.21 (3,704)
Time in industry, yr 25.3 ± 2.54 (15) 14.5 ± 0.64 (258) 13.0 ± 0.82 (119) 12.9 ± 0.20 (2,466) 9.2 ± 0.32 (855) 12.2 ± 0.16 (3,713)
4
Age, yr N/A 45.2 ± 0.77 (182) 39.9 ± 1.83 (39) 36.8 ± 0.30 (1,347) 38.9 ± 0.31 (1,264) 38.3 ± 0.21 (2,832)
Height, cm N/A 171.6 ± 0.60(184) 175.6 ± 0.91 (50) 175.4 ± 0.18 (1,344) 177.2 ± 0.19 (1,264) 176.0 ± 0.13 (2,842)
Weight, kg N/A 76.8 ± 0.87 (183) 85.5 ± 1.86 (50) 84.0 ± 0.38 (1,342) 83.0 ± 0.34 (1,264) 83.1 ± 0.25 (2,837)
Time in industry, yr N/A 17.0 ± 0.73 (183) 14.7 ± 1.41 (43) 13.2 ± 0.25 (1,336) 13.2 ± 0.25 (1,264) 13.5 ± 0.17 (2,822)
5
Age, yr N/A 43.0 ± 1.00(141) 38.4 ± 3.40 (15) 36.5 ± 0.24 (1,909) 39.4 ± 0.31 (1,004) 37.7 ± 0.19 (3,070)
Height, cm N/A 172.5 ± 0.68 (41) 176.9 ± 2.11 (15) 175.9 ± 0.16 (1,902) 176.4 ± 0.23 (1,001) 175.9 ± 0.13 (3,060)
Weight, kg N/A 78.9 ± 1.09(140) 87.4 ± 4.62 (15) 85.9 ± 0.33 (1,896) 86.1 ± 0.43 (995) 85.6 ± 0.25 (3,047)
Time in industry, yr N/A 15.8 ± 1.01 (41) 13.5 ± 2.99 (15) 13.4 ± 0.20 (1,903) 14.1 ± 0.28 (999) 13.8 ± 0.16 (3,059)

Table 3—Smoking Status Stratified by Region and Cycle

Cycles Regions
Atlantic St. Lawrence Great Lakes Central Mountain Total
1
Nonsmokers 12 (17.4) 118 (21.5) 69 (26.6) 1,094 (27.2) 250 (34.4) 1,543 (27.4)
Ex-smokers 18(26.1) 97(17.7) 49(18.9) 858 (21.3) 203 (28.0) 1,225 (21.8)
Current smokers 39 (56.5) 334 (60.8) 141 (54.4) 2,068 (51.4) 273 (37.6) 2,855 (50.8)
2
Nonsmokers 18 (19.0) 143 (25.4) 46 (22.8) 1,045 (29.0) 360 (37.3) 1,612 (29.7)
Ex-smokers 24 (25.3) 127 (22.5) 57 (28.2) 892 (24.7) 228 (23.6) 1,328 (24.4)
Current smokers 53 (55.8) 294 (52.1) 99 (49.0) 1,670 (46.3) 378 (39.1) 2,494 (45.9)
3
Nonsmokers 5 (33.3) 53 (20.5) 29 (24.4) 705 (28.6) 392 (45.9) 1,184(31.9)
Ex-smokers 3 (20.0) 87 (33.7) 33 (27.7) 714 (29.0) 220 (25.7) 1,057 (28.5)
Current smokers 7 (46.7) 118 (45.7) 57 (47.0) 1,047 (42.5) 243 (28.4) 1,472 (39.6)
4
Nonsmokers N/A 75 (41.4) 12 (24.0) 433 (32.2) 383 (30.3) 903(31.8)
Ex-smokers N/A 52 (28.7) 18 (36.0) 371 (27.7) 403 (31.9) 845 (29.8)
Current smokers N/A 54 (29.8) 20 (40.0) 540 (40.2) 477 (37.8) 1,091 (38.4)
5
Nonsmokers N/A 66 (48.2) 4 (26.7) 674 (35.6) 323 (32.5) 1,067 (35.1)
Ex-smokers N/A 35 (25.7) 8 (53.3) 538 (28.4) 358 (36.0) 939 (30.9)
Current smokers N/A 36 (26.3) 3 (20.0) 682 (36.0) 313(31.5) 1,034 (34.0)

Table 4—Prevalence in Percentage of Respiratory Symptoms in Grain Workers Stratified by Cycle

Respiratory Symptoms Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5
Chronic wheeze 28.5 29.3 22.0 9.6 10.6
Chronic cough 28.8 31.9 15.7 12.6 11.0
Chronic dyspnea 29.3 29.2 17.1 13.4 11.1
Chronic sputum 27.1 29.7 18.3 13.4 11.5

Table 5—Prevalence in Percentage of Chronic Respiratory Symptoms Before (Cycle 1 Through Cycle 3) and After (Cycle 4 and Cycle 5) Dust Control

Regions Chronic Cough Chronic Wheeze Chronic Dyspnea Chronic Sputum
IBefore After Before After IBefore After Before After
Atlantic 32 (31.1) N/A 14 (13.6) N/A 14 (13.6) N/A 19(18.5) N/A
St. Lawrence 297 (27.8) 26 (8.4) 151 (14.2) 11 (3.6) 207 (19.4) 191 (16.1) 223 (20.9) 18 (5.8)
Great Lakes 52 (15.8) 5 (9.3) 36(11.0) 5 (9.3) 32 (9.9) 5 (9.4) 48(14.6) 7 (13.0)
Central 1,558 (16.9) 397 (12.4) 1,526 (16.6) 255 (8.0) 1,051 (11.5) 194 (6.1) 1,475 (16.0) 325 (10.1)
Mountain 279 (17.0) 291 (13.0) 171 (10.4) 209 (9.4) 225 (13.7) 256(11.5) 300 (18.2) 335 (15.0)
X2 (p value) 92.4 (< 0.0001) 5.97NS 47.7 (< 0.0001) 13.1 (< 0.01) 59.9 (< 0.0001) 52.7 (< 0.0001) 20.6 (< 0.001) 41.2 (< 0.0001)

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