3.1 Comparison of the characteristics between high-temperature exposure and non-high-temperature exposure petrochemical workers
Table 1 shows the distribution of sociodemographic characteristics and other occupational hazards in the workers with and without occupational heat exposure. “Overall” is the summary of all health examination records, “between” is the between-group statistic (between individuals), and “within” is within the group (the same individual in different periods) (26). The results showed that there were significant differences in gender, age, and smoking status among workers with and without heat exposure, while no significant differences were observed in BMI and alcohol consumption. Workers exposed to heat in the workplace had a significantly higher proportion of being exposed to gasoline, nitrogen oxides, sulfur dioxide, hydrogen sulfide, benzene, methanol, and carbon monoxide, compared to those without occupational heat exposure.
Table 1. Characteristics of occupational heat exposure by demographic factors among petrochemical workers
|
Variable
|
Heat exposure
|
Non-heat exposure
|
P-value
|
Overall n(%)
|
Between n(%)
|
Within %
|
Overall n(%)
|
Between n(%)
|
Within %
|
Gender
|
|
|
|
|
|
|
|
Male
|
10090 (85.0)
|
4508 (83.6)
|
100.0
|
14130 (74.5)
|
4273 (78.5)
|
100.0
|
<0.001
|
Female
|
1782 (15.0)
|
882 (16.4)
|
100.0
|
4845 (25.5)
|
1172 (21.5)
|
100.0
|
Age group (years)
|
|
|
|
|
|
|
≦24
|
2351 (19.8)
|
1547 (28.7)
|
85.2
|
1354 (7.1)
|
914 (16.8)
|
78.2
|
<0.001
|
25-34
|
3865 (32.6)
|
2021 (37.5)
|
86.3
|
4127 (21.8)
|
1921 (35.3)
|
86.6
|
35-54
|
5267 (44.4)
|
2233 (41.4)
|
96.2
|
12256 (64.6)
|
2945 (54.1)
|
93.7
|
≥55
|
387 (3.3)
|
205 (3.8)
|
83.9
|
1231 (6.5)
|
407 (7.5)
|
74.6
|
BMI (kg/m2)
|
|
|
|
|
|
|
|
<24
|
6451 (54.3)
|
3251 (60.3)
|
93.0
|
10316 (54.4)
|
3341 (61.4)
|
87.9
|
0.961
|
≥24
|
5421 (45.7)
|
2588 (48.0)
|
91.4
|
8659 (45.6)
|
2919 (53.6)
|
85.9
|
Smoking
|
|
|
|
|
|
|
|
No
|
6986 (67.0)
|
3097 (71.2)
|
96.4
|
10548 (66.0)
|
2927 (69.7)
|
96.0
|
0.008
|
Occasionally
|
1447 (13.9)
|
798 (18.3)
|
74.5
|
2132 (13.3)
|
753 (17.9)
|
73.7
|
Heavy
|
2002 (19.2)
|
883 (20.3)
|
87.6
|
3312 (20.7)
|
960 (22.9)
|
86.7
|
Drinking
|
|
|
|
|
|
|
|
No
|
5220 (50.6)
|
2498 (58.2)
|
90.0
|
7885 (49.8)
|
2339 (56.5)
|
88.0
|
0.187
|
Yes
|
5091 (49.4)
|
2256 (52.6)
|
90.6
|
7951 (50.2)
|
2301 (55.6)
|
90.5
|
Gasoline
|
|
|
|
|
|
|
|
Yes
|
7236 (61.0)
|
3666 (68.0)
|
92.2
|
5078 (26.8)
|
2426 (44.6)
|
77.7
|
<0.001
|
No
|
4636 (39.1)
|
2334 (43.3)
|
86.1
|
13897 (73.2)
|
4024 (73.9)
|
88.5
|
Oxynitrides
|
|
|
|
|
|
|
|
Yes
|
1516 (12.8)
|
1128 (20.9)
|
72.6
|
620 (3.3)
|
526 (9.7)
|
63.1
|
<0.001
|
No
|
10356 (87.2)
|
4790 (88.9)
|
95.4
|
18355 (96.7)
|
5227 (96.0)
|
97.8
|
SO2
|
|
|
|
|
|
|
|
Yes
|
1313 (11.1)
|
797 (14.8)
|
79.2
|
647 (3.4)
|
476 (8.7)
|
67.1
|
<0.001
|
No
|
10559 (88.9)
|
4886 (90.7)
|
97.4
|
18328 (96.6)
|
5220 (95.9)
|
98.2
|
H2S
|
|
|
|
|
|
|
|
Yes
|
6713 (56.5)
|
3688 (68.4)
|
89.8
|
5248 (27.7)
|
2600 (47.8)
|
67.9
|
<0.001
|
No
|
5159 (43.5)
|
2426 (45.0)
|
85.7
|
13727 (72.3)
|
4326 (79.5)
|
85.1
|
Benzene
|
|
|
|
|
|
|
|
Yes
|
3107 (26.2)
|
2093 (38.8)
|
81.4
|
3208 (16.9)
|
1579 (29.0)
|
71.6
|
<0.001
|
No
|
8765 (73.8)
|
3996 (74.1)
|
92.3
|
15767 (83.1)
|
4616 (84.8)
|
93.5
|
Methanol
|
|
|
|
|
|
|
|
Yes
|
2336 (19.7)
|
1502 (27.9)
|
84.0
|
1459 (7.7)
|
664 (12.2)
|
70.3
|
<0.001
|
No
|
9536 (80.3)
|
4321 (80.2)
|
95.6
|
17516 (92.3)
|
5129 (94.2)
|
97.1
|
CO
|
|
|
|
|
|
|
|
Yes
|
3967 (33.4)
|
2485 (46.1)
|
81.2
|
2225 (11.7)
|
1392 (25.6)
|
65.7
|
<0.001
|
No
|
7905 (66.6)
|
3756 (69.7)
|
89.8
|
16750 (88.3)
|
4824 (88.6)
|
93.9
|
Total
|
11872 (38.5)
|
5390 (78.0)
|
59.8
|
18975 (61.5)
|
5445 (78.8)
|
67.8
|
3.2 Comparison of dyslipidemia between heat exposure and non-heat exposure petrochemical workers
Table 2 shows the distribution of the four types of dyslipidemia among petrochemical workers with/without heat exposure. Overall, the proportions of HTC, HTG, HLDL-C, and LHDL-C among heat exposure workers were 3.7%, 10.6%, 6.3%, and 6.3%, respectively. By contrast, the overall proportions of HTC (6.0%), HTG (19.3%), HLDL-C (9.4%), and LHDL-C (9.4%) in non-heat exposure workers were higher than their heat exposure workmates. Differences in TC, TG, LDL-C, and HDL-C between heat and non-heat exposure groups were statistically significant. As to the variation across individuals, the proportions of HTC (8.3% vs. 5.1%), HTG (19.9% vs. 12.4%), HLDL-C (12.8% vs. 9.2%), and LHDL-C (12.8% vs. 9.2) were also higher in the non-heat exposure workers than that in the heat exposure workers.
Table 2. Blood lipid disorders by heat exposure among petrochemical workers
|
Variable
|
Heat exposure
|
Non-heat exposure
|
P-value
|
Overall n (%)
|
Between n (%)
|
Within %
|
Overall n (%)
|
Between n (%)
|
Within %
|
High TC
|
|
|
|
|
|
|
|
Yes
|
444 (3.7)
|
274 (5.1)
|
66.3
|
1135 (6.0)
|
453 (8.3)
|
45.8
|
<0.001
|
No
|
11428 (96.3)
|
5287 (98.1)
|
98.5
|
17840 (94.0)
|
5377 (98.8)
|
97.4
|
High TG
|
|
|
|
|
|
|
|
Yes
|
1258 (10.6)
|
667 (12.4)
|
72.8
|
3662 (19.3)
|
1086 (19.9)
|
59.7
|
<0.001
|
No
|
10614 (89.4)
|
5068 (94.0)
|
96.8
|
15313 (80.7)
|
5139 (94.3)
|
93.3
|
High LDL-C
|
|
|
|
|
|
|
|
Yes
|
752 (6.3)
|
494 (9.2)
|
62.9
|
1775 (9.4)
|
697 (12.8)
|
46.2
|
<0.001
|
No
|
11120 (93.7)
|
5228 (97.0)
|
97.2
|
17200 (90.7)
|
5350 (98.3)
|
95.8
|
Low HDL-C
|
|
|
|
|
|
|
|
Yes
|
752 (6.3)
|
494 (9.2)
|
62.9
|
1775 (9.4)
|
697 (12.8)
|
46.2
|
<0.001
|
No
|
11120 (93.7)
|
5228 (97.0)
|
97.2
|
17200 (90.7)
|
5350 (98.3)
|
95.8
|
3.3 One-factor exposure model to analyze the association between occupational hazards and dyslipidemia
Figure 1 shows the effects of heat, gasoline, nitrogen oxides, sulfur dioxide, hydrogen sulfide, benzene, methanol, and carbon monoxide on blood lipid levels in petrochemical workers. Results of univariate exposure models showed that gasoline (IRR=0.563, 95%CI 0.670-0.473), oxynitrides (IRR=0.595, 95%CI 0.439-0.807), sulfur dioxide (IRR=0.741, 95%CI 0.552-0.993), hydrogen sulfide (IRR=0.784, 95%CI 0.682-0.901), and methanol (IRR=0.774, 95%CI 0.611-0.980) were negatively associated with TC. Negative effects were also observed in TG, including heat exposure (IRR=0.787, 95%CI 0.835-0.742) and other seven occupational hazards. Similarly, LDL-C was negatively associated with heat exposure (IRR=0.863, 95%CI 0.795-0.937), gasoline (IRR=0.548, 95%CI 0.629-0.478), oxynitrides (IRR=0.281, 95%CI 0.186-0.423), sulfur dioxide (IRR=0.671, 95%CI 0.513-0.878), hydrogen sulfide (IRR=0.789, 95%CI 0.707-0.881), benzene (IRR=0.779, 95%CI 0.666-0.912) and methanol (IRR=0.746, 95%CI 0.604-0.922). By contrast, heat exposure (IRR=0.994, 95%CI 0.989-0.998), gasoline (IRR=0.994, 95%CI 0.989-0.998), oxynitrides (IRR=0.990, 95%CI 0.985-0.994), sulfur dioxide (IRR=0.993, 95%CI 0.987-0.999), methanol (IRR=0.993, 95%CI 0.986-0.999) and carbon monoxide (IRR=0.992, 95%CI 0.986-0.998) were negatively associated with HDL-C.
3.4 Two-factor exposure model to analyze the association between heat combined with other occupational hazards and dyslipidemia
Table 3 shows that heat exposure coupled with gasoline could increase the risk of high cholesterol (IRR=1.267, 95%CI 1.117-1.437). Similarly, heat exposure coupled with hydrogen sulfide could increase the risk of high cholesterol (IRR=1.324, 95%CI 1.166-1.505). However, workplace heat exposure combined with nitrogen oxides, sulfur dioxide, benzene, methanol, carbon monoxide had no significant impact on the four types of dyslipidemia.
Table 3. Effects of heat exposure on blood lipid levels among petrochemical workers, using multi-exposure models
|
|
IRR (95%CI)
|
|
TC
|
TG
|
LDL-C
|
HDL-C
|
Heat+gasoline
|
|
|
|
|
Heat
|
1.267 (1.117-1.437)
|
0.847 (0.788-0.911)
|
1.077 (0.976-1.187)
|
0.997 (0.990-1.003)
|
Gasoline
|
0.644 (0.529-0.785)
|
0.583 (0.527-0.645)
|
0.667 (0.578-0.769)
|
0.997 (0.989-1.006)
|
Heat+Oxynitrides
|
|
|
|
|
Heat
|
1.073 (0.962-1.198)
|
0.800 (0.753-0.851)
|
0.897 (0.824-0.977)
|
0.993 (0.989-0.998)
|
Oxynitrides
|
0.807 (0.484-1.343)
|
0.798 (0.637-1.000)
|
0.414 (0.214-0.803)
|
0.983 (0.974-0.991)
|
Heat+SO2
|
|
|
|
|
Heat
|
1.077 (0.966-1.200)
|
0.812 (0.764-0.863)
|
0.918 (0.844-0.998)
|
0.993 (0.989-0.998)
|
SO2
|
1.002 (0.649-1.545)
|
1.041 (0.848-1.278)
|
1.307 (0.984-1.734)
|
0.988 (0.977-0.999)
|
Heat+H2S
|
|
|
|
|
Heat
|
1.324 (1.166-1.505)
|
0.865 (0.808-0.926)
|
1.076 (0.974-1.188)
|
0.994 (0.988-1.000)
|
H2S
|
0.923 (0.792-1.076)
|
0.836 (0.775-0.901)
|
0.945 (0.842-1.060)
|
0.997 (0.989-1.004)
|
Heat+Benzene
|
|
|
|
|
Heat
|
1.059 (0.948-1.182)
|
0.782 (0.734-0.833)
|
0.863 (0.790-0.942)
|
0.993 (0.988-0.999)
|
Benzene
|
1.004 (0.823-1.225)
|
0.861 (0.774-0.957)
|
0.777 (0.648-0.932)
|
0.994 (0.985-1.003)
|
Heat+Methanol
|
|
|
|
|
Heat
|
1.092 (0.979-1.217)
|
0.781 (0.733-0.831)
|
0.906 (0.831-0.986)
|
0.994 (0.989-0.998)
|
Methanol
|
0.951 (0.709-1.277)
|
0.723 (0.609-0.857)
|
0.975 (0.772-1.232)
|
0.992 (0.981-1.003)
|
Heat+CO
|
|
|
|
|
Heat
|
1.119 (0.998-1.126)
|
0.798 (0.749-0.849)
|
0.920 (0.842-1.006)
|
0.992 (0.987-0.998)
|
CO
|
1.126 (0.924-1.372)
|
0.888 (0.806-0.979)
|
1.068 (0.907-1.259)
|
0.989 (0.981-0.998)
|
3.5 Stratified analysis to identify vulnerable sub-groups
To further analyze the effect of workplace heat exposure on high cholesterol among petrochemical workers and identify vulnerable sub-groups, stratified analyses by gender, age, smoking and drinking habits were conducted. As shown in Table 4, we found that female petrochemical workers (IRR=2.240, 95%CI 1.639-3.062), aged ≥35 years (IRR=1.317, 95%CI 1.158-1.497), without smoking (IRR=1.536, 95%CI 1.285-1.836) and drinking habits (IRR=1.587, 95%CI 1.276-1.973) were at high risk of high cholesterol if exposed to heat and gasoline in the workplace. Moreover, exposure to heat and hydrogen sulfide may increase the risk of high cholesterol among female petrochemical workers (IRR=2.347, 95%CI 1.736-3.173), aged ≥35 years (IRR=1.385, 95%CI 1.215-1.578), without smoking (IRR=1.602, 95%CI 1.344-1.911) and drinking habits (IRR=1.647, 95%CI 1.323-2.051).