Table 2.
Unadjusted estimates of GM benzene exposure in ppm, GM (95% CI), and GM ratio using the mixed-effect model. In addition, the minimum and maximum exposure in each group and arithmetic mean in ppm.
| Characteristic | N | % <LOD | Min–max | AM (95% CI) | GM (95% CI) | GM ratio (95% CI) |
|---|---|---|---|---|---|---|
| ppm | ||||||
| Overall exposure | 763 | 23 | <LOD–22.2 | 0.35 (0.26–0.43) | 0.02 (0.01–0.04) | |
| Work operation | ||||||
| Sampling | 355 | 29 | <LOD–11.7 | 0.32 (0.19–0.49) | 0.01 (0.01–0.03) | Reference |
| Disassembling/assembling | 154 | 8 | <LOD–22.2 | 0.50 (0.20–0.80) | 0.04 (0.02–0.10) | 3.56 (2.03–6.26) |
| Laboratory work | 71 | 49 | <LOD–1.50 | 0.07 (0.02–0.13) | 0.01 (0.002–0.01) | 0.38 (0.17–0.86) |
| Control of sand trap | 33 | 55 | <LOD–0.40 | 0.08 (0.38–0.12) | 0.01 (0.002–0.02) | 0.51 (0.17–1.54) |
| PIG operation | 28 | 0 | 0.03–10.0 | 0.59 (0.24-0.95) | 0.33 (0.09–1.14) | 26.72 (8.56–83.5) |
| Skimming | 23 | 13 | <LOD–0.71 | 0.13 (0.05–0.22) | 0.03 (0.01–0.12) | 2.48 (0.76–8.29) |
| Work on hydrocyclones | 19 | 21 | <LOD–0.62 | 0.14 (0.52–0.23) | 0.03 (0.01–0.11) | 2.06 (0.55-7.69) |
| Work on flotation cells | 16 | 6 | <LOD–2.33 | 0.73 (0.32–1.13) | 0.16 (0.03–0.82) | 13.05 (2.76–61.8) |
| Pressure release | 15 | 13 | <LOD–2.00 | 0.42 (0.11–0.72) | 0.03 (0.01–0.13) | 2.02 (0.41–9.89) |
| Other | 49 | 21 | <LOD–5.23 | 0.57 (0.22–0.91) | 0.03 (0.01–0.07) | 2.35 (1.00–5.49) |
| Source | ||||||
| Crude oil | 219 | 15 | <LOD–22.2 | 0.66 (0.37–0.94) | 0.04 (0.02–0.10) | Reference |
| Production water | 280 | 31 | <LOD–3.45 | 0.20 (0.14–0.25) | 0.01 (0.01–0.04) | 0.42 (0.25–0.71) |
| Wet glycol | 65 | 26 | <LOD–3.00 | 0.30 (0.16–0.44) | 0.04 (0.01–0.11) | 1.00 (0.40–2.46) |
| Natural gas | 39 | 26 | <LOD–2.00 | 0.22 (0.07–0.37) | 0.01 (0.003–0.03) | 0.27 (0.10–0.76) |
| Condensate | 21 | 10 | <LOD–2.10 | 0.32 (0.07–0.58) | 0.07 (0.02–0.31) | 1.74 (0.44–6.86) |
| Mixed benzene sources | 53 | 25 | <LOD–0.83 | 0.07 (0.03–0.11) | 0.01 (0.004–0.04) | 0.30 (0.12–0.70) |
| Other benzene sources | 86 | 24 | <LOD–3.70 | 0.31 (0.17–0.46) | 0.01 (0.004–0.03) | 0.24 (0.11–0.51) |
| Season | ||||||
| Winter | 431 | 28 | <LOD–22.2 | 0.36 (0.25–0.47) | 0.02 (0.01–0.04) | Reference |
| Summer | 332 | 21 | <LOD–11.7 | 0.33 (0.18–0.48) | 0.02 (0.01–0.05) | 1.29 (0.77–2.17) |
| Indoor/outdoor | ||||||
| Outdoor | 679 | 21 | <LOD–22.2 | 0.38 (0.28–0.48) | 0.02 (0.01–0.05) | Reference |
| Indoor | 84 | 43 | <LOD–1.51 | 0.08 (0.03–0.13) | 0.01 (0.02– 0.01) | 0.22 (0.11–0.45) |
| Design of process area | ||||||
| Restricted | 278 | 34 | <LOD–10.0 | 0.20 (0.13–0.26) | 0.02 (0.01–0.06) | Reference |
| Partially restricted | 385 | 17 | <LOD–22.2 | 0.45 (0.30–0.61) | 0.05 (0.01–0.15) | 2.71 (0.48–15.3) |
| Open | 100 | 24 | <LOD–11.5 | 0.35 (0.08–0.63) | 0.07 (0.002–0.03) | 0.43 (0.06–1.0) |
| Year of production start | ||||||
| 1979–1989 | 289 | 23 | <LOD–10.0 | 0.29 (0.21–0.38) | 0.03 (0.01–0.22) | Reference |
| 1990–1999 | 396 | 27 | <LOD–22.2 | 0.37 (0.22–0.51) | 0.02 (0.01–0.06) | 0.60 (0.11–3.40) |
| 2000–2018 | 78 | 19 | <LOD–4.50 | 0.44 (0.09–0.79) | 0.01 (0.001–0.05) | 0.24 (0.03–2.24) |
| Sampling method | ||||||
| Active | 743 | 25 | <LOD–22.2 | 0.34 (0.25– 0.43) | 0.05 (0.01–0.22) | Reference |
| Passive | 20 | 20 | <LOD–4.25 | 0.57 (−0.05–1.19) | 0.02 (0.01–0.04) | 2.52 (0.60–10.7) |
N, number of measurements; LOD, Limit of detection; AM, arithmetic mean; GM, geometric mean; CI, Confidence intervals.