Exposure-response analyses of butadiene or styrene ppm-years and leukaemia: number (N) of cases, adjusted rate ratio (RR) with 95% CI by exposure quartile, beta-coefficient (β) with 95% CI and trend p value
| Model* | Butadiene | Styrene | ||||
| N | RR | 95% CI | N | RR | 95% CI | |
| Quartile† | ||||||
| Unexposed | 29 | 1.0 | ref | 23 | 1.0 | ref |
| 1 | 26 | 1.04 | 0.60 to 1.83 | 27 | 1.12 | 0.61 to 2.07 |
| 2 | 26 | 1.37 | 0.76 to 2.46 | 27 | 1.12 | 0.59 to 2.11 |
| 3 | 25 | 1.60 | 0.87 to 2.94 | 28 | 1.79 | 0.93 to 3.45 |
| 4 | 26 | 2.53 | 1.37 to 4.67 | 27 | 1.96 | 1.00 to 3.82 |
| Trend | ||||||
| All person-time: β(95% CI), trend p value, AIC, N‡ | 2.55×10−4((0.52 to 4.57)×10−4), p=0.014, AIC=2384, N=132 | 1.04×10−3((−0.26 to 2.33)×10−3), p=0.116, AIC=2386, N=132 | ||||
| Exposed person-time: β(95% CI), trend p value, AIC, N | 2.50×10−4((0.27 to 4.73)×10−4), p=0.028, AIC=1786, N=103 | 0.84×10−3((−0.51 to 2.20)×10−3), p=0.220, AIC=1912, N=109 | ||||
| Exposed person-time ≤95th percentile§: β(95% CI), trend p value, AIC, N | 9.94×10−4((1.88 to 18.00)×10−4), p=0.016, AIC=1673, N=97 | 4.05×10−3((−1.19 to 9.30)×10−3), p=0.130, AIC=1795, N=103 | ||||
| Stratified¶ | ||||||
| Styrene <27.00 ppm-years | 1.34×10−4((−20.94 to 23.61)×10−4), p=0.907, AIC=1340, N=77 | |||||
| Styrene ≥27.00 ppm-years | 2.86×10−4((0.38 to 5.34)×10−4), p=0.024, AIC=870, N=55 | |||||
| Butadiene <121.28 ppm-years | −7.26×10−4((−120.19 to 105.67)×10−4), p=0.900, AIC=1415, N=80 | |||||
| Butadiene ≥121.28 ppm-years | 4.17×10−4((−12.45 to 20.78)×10−4), p=0.623, AIC=803, N=52 | |||||
*All models used attained age as of each day of follow-up as the time scale. Covariates were age at hire, year of hire, race, sex, plant and ever hourly status, except as follows: for the model ofbutadiene within the stratum ≥27.00 styrene ppm-years, sex was not included due to the lack of events among women; for the model of styrene within the stratum ≥121.28 butadiene ppm-years, sex and ever-hourly were not included due to the lack of events among women and never hourly workers.
†Quartile (Q) cutpoints were at the following values of ppm-years: (1) butadiene: Q2, 34.00; Q3, 121.28; Q4, 363.64, maximum=7741.41; (2) styrene: Q2, 5.76; Q3, 27.00; Q4, 60.53, maximum=1203.21.
‡AIC, Akaike information criterion; N, number of cases included in the model.
§95th percentile was at 1144 ppm-years for butadiene and 171 ppm-years for styrene.
¶Models used all person-time.