Chang et al. collected by an audiologist who was blinded to the participants’ subject group. All participants received pure-tone audiometry tests with a Beltone 2000 audiometer (Beltone Co., Chicago, IL, USA). Both ears were tested using the method of ascending at 1, 2, 3, 4, and 6 kHz and then descending to 1 and 0.5 kHz, following IOSH (1999) requirements; the test for 1 kHz was repeated. Frequency spectrum calibration in decibel hearing level fulﬁlled the International Organization for Standardization (ISO) 8253-1 criteria for audiometric testing environment (ISO 1989) adapted to the American National Standards Institute (ANSI) S 3.6-1968 requirement (ANSI 1970). Each person received the test 14 hr after the end of the previous work day. Daily calibration checks were conducted before subjects were tested. On-site environmental noise levels for areas of the three study groups were assessed using sound pressure level meters (model B&K 2260; Bruel and Kjaer, Naerum, Denmark) based on the Taiwan Council of Labor require- ments (IOSH 1999). Noise levels were meas- ured in various locations throughout the study areas as well as in the same locations where air samples were collected for the toluene-exposed divisions. The measurements showed noise lev- els ranging from < 70 dB(A) to 90 dB(A) among the study areas. Most of the noise in the toluene plus noise group and the noise- only group was continuous. Time-weighted averages of noise levels were calculated for each group. Electroacoustic calibration was per- formed before data collection. Statistical analyses. We compared selected sociodemographic and lifestyle variables among the three study groups to identify potential confounding factors. The prevalence of hearing loss was calculated in percentage distribution based on the worse ear. The prevalence of hearing loss ≥ 25 dB and age- adjusted odds ratios (ORs) and 95% confi- dence intervals (CIs) of hearing loss ≥ 25 dB Table 1. Selected characteristics of the study population [no. (%)] by study group. Exposure group Toluene + noise (n = 58) Reference group Noise only (n = 58) Administrative (n = 60) 40.0 ± 9.7 22 (37.9) 29 (50.0) 7 (12.1) 26 (44.8) 30 (51.7) 2 (3.4) 16 (25.8) 43 (74.1) 12.3 ± 8.81 24 (41.4) 18 (31.0) 16 (27.6) 18 (31.0) 40 (69.0) 0 (0.0) 34 (58.6) 17 (29.3) 7 (12.1) 83.9 ± 1.3 78.6–87.1 8 (13.8) 41.5 ± 3.1 16 (27.6) 38 (65.5) 4 (6.9) 24 (41.4) 30 (51.7) 4 (6.9) 4 (6.9) 54 (93.1) 11.5 ± 5.73 22 (37.9) 31 (53.4) 5 (8.6) 19 (32.8) 34 (58.6) 5 (8.6) 25 (43.1) 24 (41.4) 9 (15.5) 85.0 ± 4.2 83.5–90.1 7 (12.1) 40.9 ± 3.4 24 (40.0) 32 (53.3) 4 (6.7) 16 (26.6) 14 (23.3) 30 (50.1) 7 (11.7) 53 (88.3) 9.52 ± 5.26 27 (45.0) 30 (50.0) 3 (5.0) 20 (33.3) 36 (60.0) 4 (6.7) 27 (45.0) 28 (46.7) 5 (8.3) 70.0 ± 1.1 67.9–72.6 0 Variable Age (years)a < 40 40–49 ≥ 50 Education (years) ≤ 9 10–12 ≥ 13 Marital status Unmarried Married Employment (years)a 1–9 10–19 ≥ 20 Smoking tobacco No Yes Quit Drinking alcohol No Yes Quit Noise level dB(A)a Range Use hearing protection aMean ± SD. Exposure status Administrative Noise only Toluene/noised 33.0/83.2 107.6/84.1 164.6/84.1 No. 60 58 58 13 22 23 1284 Table 2. Mean ± SD, prevalence, and corresponding age-adjusted OR for hearing loss ≥ 25 dB in study groups by model. Model 1 Mean ± SD (%)b 14.6 ± 6.01 (5.0)* 26.2 ± 11.1 (44.8)* 29.8 ± 6.8 (86.2)* 30.9 ± 7.8 (84.6) 29.9 ± 7.9 (86.4) 29.1 ± 5.1 (87.0) OR (95% CI)c 1.0 15.4 (4.3 –54.9) 119 (29.8–471) 104 (15.6 –699) 120 (22.3–646) 127 (23.6 –678) Model 2a Mean ± SD (%)b 14.6 ± 6.4 (8.3)* 23.9 ± 11.8 (32.8)* 27.7 ± 7.9 (67.2)* 28.1 ± 8.3 (76.9) 28.1 ± 9.5 (72.7) 27.1 ± 6.0 (56.5) OR (95% CI)c 1.0 5.4 (1.8–15.6) 22.6 (7.8–65.6) 36.7 (7.5–178) 29.3 (7.9–109) 14.3 (4.2–49.0) aPure tone of 0.5 kHz was excluded. bPrevalence of hearing loss ≥ 25 dB. cOR of hearing loss ≥ 25 dB. dToluene levels given in ppm, and noise levels given in dB(A). *p < 0.001 for comparison between any two groups. p-Value 0.418 < 0.001 0.023 0.071 0.794 0.335 < 0.001 < 0.001 were estimated for the toluene plus noise and noise-only groups, using the administrative workers as the reference. Workers in the toluene plus noise group were also stratified by toluene exposure levels (average, 33.0 ppm in toluene recovery division, 107.6 ppm in the adhesive materials manufacturing divi- sion, 164.6 ppm in the adhesive application division). Average hearing loss levels were also calculated for each group. Because of a less reli- able threshold for 0.5 kHz, we calculated the prevalence of and the OR for hearing loss ≥ 25 dB among the study groups using two models: the pure tone of 0.5 kHz was included in model 1 but excluded from model 2. To differentiate the pure-tone impact among study subjects, we also plotted the mean hearing thresholds at frequencies of 1, 2, 3, 4, and 6 kHz and compared these plots among the study groups (Morata et al. 2002; Sliwinska-Kowalska et al. 2004). The toluene plus noise group was further stratiﬁed into sub- groups based on the environmental noise levels [< 85 dB(A) and ≥ 85 dB(A)]. In order to estimate the dose–response effect of toluene on hearing loss for workers exposed to toluene plus noise, we calculated the cumulative exposure index (CEI) of toluene for each person in this group. The CEI was the product of the average toluene level in each division multiplied by the years of employment given as year-ppm. For example, an individual who had worked in the division for 10 years and had an average toluene level of 164.6 ppm received a 1,646 year-ppm cumula- tive exposure. We estimated and plotted the prevalence rates of hearing loss at 25–39, 40–54, and ≥ 55 dB and the mean hearing loss by stratiﬁed CEI. To estimate the potential exposure thresh- old leading to a significant hearing loss, multivariate logistic analysis was performed to evaluate the dose–response effect based on CEI quartile distribution. Results of compar- isons are reported with the statistical signifi- cance set at the 0.05 level. Data analyses were performed with SAS software (version 8.2; SAS Institute Inc., Cary, NC, USA). Results There was no significant difference in age among the three study groups, with an average age range of 40.0–41.5 years (Table 1). Administrative clerks had received more school education (p < 0.001) but had a shorter employment history (p = 0.07). Approximately 28% of the workers exposed to toluene plus noise had worked for ≥ 20 years. The average noise exposure levels were 83.9 dB(A) in the toluene plus noise sites, 85.0 dB(A) in the noise-only sites, and 70.0 dB(A) in the administrative offices. Fewer than 15% of workers with noise exposure used hearing protectors. VOLUME 114 | NUMBER 8 | August 2006 • Environmental Health Perspectives