Chang et al. 1997; Lataye et al. 1999; McWilliams et al. 2000). In humans, limited studies on this type of ototoxic effect have been conducted in occupational settings (Abbate et al. 1993; Morata et al. 1993, 1995, 1997; Schaper et al. 2003). However, the dose–response relation- ship for hearing loss had not been established in these studies. With simultaneous exposure to toluene and noise, the prevalence rate of hearing loss in workers at a printing and paint manufac- turing plant was lower in the study of Morata et al. (1993) than in the present study, even based on high frequency sounds (53% vs. 84%). The risk for hearing loss at ≥ 25dB was also much greater in the present study than in their study. Our study participants were older (40.0 vs. 32.5 years on average) and had a longer work history (12.3 vs. 8.1 years on average). In another study by Morata et al. (1997), their participants were also younger and had shorter work histories than in our study. They may have had less cumulative exposure to toluene. Many of our study par- ticipants had a longer employment history. This may explain why the rate of hearing loss was also profound in the noise-exposed group. To our knowledge, the present study is the ﬁrst to identify such a strong effect of hearing impairment from simultaneous exposure to toluene and noise in humans. In this study, the average noise exposure levels were similar between the toluene plus noise group and the noise-only group. However, the risk for hear- ing loss at ≥ 25 dB was much greater in the toluene plus noise group than in the noise- only group. The overall ORs adjusted for covariates were 140 versus 12.8 with 0.5 kHz included in the measurement (model 1) and 29.1 versus 5.0 with this pure tone excluded (model 2). This indicates that the risk for hearing loss boosted by toluene exposure may be more than six times greater than the risk induced by noise only. The other unique ﬁnding in this study is that the magnitudes of ototoxic effect were different for various tested pure-tone fre- quencies among workers exposed to toluene plus noise, noise only, and administrative clerks. This finding has not been reported previously for toluene. It is worthwhile to note that the patterns of hearing impairment, measured by the pure-tone frequencies, asso- ciated with toluene plus noise exposure are similar to those associated with the simulta- neous exposure to carbon disulﬁde and noise (Chang et al. 2003). Both toluene and car- bon disulfide have greater impact on the speech frequencies than does noise alone, with the gap the largest at the frequency of 500 Hz. Therefore, the toluene plus noise group had poorer thresholds than did the noise-only group at 1 kHz frequencies, but not necessarily at high frequencies. However, the poorest mean hearing threshold in the toluene plus noise group was at 6 kHz. This was similar to the mean hearing threshold pattern found for the ototoxicity of styrene (Morata et al. 2002). We suspect that other types of ototoxic solvents may have other types of effects on hearing measured by pure- tone frequency. The average air concentrations of toluene at work sites for the three divisions of the toluene plus noise group were 33.0 ppm, 107.6 ppm, and 164.6 ppm, but with similar noise exposure levels. It was surprising to ﬁnd that the risk for hearing loss in workers with the lowest toluene exposure was only slightly lower than that for those with higher levels of toluene exposure. The dose–response analysis based on measures of toluene CEI showed a peak effect at the cumulative exposure level of 200–530 year-ppm and failed to estimate the threshold dose of toluene on the hearing loss effect due to the solvent. This observation might reﬂect variations in exposure history and healthy worker effect. Most of our study par- ticipants in the toluene plus noise group (all three areas) may have been exposed to higher levels of toluene during their long employ- ment. Those who had a CEI > 200–530 year-ppm may have quit their jobs because of hearing problems or other reasons, which would lower the estimated ORs. This is one of the limitations of this study. Another limi- tation of this study was the sample size. No data were available for estimating the impact of hearing loss for workers due to toluene- only exposure. Results from this study showed that there was an elevated hearing impairment for workers who were exposed to toluene plus noise compared with those exposed to noise alone. Although the overall hearing loss was rarely > 55 dB, the impact was greater for the speech frequencies than for the higher fre- quencies. 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