Rats exposed to Toluene and Noise may develop Loss of Auditory Sensitivity due to Synergistic Interaction

Hearing loss in workers exposed to organic solvents has been shown to be the effect of interaction between the exposure to solvents and noise. Synergistic interaction has been demonstrated in rats following simultaneous exposure to toluene and noise, but only at high-level toluene exposure. The present study was initiated to investigate the potential interaction of exposure to noise and toluene on the auditory system of the rat, covering a dose-range of toluene exposure (0, 500, 1000, 1500, and 2000 ppm, 6 h/d, 10 d). Exposed to toluene only, the rats exposed at the 1500 and 2000 ppm level developed a mid-frequency ABR threshold shift, whereas rats exposed to 0, 500, and 1000 ppm did not exhibit signs of auditory impairment. Rats exposed to 500 ppm toluene and noise (96 dB SPL, 2h following the daily toluene exposure, 10 d) developed a small, but statistically significant threshold shift, equal to the hearing loss in rats exposed to noise only (0 ppm). Synergistic interaction was evident at the 1000, 1500, and 2000 ppm toluene exposure levels. There was no further hearing loss at the 2000 ppm than at the 1500 ppm level, indicating that a saturation of the auditory impairment had been reached. When acute noise exposure (105 dB SPL, 4 h) followed the toluene exposure by 30 days, interaction was noted at the 1500 ppm toluene exposure level, but not at the 1000 ppm level. However, the latter type of interaction is of indirect nature and should be distinguished from the direct interaction, taking place when toluene is physically present in the cochlea during exposure to noise. Further investigations in animal models should preferentially be carried out as long-term, low-level exposure studies, showing the possible interaction at low exposure levels, where exposure to each factor alone is without any effect.