Ethyl benzene-induced ototoxicity in rats: A dose-dependent mid-frequency hearing loss

Rats were exposed to ethyl benzene at 0, 300, 400 and 550 ppm for 8 hours/d ay for 5 consecutive days. Three to six weeks after the exposure, auditory function was tested by measuring compound action potentials (CAP) in the fr equency range of 1-24 kHz and 2f(1)-f(2) distortion product otoacoustic emi ssions (DPOAEs) in the frequency range of 4-22.6 kHz. In addition, outer ha ir cell (OHC) loss was quantified by histological examination. The lowest c oncentration ethyl benzene had no effect on any of the above measures. At 4 00 ppm, auditory thresholds were increased by 15 and 16 dB at 12 and 16 kHz , respectively: and at 550 ppm by 24, 31, and 22 dB at 8, 12, and 16 kHz, r espectively DPOAE amplitude growth with stimulus level was affected only af ter 550 ppm at 5.6, 8, and 11.3 kHz. OHC loss was found in two of the five examined locations in the cochlea. At 400 ppm, 25% OHC loss was found at th e 11- and 21-kHz region. The highest concentration evoked 40% and 75% OHC l oss at the 11- and 21-kHz location, respectively. Thus, the mid-frequency r egion of rats is affected after exposure to relatively low concentrations o f ethyl benzene (400-550 ppm). These results indicate that ethvl benzene is one of the most potent ototoxic organic solvents known today.