Combined effects of noise and styrene exposure on hearing function in the rat

Combined exposure to both noise and aromatic solvents such as styrene is co mmon in many industries. In order to study the combined effects of simultan eous exposure to both noise and styrene on hearing, male adult Long-Evans r ats were exposed either to 750 ppm styrene alone, to a 97 dB SPL octave ban d of noise centered at 8 ItHz, or to a combination of noise and styrene. Th e exposure duration was 6 h/day, 5 days/week. for 4 consecutive weeks. Audi tory function was tested over a frequency range from 2 to 32 kHz by recordi ng near field potentials From the inferior colliculus, whereas histopatholo gical analyses of the cochleae were pel formed with conventional morphometr ic approaches. Whereas both noise and styrene each caused permanent thresho ld shifts, the mechanisms of cochlear damage were different. Noise-induced hearing loss was mainly related to injuries of the stereocilia, whereas sty rene-induced hearing loss was related to outer hair cell losses. Following the combined exposure, the threshold elevations as well as the cell losses exceeded the summed loss caused by noise and by styrene alone in the range of 8-16 kHz. Therefore, these results suggest that the two ototoxicants car l cause a permanent synergistic loss of auditory sensitivity. (C) 2000 Else vier Science B.V. All rights reserved.