Cochlear injuries induced by the combined exposure to noise and styrene

Workers exposed to industrial solvents are also frequently exposed to mechanical noise. In this study, a combination of a continuous noise (100 dB SPL) and an impact noise (110 dB SPL) was used to mimic the noise exposure in the workplace. A noise band of 10-20 kHz was used to induce a cochlear injury in the same cochlear region in the rat as styrene exposure. Styrene levels of 300 and 400 mg/kg were applied to induce outer hair cell (OHC) loss limited to the third row of the middle turn, but without significant cochlear functional loss. The combined exposures of the noise and styrene for 3 weeks caused greater threshold shifts than the noise alone, although the styrene alone did not induce significant threshold shift. Correspondingly, the combined exposures induced OHC losses that were greater than the summated OHC losses induced by the noise and styrene exposure alone. Apoptosis in Deiters cells was also examined after a short-term exposure (7 days) to a combined exposure of a high-level styrene (800 mg/kg) and the noise. The styrene-noise synergistic interaction was also observed in the Deiters cells. The synergistic interaction between the noise and styrene suggests that each of the exposures alone (noise or styrene) may cause stress, temporary alteration, or nonlethal injury in cochlear cells and the combined exposure strengthens the stress leading to cell death.