Sequence of exposure to noise and toluene can determine loss of auditory sensitivity in the rat.

Rats were exposed to noise (100 dB Leq, 10 h/d, 7 d/w, 4 w), or to toluene (1,000 ppm, 16 h/d, 7 d/w, 2 w), or to noise followed by toluene. Auditory sensitivity was tested before exposure, and 1 to 4 weeks after exposure, by brainstem audiometry using a 1/3-octave filtered sine wave stimulus at the frequencies 1.6, 3.15, 6.3, 12.5 and 20.0 kHz. Some auditory impairment was observed after all exposures. The sensitivity loss after exposure to noise followed by toluene was greater than that recorded after exposure to noise alone or toluene alone, but did not exceed the summated loss caused by noise alone and toluene alone at any frequency. This result contrasts with the earlier reported effect of the same exposures in the reversed order. It is concluded that the exposure sequence can determine the extent of auditory impairment.