Intermittent noise causes less hearing loss than continuous noise of equal
intensity. The reduction in damage observed with intermittent noise may be
explained by the fact that the auditory system has time to recover between
the noise phases. Simultaneous carbon monoxide (CO) exposure produces great
er noise-induced hearing loss than does noise alone (Chen and Fechter, 1999
). In the present study, intermittent noise (octave-band with a center freq
uency of 13.6 kHz, 100 dB) of a 2 h total duration but with a different dut
y cycle (% of noise during exposure) was used. The intermittent exposure th
at had a shorter noise duty cycle induced a less permanent threshold shift
(PTS) than those that had a longer noise duty cycle (or less rest periods).
This relation between the loss in compound action potential (CAP) sensitiv
ity and the noise duty cycle (or rest period) was abolished by the presence
of CO. The cochlear microphonic (CM) amplitude revealed similar results to
those seen using the CAP. While intermittent noise that had a short noise
duty cycle did not cause hair cell loss by itself, the combined exposure to
noise and CO (1200 ppm) caused remarkable OHC loss in the basal turn. (C)
1999 Elsevier Science B.V. All rights reserved.