Abstract
Noise levels are truly continuous in relatively few occupations, with some degree
of intermittency the most common condition. The sound levels of intermittent noise
are often referred to as non-Gaussian in that they are not normally distributed in
the time domain. In some conditions, intermittent noise affects the ear differently
from continuous noise, and it is this assumption that underlies the selection of the
5-dB exchange rate (ER). The scientific and professional communities have debated
this assumption over recent decades. This monograph explores the effect of non-Gaussian
noise on the auditory system. It begins by summarizing an earlier report by the same
author concentrating on the subject of the ER. The conclusions of the earlier report
supported the more conservative 3-dB ER with possible adjustments to the permissible
exposure limit for certain working conditions. The current document has expanded on
the earlier report in light of the relevant research accomplished in the intervening
decades. Although some of the animal research has supported the mitigating effect
of intermittency, a closer look at many of these studies reveals certain weaknesses,
along with the fact that these noise exposures were not usually representative of
the conditions under which people actually work. The more recent animal research on
complex noise shows that intermittencies do not protect the cochlea and that many
of the previous assumptions about the ameliorative effect of intermittencies are no
longer valid, lending further support to the 3-dB ER. The neurologic effects of noise
on hearing have gained increasing attention in recent years because of improvements
in microscopy and immunostaining techniques. Animal experiments showing damage to
auditory synapses from noise exposures previously considered harmless may signify
the need for a more conservative approach to the assessment of noise-induced hearing
loss and consequently the practice of hearing conservation programs.
Keywords
Noise - non-Gaussian noise - hearing loss - occupational hearing loss
