Compared harmfulness of impulse noise and of steady-state noise having the same acoustic energy during co-exposure to styrene

Outline of reasons and objectives
In its recommendations on protecting the hearing of employees exposed to noise of occupational origin, the Noise Directive 2003/10/EC does not take into account the nature of the noise, i.e. whether it is steady-state noise or impulse noise, and nor does it take into account whether or not it is associated with chemicals. And yet recent studies have shown that certain aromatic solvents, such as styrene, can disrupt auditory reflexes. From the prevention point of view, such information is significant, because the acoustic energy of impulse noise, which lasts for a time shorter than the triggering latency of the stapedius reflex, will penetrate into the cochlea without being attenuated. Therefore, noise of short duration, such as impulse noise, could be more harmful than steady-state noise having the same energy. And the risk could be increased when the exposure to noise is concomitant with exposure to an aromatic solvent. In this study, the damaging effects of impulse noise were compared to those of steady-state noise, of the same spectrum, in the presence and in the absence of a styrene-rich atmosphere.

Approach
The hearing of brown Norway rats was assessed using a technique based on objective measurement of the otoacoustic emissions caused (acoustic distortion products). The selected noise was noise of octave band centred on 8 kHz. It was either continuous/steady-state noise, or impulse noise. The steady-state noise had an LEX,d Daily level of exposure constituted by the acoustic energy corresponding to steady-state noise for 8 hours. of 85 dB, while the impulse noise’s was 80 dB. For the impulse noise, the sequence of noise impulses was chosen in such a manner that the protection of the middle ear reflex was ineffective. The exposures to noise and to styrene (300 or 600 ppm) lasted 6 hours per day, 5 days a week, for 4 weeks. The toxicity of the exposures was illustrated by cochleograms, which are histograms in which the percentages of cell losses are shown for the hair cells of the organ of Corti.

Main results
In view of the difference in harmfulness observed, during the initial tests, between the two types of noise, the LEX,d of the impulse noise was lowered by 5 dB (LEX,d = 80 dB) relative to the LEX,d of the continuous noise (LEX,d = 85 dB). Despite this difference, the association of the impulse noise with the styrene caused a synergy of the damaging effects (25 dB) and of the cochleotoxic effects (doubling of the losses of hair cells from the first row). This did not apply with the continuous noise. Continuous noise triggers the middle ear muscle reflex, whereas impulse noise triggers it only once the acoustic energy has penetrated into the cochlea.
Discussion
The study highlighted, in the rat, the exacerbated harmfulness of impulse noise compared with the harmfulness of continuous noise. This difference can be estimated at 5 dB over an LEX,d of 8 hours. In addition, the styrene potentiates the damaging effects of the impulse noise. The middle ear reflex thus constitutes a significant variable that, so far, has not been taken into account in the legislation on protection from the effects of noise. INRS will propose modifications of the action values (values at which action should be triggered) recommended by the Noise Directive 2003/10/EC or adaptations of the exposure time depending on the nature of the noise (“continuous versus impulse”).