Fatigue and Hearing Performance of Healthy Volunteers Acutely Exposed to Occupational Noise ( FATIGAUDIT)

Study rationale:

Individuals routinely endure daily noise exposure related to transportation as well as occupational and recreational (portable music players, video games, concerts) activities. European and United States regulations have limited occupational noise exposure to 85 dB(A) (A-weighting - the sound pressure level filtered appropriately to the human audiogram) based on an 8h working day. This level is commonly encountered in the manufacturing, construction, and music industries, while noise levels are constantly increasing in the tertiary sector.

Overall, 13% of people are exposed to a cumulative daily dose of noise equal to or exceeding 85 dB(A) approved by law. This is not without a harm: excessive noise exposure rises stress levels, induces physical tiredness and causes headaches leading to increased workplace accident rates. It can also stimulate aggression and other antisocial behaviours.

Importantly, a daily noise exposure of 85 dB(A) is responsible for "auditory fatigue" - a complex phenomenon correlated to a temporary elevation of hearing thresholds or TTS (temporary threshold shift). According to recent investigations, daily TTS can reach 5 dB elevations (i.e. a 68% decrease in sound intensity perceived) in factory workers and sound engineers at the end of every single workday.

Auditory fatigue has received little attention in the scientific literature because TTS is considered fully reversible and therefore inoffensive. This complacency might be misplaced: auditory fatigue repeated throughout a career may contribute to:

* the permanent hearing loss experienced by 10-15% of people exposed to occupational noise below the legal exposure limit * the increased rate of chronic auditory disorders such as tinnitus (a perception of phantom sounds) or hyperacusis (a reduced tolerance for moderate sound levels) in professional musicians and noise-exposed workers * a decrease in auditory performance (e.g. speech-in-noise or frequency discrimination), a heavier cognitive load or an adaptive modification of brain circuits under the prolonged sensory constraints of noise masking and degraded hearing sensitivity

Recent rodent studies show that a TTS can have damaging consequences on sensorineural inner ear structures despite a long-term recovery of hearing thresholds. For instance, weeks to months of continuous exposure at 85 dB Sound Pressure Level (SPL) may induce ultrastructural abnormalities within the cochlea, loss of synaptic ribbons in sensory inner hair cells, an amplitude decrease of auditory nerve activity and delayed degeneration of auditory nerve fibres. These symptoms, referred to as synaptopathy, disrupt the signal transmission. This could potentially explain why some subjects experience poor speech perception in noise despite normal auditory thresholds.

Whether auditory fatigue, i.e. the daily noise-induced TTS after 8 hours at 85 dB(A), involves one of the above insults (or other ones) remains unknown. The possibility of preventing auditory fatigue, for example by rest periods, also depends on the mechanisms involved and their kinetics. Due to its fast recovery at the scale of minutes or hours, it is scientifically and technically challenging to detect auditory fatigue and then establish its relationship with immediate- and long-term consequences on the molecular, functional, and perceptive levels. Nevertheless, auditory fatigue is an important and neglected public and occupational health issue.

The aim of the FATIGAUDIT study is:

* to better evaluate the temporary changes in auditory and cognitive performances of healthy volunteers after an acute noise exposure of 85 dB(A) for 2 and a half hours (the sound level in a noisy canteen or during choir rehearsal for a duration well below the legal threshold of 8 hours) * to determine whether the introduction of silent breaks limits these temporary changes and reduces a cognitive load

From an ethical point of view, it is not possible to propose prolonged and/or chronic exposure to this type of noise as it might damage hearing irreversibly.

• Stroop test to assess cognitive load [Time frame: End of follow-up period (up to 1 month) for all subjects included in the study]
• Baseline tone audiometry [Time frame: Up to 1 month, for subjects included in experimental groups (1, 2 and 3)]
• Assessment of speech perception in noise [Time frame: Up to 1 month, for all subjects included in the study]
• Assessment of Acoustic Distortion Products (ADP) [Time frame: Up to 1 month, for all subjects included in the study]