Occupational noise exposure is regulated by law, and protective measures are applied if the noise level exceeds 90 dB(A). The inferior exposure levels can cause an auditory fatigue which is defined as temporary and reversible hearing loss. Its mechanisms, however, remain largely underexplored with recent animal data suggesting that the long-term exposure (over several weeks) to noise above 85 dB Sound Pressure Level (SPL) may provoke a permanent damage to hearing. Furthermore, there is also a lack of research on cognitive and attentional repercussions of this phenomenon. Yet, this information could be important to prevent work-related errors and accidents. Lastly, although silent breaks (noise-free resting periods) throughout a day of work are preconized to limit the noise exposure, their effect on auditory fatigue has never been demonstrated.

Being a part of a larger project involving fundamental researchers (animal experimentation) and occupational health specialists (investigations at the workplace), this study aims:

• to evaluate how auditory fatigue caused by acute noise exposure in doses tolerated by law impacts auditory and cognitive performances in humans
• to determine the effect of silent breaks proposed by occupational legislation on cognitive functioning
• to issue new recommendations for the protection of workers exposed to noise

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.

Inclusion Criteria:

• Healthy subjects aged between 18 and 35
• With normal pure-tone audiometry data (air and bone conductions) on both ears: hearing thresholds ≤20 dB HL for each frequency tested - 125, 250, 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz
• With normal Speech-in-noise test (the speech reception threshold (SRT) at which 50% recognition of the speech stimuli presented is achieved must be 3 dB higher than the standard test value established in subjects with normal hearing)
• Without history of excessive noise exposure (1-MINUTE NOISE SCREEN ≤4)

Exclusion Criteria:

• History of sudden deafness or tinnitus lasting more than 48 hours
• Conditions likely to affect hearing (tumours of the ear or base of the skull, etc.)
• History of unresolved chronic otitis
• Work in a noisy environment requiring workplace soundproofing or the use of personal protective equipment
• Migraine with or without aura or tension-type headaches (International Headache Society (IHS) classification)
• Pregnant or breastfeeding women or women of childbearing age not using contraception
• Participation in another interventional clinical trial
• A person whose physical and/or psychological health is severely impaired
• Individuals deprived of their legal capacity, or under guardianship or trusteeship
• Persons deprived of liberty (by judicial or administrative decision)
• Not affiliated to a French social security system or not a beneficiary of this system
• Absence written informed consent to participate in the study