Describe noise-induced hearing loss with its prevention.

Mechanism and Features

• Noise-induced hearing loss (NIHL) is sensorineural hearing loss caused by exposure to excessive noise.
• It is the commonest preventable cause of sensorineural deafness and the commonest occupational disease in the world.
• Two distinct mechanisms — acoustic trauma, from a single intense impulse (a blast, gunshot or firework) causing immediate mechanical destruction; and chronic noise exposure, causing metabolic exhaustion of the outer hair cells with free radical damage and eventual cell death
• The outer hair cells of the basal turn are affected first.
• Temporary threshold shift (TTS) — the reversible dullness and tinnitus after a concert or a noisy shift, recovering in hours to days. permanent threshold shift (PTS) follows repeated exposure and does not recover. Repeated temporary shifts predict permanent damage, and are the warning that should be acted upon
• The audiogram IS characteristic and essentially diagnostic: a notch at 4000 Hz (sometimes 3000 or 6000) with recovery at 8000 Hz.

Prevention and Management

• The loss IS irreversible.
• 1. Eliminate or substitute the noise source — quieter machinery, which is the only measure that protects everyone
• 2. Engineering controls — enclosure, isolation, damping, silencers and maintenance
• 3. Administrative controls — limiting exposure time, job rotation, distance from the source, and quiet rest areas
• 4. Personal protection last — ear plugs (20 to 30 dB) and ear muffs (30 to 40 dB).
• Audiometric surveillance — a baseline audiogram before employment, which is essential both clinically and medicolegally, and periodic audiograms thereafter, acting on an early notch before the speech frequencies are involved
• Education of workers and employers, and enforcement of the Factories Act and Noise Pollution (Regulation and Control) Rules
• Management of established loss — remove from further exposure; hearing AIDS; tinnitus management; and certification and compensation under the Employees State Insurance and workmen’s compensation provisions, for which NIHL is a notifiable occupational disease in India
• Acoustic trauma is treated as an emergency like sudden sensorineural loss — high-dose systemic steroids as early as possible, with the drum examined for perforation
• Counsel the young about personal audio devices, using the "60:60" rule — no more than 60% volume for no more than 60 minutes at a time
• Tinnitus after noise precedes the measurable loss, and is the earliest warning the ear gives