Noise-Induced Hearing Loss

Permanent damage from loud sound exposure, often the 4 kHz notch

Overview

Noise induced hearing loss is permanent sensorineural hearing loss caused by exposure to loud sound, either a single intense impulse such as an explosion or prolonged exposure to loud noise at work or during leisure. Loud sound damages the stereocilia and outer hair cells of the cochlea, and in mammals these cells do not regenerate. A classic audiogram shows a notch around 4 kHz, with better hearing at neighbouring frequencies. The risk depends on both the level and the duration of exposure, which is why occupational and WHO guidance relate permissible exposure to loudness and time together. Temporary muffled hearing or ringing after a loud event, known as a temporary threshold shift, reflects cochlear stress, and animal research suggests such exposures can damage nerve synapses even when thresholds recover. Because it is largely preventable, it is a major target of hearing conservation programs, and WHO has estimated that over a billion young people are at risk from unsafe listening practices.