Occupational Noise-Induced Hearing Loss is defined as partial or complete hearing loss in one or both ears as a result of one's employment.
The Magnitude of the Problem
Hearing loss due to noise exposure in the workplace is a significant health problem worldwide. For example, according to the National Institution of Occupational Safety and Health (NIOSH), in the U.S.A., about one-fourth of all workers have been exposed to loud noise, and 12% of them report some hearing difficulty or tinnitus. Furthermore, of the workers who underwent testing for hearing loss due to noise exposure, about 16% had substantial hearing impairment affecting their day-to-day activities—the annual compensation for hearing loss resulting from noise in the U.S.A. is approximately $242.4 million.
Pathophysiology
The mechanism of noise-induced hearing loss (NIHL) involves the destruction of hair cells in the organ of Corti within the cochlea of the inner ear. Chronic exposure to loud noise at the beginning damages the hair cells responsible for receiving high-frequency sounds. We have, unfortunately, limited ability to detect the beginning stages of noise-induced hearing loss. By the time it is noticeable, a loss of a sufficient number of hair cells has already occurred.
Therefore, NIHL is an irreversible condition and will continue to worsen with ongoing noise exposure.
Risk Factors
The major environmental factor which causes NIHL is continuous and long-term exposure to loud sounds. Workers most at risk of occupational NIHL include those employed in construction, manufacturing, mining, agriculture, utilities, transportation, the military, and musicians.
Sound intensity is measured in decibels (dB), which are used to indicate how humans hear a given sound. For example, a dB of zero is considered to be when a person starts to hear a sound, a whisper at 3 feet is equal to 30 dB, and outside a jet engine aircraft, noise at 100 feet can measure up to 140 dB. NIOSH defines hazardous noise as sound exceeding 85 dB over a typical 8-hour day.
Adverse effects of noise depend upon intensity and duration of exposure. It may result in a temporary threshold shift, which is a change in hearing that a worker can recover over time, or a permanent threshold shift, which involves loss of cochlear hair cells and permanent hearing loss.
Factors that are found to be involved in accelerating the development of NIHL include:
● Genetic predisposition
● Smoking Cigarettes
● Diabetes
● Sedentary Lifestyle and low levels of exercise
● Male Gender
Symptoms and Complications
Workers with NIHL often report tinnitus and the inability to tolerate loud sounds. They may also undergo altered speech and impaired verbal communication.
Loud noise has other effects on the body unrelated to hearing. Non-hearing-related effects may include irritability, elevated blood pressure, sleep disturbance, cognitive impairment, and adverse cardiovascular health.
Excessive noise hampers a person’s ability to hear warning signals, monitor equipment, respond to environmental sounds, and communicate with other workers. In addition, possibly aggravated by these environmental factors, occupational NIHL increases the risk of injuries at work.
Diagnosis and Characteristics of NIHL
The diagnosis of NIHL is based on evaluating a person's personal history, including hobbies, past trauma and surgeries, and occupational history. The diagnosis is highly likely if, along with a history of working in a noisy environment for a prolonged period, the individual has characteristic symptoms and audiometric findings of NIHL.
Occupational NIHL is typically bilateral and symmetric. However, hearing loss may have some asymmetry, especially if there is differential exposure to sound between the two ears. When a discrepancy is present, occupational NIHL is commonly more severe in the left ear, due to unknown reasons.
Audiometry
Pure Tone Audiometry is the primary method of diagnosing hearing loss. The test assesses and identifies the patient's hearing threshold and is 92% sensitive and 94% specific in detecting hearing defects. Noise exposure typically affects the hair cells near the basal turn of the cochlea, leading to a characteristic decrease in hearing thresholds between 3 kHz and 6 kHz. This pattern of hearing loss will usually produce a distinctive dip at 4 kHz on an audiogram, often termed the '4K notch.'
The lower frequencies, including the main speech frequencies, are often spared initially. However, the noise notch will flatten with progressive damage as thresholds decrease for the surrounding frequencies. As a result, individuals with severe NIHL may have audiograms indistinguishable from other causes of hearing loss.
Management of Occupational NIHL
Noise conservation Program
The Occupational Safety and Health Administration (OSHA) mandates that a hearing conservation program be provided for workers exposed to the noise of more than 85 dB for over 8 hours. This hearing conservation program must include the following elements:
Employers must measure noise levels on the premises.
If noise exceeds exposure limits, the employer must provide free hearing protective devices.
Proper training of employees regarding noise exposure and personal hearing protective devices.
Baselines and annual audiograms of employees to assess their hearing
Records of all test results must be kept and made available to employees
Concerning annual audiogram surveillance, the employer must monitor individuals' audiograms for a "standard threshold shift" (STS), defined as 10 dB or greater, worsening over time in the average hearing threshold level for 2, 3, and 4 kHz tones in either ear. When an STS is confirmed and documented, the employer must notify the individual worker, provide retraining and refitting of hearing protectors, and make any necessary referral for the specialist evaluation.
Hearing Protection
Personal hearing protective devices include both earmuffs and earplugs. Expandable foam plugs provide similar noise attenuation as earmuffs, with other types of earplugs providing less protection. Most earmuffs and expandable foam plugs will provide 10–15 dB attenuation. Earmuffs and plugs together improve attenuation by 5–10 dB over either device alone. A 10–15 dB protective effect is relevant to a noise-exposed worker, as 10 dB of attenuation will bring noise levels into the acceptable range for more than 90% of exposed workers.
Due to a lack of effective treatment and the non-reversible nature of the condition, noise conservation and hearing protection are the foundations for preventing NIHL among workers.
National Institution of Occupational Safety and Health (NIOSH). Occupational Hearing Loss Surveillance. Overall Statistics – All U.S. Industries. https://www.cdc.gov/niosh/topics/ohl/overall.html (access 3/31/2022)
Chen, KH., Su, SB. & Chen, KT. An overview of occupational noise-induced hearing loss among workers: epidemiology, pathogenesis, and preventive measures. Environ Health Prev Med 25, 65 (2020). https://doi.org/10.1186/s12199-020-00906-0 (access 3/31/2022)
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