Noise exposure can cause two kinds of health effects. These effects are non-auditory effects and auditory effects. Non-auditory effects include stress, related physiological and behavioural effects, and safety concerns. Auditory effects include hearing impairment resulting from excessive noise exposure. Noise-induced permanent hearing loss is the main concern related to occupational noise exposure.
The main auditory effects include:
Sudden hearing damage caused by short burst of extremely loud noise such as a gun shot.
Ringing or buzzing in the ear.
Also known as temporary threshold shift (TTS) which occurs immediately after exposure to a high level of noise. There is gradual recovery when the affected person spends time in a quiet place. Complete recovery may take several hours.
Permanent hearing loss, also known as permanent threshold shift (PTS), progresses constantly as noise exposure continues month after month and year after year. The hearing impairment is noticeable only when it is substantial enough to interfere with routine activities. At this stage, a permanent and irreversible hearing damage has occurred. Noise-induced hearing damage cannot be cured by medical treatment and worsens as noise exposure continues.
When noise exposure stops, the person does not regain the lost hearing sensitivity. As the employee ages, hearing may worsen as "age-related hearing loss" adds to the existing noise-induced hearing loss.
The main characteristics of noise-induced hearing loss are:
Hearing sensitivity declines as people become older. This medical condition is called presbycusis. Again, just like noise-induced hearing loss, everyone is not affected equally. Age-related hearing loss adds to noise-induced hearing loss. Hearing ability may continue to worsen even after a person stops work in a noisy environment.
Noise affects the hearing organs (cochlea) in the inner ear. That is why noise-induced hearing loss is sensory-neural type of hearing loss. Certain medications and diseases may also cause damage to the inner ear resulting in hearing loss as well. Generally, it is not possible to distinguish sensory-neural hearing loss caused by exposure to noise from sensory-neural hearing loss due to other causes. Medical judgement, in such cases, is based on the noise exposure history. Workers in noisy environments who are also exposed to vibration (e.g., from a jack hammer) may experience greater hearing loss than those exposed to the same level of noise but not to vibration.
Some chemicals are ototoxic; that is, they are toxic to the organs of hearing and balance or the nerves that go to these organs. This means that noise-exposed workers who are also exposed to ototoxic chemicals (e.g., toluene, carbon disulfide) may suffer from more hearing impairment than those who have the same amount of noise exposure without any exposure to ototoxic chemicals.
Hearing loss is measured as threshold shift in dB units using an audiometer. The 0 dB threshold shift reading of the audiometer represents the average hearing threshold level of an average young adult with disease-free ears. The PTS (permanent threshold shift), as measured by audiometry, is dB level of sounds of different frequencies that are just barely audible to that individual. A positive threshold shift represents hearing loss and a negative threshold shift means better than average hearing when compared with the standard.
Several methods of calculating the percentage of hearing disability are in use. The American Medical Association (AMA)/ American Academy of Otolaryngology (AAO) formula is widely accepted in North America. The current method recommended by AMA/AAO is as follows:
1. The average hearing threshold level at 500, 1000, 2000, and 3000 Hz should be calculated for each ear.
2. Multiplying should calculate the percentage of impairment for each ear (the monaural loss) by 1.5 times the amount by which the above average exceeds 25 dB (low fence). Hearing impairment is 100% for 92 dB average hearing threshold level.
3. The hearing disability (binaural assessment) is calculated by multiplying the smaller percentage (better ear) by 5, adding it to the larger percentage (poorer ear), and dividing the total by 6.
The data in the table below show how audiometric readings are converted into percent hearing impairment.
|Table 1 |
Hearing Disability Calculation
Using the American Medical Association Formula
|Frequency, Hz||Hearing Threshold Level, dB|
|Left Ear||Right Ear|
Exceeds low fence
|Disability||[45 + (5x15)]/6 = 20%|
The actual dollar value of compensation for a given percentage of hearing loss varies from one jurisdiction to another.
From the scientific data accumulated to date, it is possible to determine the risk of hearing loss among a group of noise exposed persons. To do this we need the following data:
Figure 1. Percent of exposed population with hearing loss greater than 25 dB for various noise levels and years of exposure as given by the ISO 1999-1990 method. Hearing loss is defined as average threshold shift at 500, 1000, 2000 and 3000 Hz.
Given a noise-exposed person, it is not possible to determine whether or not his or her hearing loss is due to noise. However, in a group of exposed persons, the percentage of population with hearing loss depends on the level of noise exposure and the duration of exposure. For higher noise exposure levels, and longer durations of exposures, a larger percentage of exposed persons acquire hearing loss. This observation forms the basis for calculating noise-induced hearing loss as outlined in the international standard ISO1999-1990 and American National Standard ANSI S3.44 - 1996.
Document last updated on August 15, 2008