Hearing LossHearing loss can arise from a number of causes and diseases.  Primarily two types of damage may occur:  Conductive hearing loss, and Sensorineural hearing loss. A conductive hearing loss means that the vibrations of the ear drum are not transferred effectively to the inner ear sensory organ, the cochlea.  This can happen, for example, if there is traumatic damage that breaks the connection between the fine bones of the inner ear.  A sensorineural hearing loss results from damage to the cochlea itself, which is composed of sensory hair cells and nerves.  Noise-induced hearing loss (NIHL) results from sensorineural damage, NOT conductive damage.  In NIHL, the sensory hair cells of the cochlea are damaged and die.  Because of the spiral conical structure of this organ and its internal resonances, certain areas of this organ are more damaged by noise than others.  The area of the cochlea that is most damaged by noise are the hair cells that detect sound frequencies around 4000 Hz (also called 4 kHz).  Therefore, when measuring hearing loss (usually displayed on a graph called an audiogram), those speech frequencies around 4 kHz are most affected.  The audiogram below shows both normal hearing (straight line) and a person with NIHL (showing a 4 kHz “notch”).

Another unique feature of NIHL is that noise exposure causes most damage in early years of exposure.  In contrast, age-related hearing loss (called presbycusis) is very small at young ages, but accelerates with increasing age.  The following graph shows the average hearing loss for four frequencies (0.5, 1, 2, and 3 kHz) in males without any ear diseases due to continuous noise exposures beginning at age 20.  The noise curves are the same when shifted to a later starting age.  This graph indicates that NIHL develops over many years of exposure.The graph also displays the hearing loss due to age (presbycusis).

Noise and age-related hearing loss at 0.5, 1, 2, 3 kHz in the median (50th percentile: which means half of the people will have better, half worse hearing) of an otologically normal male population.  Derived from models developed by the International Standards Organization, ISO-1999 standard, published in 1990.There is also a temporary hearing loss that can occur after short exposures to loud noise, such as after a music concert.  You may hear this referred to as a “temporary threshold shift”.  The inner ear is able to recover from such acute hearing loss in a matter of several hours away from further noise exposure.   However, if noise exposure continues for long periods, the hearing loss can become permanent.   Long-term noise exposures can permanently damaged the cochlea and this damage cannot be reversed by any treatments.   When the term “NIHL” is used, we are talking about such permanent damage. NIHL is a gradual process that occurs slowly over many years of exposure to high noise levels. Since the process is slow and gradual, by the time a worker recognizes that they have some hearing loss it is often too late to change it. Symptoms include a muffling of sounds or a distortion of sound. People start losing their ability to hear sounds such as t, ch, and s, and are unable to carry on conversations in a group or on the telephone. As further damage occurs, the person begins to lose the ability to understand normal speech.Permanent hearing loss can also be caused by acoustic trauma. This occurs when there is hearing loss due to a single, very loud sound (extreme intensity) such as an explosion or a gunshot blast which immediately and permanently destroys the hairs in the inner ear.  This is referred to as “traumatic hearing loss”.  In contrast, NIHL results from chronic exposure to hazardous noise levels over many years.8Other Effects of Noise ExposureTinnitusLike NIHL, there are many causes and diseases that can produce tinnitus, with hazardous noise being one cause.  Tinnitus is a symptom that is usually described as “ringing in the ears”.  The noise experienced varies from person-to-person and may be characterized as a ringing, whistling,

buzzing, whooshing, or humming sensation in the ears.  While many people can experience some tinnitus on a temporary or intermittent basis, a permanent tinnitus that is constant can be quite disabling.  Such disabling tinnitus is more common in those with NIHL.Hearing loss and chemicalsChemicals and drugs that are toxic to the ear are referred to as ototoxic. Workers exposed to some chemicals and to hazardous noise at the same time may have a greater risk of hearing loss than those exposed to noise alone.Some potential ototoxic agents include:

some organic solvents Carbon monoxide Heavy metals such as lead and manganese:

Hearing loss and smokingThe effects of tobacco smoking may also combine with other factors to increase hearing damage.  If you smoke and are exposed to hazardous noise, the effects of both can add together to produce more severe hearing loss than noise exposure alone. Noise and vibrationPower tools such as impact wrenches, pneumatic hammers, and disc grinders that produce high noise levels can also produce high levels of vibration. Exposure to vibration can lead to a vibration-related disease known as hand/arm vibration syndrome (HAVS).Other health effects caused by noise exposureIn addition to hearing loss and tinnitus, noise is also associated with adverse health effects such as:

high blood pressure increased risk of heart disease increased stress levels tiredness irritability hormonal changes low birth weight in pregnant women’s children.

Noise and AccidentsHigh noise levels may be associated with higher accident rates because:

noise makes it more difficult for workers to hear warning signals such as warning shouts, sirens, back up alarms, and moving machinery

noise can interfere with a worker’s ability to hear and understand safety instructions. noise can be distractive and affect the ability of people to focus on the tasks they are performing. 

This effect often increases with age.Noise at Work and at HomeHearing loss prevention efforts need to focus on reducing high noise exposures regardless of whether they occur at work or away from work.  Examples of sources of non-work-related noise exposure include lawn mowers, chain saws, power tools, amplified music, rock concerts, personal listening devices, guns, snowmobiles, motorcycles, and motor racing.Impact of Hearing LossNIHL is a gradual process that occurs over many years of prolonged exposure to high noise levels. Although it is not life threatening, it can profoundly affect a person’s life in many ways.  For example:

Conversations become difficult to follow when two or more people are speaking at the same time. This may cause some people to withdraw socially. This social isolation may cause a person to become lonely and depressed.

Other people often complain that the volume on the television or radio is too high. It becomes necessary to strain to understand conversations. Hearing becomes difficult when there is a lot of background noise. It is frequently necessary to ask people to repeat themselves. Other people sound like they are mumbling or are speaking unclearly. Conversations over the telephone become difficult. Conversations may be misunderstood, so responses may be inappropriate.  Others may interpret

this as mental slowness in those with hearing impairments.  Confidence levels can decrease and the person relies more on others to perform simple tasks that

involve contact with other people. Career choices may become restricted due to the hearing impairment.

Like NIHL, tinnitus also has a profound impact on a person’s life.  It can cause: depression anxiety sleep disorders emotional problems.

9Health SurveillanceIt is important to monitor workers’ hearing over time to ensure that engineering, administrative or protective equipment controls in the workplace are effective.  Further investigations will be necessary if early signs of hearing loss continue to progress.Hearing tests and audiogramsHearing tests are designed to identify the softest sounds a person can hear. Hearing tests require specialized equipment and must be carried out by someone who has the appropriate education and training. Larger companies with in-house occupational health services may have access to trained professionals. However, most organizations will have to contract the services from an external audiometric service.The simplest test of hearing ability is done by having the test subject listen to a range of beeps at different tones. The test subject indicates when he or she can hear the sounds by pressing a button. The loudness of each tone is reduced until the test subject can barely hear the tone. The softest sounds the test subject can hear are then marked on a graph called an audiogram.Audiograms are interpreted by trained professionals (individuals trained as hearing conservationists or audiologists) to determine if someone has hearing loss.Baseline Hearing TestA baseline hearing test is a reference point against which subsequent hearing test results can be compared. Workers should ideally have a hearing test before they are exposed to noise. This will establish each worker’s hearing abilities before noise exposure.Periodic Hearing TestSubsequent re-testing (usually every year or two) will detect changes to the worker’s hearing that may have been caused by noise exposure.  The audiogram from each periodic test should be compared to the baseline audiogram to determine if hearing loss has occurred.   The hearing conservationist or audiologist will be able to determine if any follow-up is needed.  Audiograms should be treated as medically confidential records10Noise Levels of Common Equipment

Some examples of occupations that may involve hazardous noise levels that can produce NIHL include carpenters, labourers, ironworkers, operating engineers, musicians, welders, machinists, textile workers, miners, pulp and paper workers, press operators, sawmill workers, and foundry workers.Typical Noise Levels of Common Construction ToolsNoise levels represent exposures at an operator’s ear, except where otherwise indicated.

ToolNoise level (dBA) will probably exceed...

Abrasive Blasting 112Air compressor 98Air gun 108Air hammer 110Air track drill 110Asphalt grinder 111

Backhoe