Noise induced hearing loss (NIHL) is one of the most common work-related illnesses worldwide but can be prevented by effective workplace hearing conservation programs. Occupational hearing loss is a permanent, non-treatable condition which leads to reduced quality of life when the person’s ability to communicate and socialize becomes impaired. Baseline and subsequent regular audiometric screening of employees exposed to noise allows for early detection of NIHL and provides information for employer investigation and intervention to prevent further hearing loss in employees.

How Do We Hear?

Sound is transmitted in the form of sound waves that are picked up by the outer ear and directed through the ear canal to the eardrum. The pressure changes on the eardrum causes it to vibrate and these vibrations are transmitted mechanically by three small bones (commonly known as the hammer, anvil, and stirrup) to the round window between the middle and inner ear. The three bones in the middle ear strengthen and concentrate the force of the vibrations so that the pressure on the oval window is about 22 times greater than on the eardrum. The vibration of the stapes (the stirrup) acts like a piston on the oval window causing the fluid in the inner ear to vibrate. This in turn stimulates hair-like hearing receptor cells that are connected to the nerve fibers which conduct the impulses to the brain’s hearing center. Different hearing receptor cells respond to different sound frequencies.

Types of Hearing Loss

There are two main types of hearing loss.

• Conduction hearing loss – In this type of hearing loss the cause is related to the mechanical conveyance of sound in the outer and middle ear. This type of hearing loss can usually be treated.
• Sensorineural hearing loss – In this type of hearing loss there is damage to the structures of the inner ear that transmit sound to the brain. This hearing loss is usually permanent, irreversible, and mostly untreatable.

How Does Audiometric Testing Find Hearing Loss?

Audiometric testing allows trained medical professionals to detect early signs of degradation in the ability of a person to hear much earlier than the employee would notice a loss of hearing in everyday activities. Early detection is vital so that cause can be determined and further damage prevented.

The sounds we hear are measured scientifically according to both intensity and pitch.

• Intensity (loudness) is measured in decibels (dB), with 0 dB being defined as the quietest sound that the average human ear can hear. Normal speech is at the level of around 60 dB and a gun muzzle blast can reach up to 140 dB.
• Pitch is measured in Hertz (Hz) and refers to the frequency of sound vibrations in seconds. The higher the sound, the higher the frequency of the vibrations. Human speech ranges from about 300 to 4,000 Hz with male voices on the lower end of the scale and female voices towards the higher end of the scale.

Hearing tests, or audiograms, assess the levels of loudness at which a person can hear different pitches of sound. The results are typically displayed in graph form.

What is Noise Induced Hearing Loss?

NIHL is when exposure to noise affects the hearing receptor cells in the inner ear, leading to sensorineural hearing loss. Temporary NIHL occurs when exposure to noise leads to some loss of hearing ability which improves after rest. This is sometimes accompanied by pain and ringing in the ears. Sudden extreme noise can cause NIHL through trauma to the mechanical and/or sensorineural parts of the ear.

The type of NIHL generally seen in occupational settings is where continuous and regular exposure to high levels of noise (over 85dB) leads to permanent damage to hearing receptor cells over a period of years. Occupational NIHL is therefore considered an occupational injury rather than an illness. Development of NIHL depends not only on the intensity of the noise, but also on the duration of exposure over the course of the working day. NIHL can be recognized on an audiogram because it occurs first at pitches ranging from 2000-4000 Hz (higher pitched sounds), showing as a typical dip on the graph. A red flag for NIHL is where there is a loss of hearing ability, also known as a threshold shift, of more of10dB on these pitches, compared to previous audiograms.

Effects of Noise and NIHL on the Individual

Exposure to noise in the workplace appears to affect the nervous system, leading to irritation, hypertension, and poor sleeping patterns. Psychiatric disorders due to noise exposure have also been described.

Severe NIHL impairs the person’s ability to communicate with family, friends, and co-workers. Reduced ability to socialize and interact with society can result in social isolation, affecting the person’s quality of life and general well-being. It can cause irritability, decreased self-esteem and even fear, shame and depression.

Reduced hearing in the workplace raises various safety concerns, such as decreased capacity to monitor the working environment (for example, sounds from machines and warning signals) and an increased risk of injury due to impaired communication. It can also increase costs in terms of lost productivity, expenses for workman’s compensation, hearing aids, and other treatment or mitigation devices.

Employer Responsibilities

The United States Centers for Disease Control reports that “(1)Occupational hearing loss, primarily caused by high noise exposure, is the most common U.S. work-related illness. (2) approximately 22 million U.S. workers are exposed to hazardous occupational noise.” Furthermore, an estimated $242 million is spent annually on worker’s compensation for hearing loss disability. This emphasizes employers’ responsibility with regard to NIHL, which can be reduced or entirely prevented. Prevention strategies include worker education, reducing noise in the work place through work scheduling and engineering controls, the use of hearing protection devices (HPD’s) by exposed employees, and early detection of NIHL through regular audiometric screening.

Workplace health and safety legislation in most countries makes provision for hearing conservation programs. Included in these programs is the requirement that a baseline audiogram must be done on all new employees who will be exposed to average noise of 85dB or more during an 8-hour working day and that follow-up audiograms must be done annually. Because temporary NIHL occurs after noise exposure, audiograms should be done after the person has been noise free for at least 14 hours.

The baseline audiogram determines the employee’s hearing ability at the start of employment. Follow-up audiograms are compared to the baseline to identify any deterioration in hearing, specifically threshold shifts indicative of NIHL. This assessment is done by the audiologist or another medical practitioner.

The reasons for an individual’s shift in hearing threshold indicative of NIHL should be investigated. Possible causes and appropriate follow-up measures include:

• The individual was not noise free for 14 hours. The audiogram should be repeated when the employee has not had exposure to workplace noise for the required time.
• The individual fails to wear the hearing protection devices (HPD’s) according to workplace rules. This requires employee education, or re-education, with regard to NIHL and its effects. Supervisors or safety personnel should also follow up on compliance regularly.
• The HPD’s do not fit properly. In this case the employee will often complain either that he has difficulty in inserting the ear plug, or that it is loose or falls out. Here the person often has either a very small or very large ear canal, which a health care practitioner can confirm though and ear examination. The employee may need to be issued with a different type of HPD’s.
• The HPD’s does not provide sufficient noise reduction for the level of noise that the employee is exposed to. This is identified by comparing the noise levels measured in the workplace with the noise reduction specified for the particular HPD by the manufacturer. Where noise exceeds 105dB combined use of ear plugs and ear muffs can be considered.
• The employee may have a physical ear problem, such as a perforated eardrum of even a wax impaction. This can be confirmed through examination by a health care practitioner, who can either treat the problem or advise whether the employee should in future avoid all noise exposure.

Employers should also compile statistics on the shifts in hearing thresholds for all noise-exposed employees. These figures will indicate the overall effectiveness of the hearing conservation program and whether there are specific sections or occupations that require investigation and stepping up of measures to prevent NIHL.