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Dangers of work-induced hearing loss


Industrial machinery exposes many workers to high noise levels that may damage their hearing. It is important that efforts are made to minimise noise levels and their impact on those exposed to them.

Report by NSSA

Jack hammers, punch presses, weaving machines, most woodworking machinery, grinders, heavy diesel engines, compressors, disc cutters, riveting machines, crushers, blasting equipment, forges and ball mills are just some of the many noise producers that can endanger workers’ hearing.

Zimbabwe’s noise problem is further compounded by the use of old machinery with little or no soundproofing. As a result, many workers end up suffering from work-related hearing loss.

Many businesses fail to appreciate the negative impact of work-related hearing loss on productivity levels or to comply with safety and health regulations in relation to noise.

Sadly many employees overlook the impact noise can have on their hearing. They assume it is less dangerous than other work-related accidents and diseases because loss of hearing is generally not painful.

The ear is made up of the outer, middle and inner ear. The outer part is composed of the pinna, the ear canal and the eardrum (tympanic membrane).  Three tiny bones called the hammer, anvil and stirrup make up the middle ear. A snail-shaped organ called the cochlea, inside which is Corti’s organ (the organ of hearing), completes the set up.

Sound waves from the surrounding medium (usually air) are channelled through the ear canal, causing the eardrum to vibrate.  These vibrations are picked up by the bones (ossicles), which amplify the sound and transmit it to the inner ear.  Here tiny hair cells in the organ of Corti move in response to different sound frequencies. This movement is picked up by the auditory nerve, which sends signals to the brain, which interprets the sound.

There are about 15 000 hair cells in the ear capable of responding to a large frequency range from as low as 20Hertz (Hz) up to around 20 000Hz. Exposure to high noise levels may damage these hair cells. Once damaged, they do not regenerate. Thus permanent hearing loss is incurable.

Exposure to high levels of noise for a short time may result in temporary hearing loss. The hair cells  suffer fatigue, but after resting they recover and normal hearing returns. When this happens at the workplace, it should be taken as a warning that a problem exists.

If the exposure becomes habitual, then the hair cells become permanently damaged. The hair cells will not regenerate and the person suffers permanent loss of hearing.  The longer the person is exposed the greater the damage.

Unfortunately, the effects are hardly noticeable at first. A person may fail at first to hear high frequency sounds, with this progressing to difficulty in speech comprehension and finally to deafness.

Warning signs that noise levels may be dangerously high include hearing loss, interference in communication such as one often experiences when speaking on the phone in a moving vehicle and  tinnitus (ringing in the ears), which is when the ear sends signals to the brain of noise that is no longer there. This is an uncomfortable condition, but usually goes away after some time.

Generally, if you have to shout to be heard or you cannot carry out normal conversation at arm’s length, then the workplace can be considered too noisy.

The best method of checking the safety of the noise level is by measuring it with an integrating sound level meter or noise dosimeter, which measure the sound in decibels on an A-scale (dB (A)). The A-scale imitates the ears response to sound.

In Zimbabwe, the first action level is 85dBA thus complying with international law and practice. However, our statutes still provide a maximum exposure limit of 90dB(A) for an eight-hour shift. Nonetheless, effective controls should be put in place once noise levels reaches 85dB(A) for an eight-hour shift.

Work-induced hearing loss can generally be prevented if the minimum statutory safety and health requirements are adhered to. The employer should put in place a hearing conservation programme, which should include conducting regular noise monitoring. Following this, all noisy areas should be designated and warning signs posted.

Engineering controls are the first point of call to reduce the noise at its source or along the transmission path. Several methods and materials are available for this such as vibration isolation or dampening, enclosures and the use of noise absorbent materials.

Only after engineering solutions have been tried and failed should ear protection devices, such as ear muffs or ear plugs, be opted for. Whatever protection is chosen, it should reduce the noise entering the worker’s ear to acceptable limits.

In Zimbabwe direct subtraction is used, taking standard deviation into consideration. A single noise reduction rating (NRR) or signal-to-noise rating (SNR) number is used to indicate the attenuation level of each device.

In all workplaces where high noise levels are emitted, baseline audiograms (hearing tests) should be carried out on workers.  Thereafter, periodic monitoring should be done and comparison be made with the baseline.

Any change warrants further investigation, as it could be an indication of failure of controls. If the shift is significant, the worker should be removed from the noisy environment.

The failure to involve inspectors when machinery is repaired or altered can be a cause of preventable work-induced hearing loss. Engineers frequently repair or tamper with machines or installations without consulting the factories inspectorate or the SHE professional employed by the organisation so as to ascertain that the sound produced from the machines is not harmful.

Repair procedures should be submitted to the regulatory authority, which is the inspectorate department of  NSSA, for approval before executing repairs. The department’s engineering personnel can offer crucial advice to ensure the repaired machinery is safe and not excessively noisy. The department is also able to measure noise levels and carry out audiometric tests.

The noise problem is widespread in Zimbabwean industry, but the awareness level is low.  Greater awareness is needed on the part of employers and workers and greater compliance with noise control regulations is a must.

Talking Social Security is published weekly by the National Social Security Authority as a public service. There is also now a weekly radio programme, Pamhepo neNSSA/Emoyeni leNSSA, discussing social security issues every Thursday at 6.50pm on Radio Zimbabwe. Readers can email issues they would like dealt with in this column to mail@mhpr.co.zw or text them to 0735 041 278. Those with individual queries should contact their local NSSA office or telephone NSSA on (04) 706517-8 or 706523 5.

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