Workers Hearing Conservation
Noise affects hundreds of workers per year and in this article we shall consider the effects of high noise level exposure and review the various types of noise induced hearing loss that may occur. Finally we will then consider the general crime of noise exposure and health and what the approach to hearing conservation should be.
Noise exposure and health
The damaging effects of noise are related to the total amount of energy or “dose” which the ear receives. The dose/energy depends on two factors: the level of noise and the duration of exposure. It is commonly accepted that equal amounts of noise energy entering the ear cause the same deafness to exposed workers irrespective of the noise and exposure profiles. Thus a short exposure to a high level of noise is considered to cause comparable hearing damage to a long exposure to a low level noise.
Exposure to high noise levels over a wide range of frequencies results in both auditory and non-auditory effects. The auditory effects are dominant, but as research findings become known the importance of occupational noise control in reducing non-auditory effects will become established.
Hearing loss, i.e. the process of losing auditory sensitivity, can be classified under two broad headings:
- Conductive Hearing Loss: Occurs due to a physical breakdown of the conducting mechanism of the ear resulting from an acoustic trauma, e.g. an explosion or gunfire. The eardrum, ossicles or the cochlea could be damaged, often beyond repair. There is no cure, although for the eardrum surgery may reduce the damage It is important to note that acute acoustic trauma is a very rare occupational trauma
- Sensorineural Hearing Loss: Occurs when the cochlea or the hair cells in the Organ of Corti are damaged. Harm may result from natural causes, e.g. an infection or by physical injury In an occupational setting, sensorineural hearing loss occurs mainly from exposure to excessive noise, resulting in varying levels of acoustic trauma. This is the main problem which, we as safety practitioners have to control
Tinnitus is a subjective condition where “noises in the head” or “ringing in the ear” are the descriptive symptoms. There are no observable external symptoms.
Tinnitus can occur after exposure to excessive noise levels as an acute condition which recedes with time. The recovery period could be 12 or more hours where high level exposure levels occur. People who have chronic noise induced hearing impairment can also suffer from the condition. The symptoms of tinnitus suggest that damage to the structure of the Organ of Corti or the auditory nerve has occurred, or possibly both.
A Threshold shift is a reduction in a person’s ability to hear, i.e. they need more sound intensity to stimulate their hearing. The condition can be temporary or permanent.
- Temporary Threshold Shift (TTS) A temporary threshold shift occurs after exposure to a high noise level; hearing acuity returns with time. The condition has been described as a fatigue of the hair cells in the Organ of Corti. The level of shift is expressed in terms of the raising sound intensity required to hear a given sound level, e.g. a 20dB shift means the sound pressure level has been increased by a value of 20dB If a person is subjected to a high sound level, say 85 dB or over for a short period, and then has an audiometer test, a dip in hearing acuity occurs at 4000Hz. It is often described as the “4kHz dip” for acoustic trauma. The amount of “dip” from the 0dB average level is used to specify the amount of threshold shift. For TTS, the amount of 4kHz dip lessens with time as recovery from exposure occurs
- Permanent Threshold Shift (PTS) Permanent threshold shift is the term used to describe the condition where there is a permanent 4 kHz dip in a persons audiogram. It is a nonreversible condition where the threshold shift does not return to the accepted norm as in TTS. It is generally accepted that PTS is a condition which follows from continual TTS exposures PTS is not a social disability while the dip does not interfere with the speech range. Unfortunately, if further high noise exposures take place, the shift worsens until the condition described as noise induced hearing loss occurs.
Presbycusis is a term used to describe a reduction in hearing acuity with frequencies at the higher end of the audio range which occurs naturally with age. An audiogram would show a smooth hearing loss beginning usually at about 1 kHz and falling to varying levels at about 8 kHz. The amount of hearing loss will vary with age. Between about 30 and 60 years of age the hearing loss can increase from about 5 dB to about 20 dB. Excessive noise can accelerate the onset of natural hearing loss.
Noise Induced Hearing Loss (NIHL)
Noise induced hearing loss is a condition which results from failure of the hair cells in the Organ of Corti to respond fully to sound intensities having frequencies within the speech range. The person does not necessarily lose the ability to hear sound, but is not able to distinguish the spoken word clearly, even if presented with a raised voice.
Noise exposure and health
Noise immission level
Research suggests that the amount of threshold shift is related to the total noise exposure, i.e. the noise dose (level and duration). This enables the definition of an indicator termed the noise immission level, which is a function of the level of noise exposure (or Leq if noise fluctuates) and the number of years exposure.
Consequently the noise Immission level (NIL) is defined as:
NIL=La+10log 10 t
Where La is the level of noise in dB(A) [Leq]
And t is the number of year’s duration
We have already seen that the maximum threshold shift occurs at 4 kHz which is characteristic of noise induced hearing loss. In the early stages of exposure the threshold shift diminishes after a few hours rest. Increased exposure leads to increasing threshold shift of which only part is recoverable after rest. The amount remaining after 40 hours is termed permanent threshold shift (PTS) and that recovered temporary threshold shift (TTS). Over many years of noise exposure the total loss increases with more of it being permanent.
The overall aim of noise control programmes is to reduce the amount of noise induced hearing loss which, as we have seen, depends on both level and duration of exposure. However, like other exposures to occupational agents noise induced hearing loss can also occur as a result of leisure or other non-occupational causes (concerts, nightclubs, etc.).
There is therefore a background level of non-occupational noise exposure below which it is non-productive to try and reduce occupational exposure. It is thus necessary to decide upon a level of exposure which affords sufficient reduction for hearing conservation whilst being realistic. If this level is set too low it will not prevent non-occupational noise induced hearing loss and will require difficult and expensive engineering solutions.
Noise induced hearing loss affects the frequencies that are required for good speech reception. Small losses at these frequencies can be tolerated towards the end of working life and the standard NIL of 104 dB over 40 years is deemed to be an acceptable compromise. Using the definition given above, this is equivalent to 88dB(A) at home and at work. Consequently if noise levels at work are controlled to <88dB(A) then the conservation criteria will not be exceeded.
Toby Hayward works for CH2M HILL as HSEQ Manager covering the Middle East and North Africa Region. CH2M HILL provides engineering, consulting, design, construction, procurement, operations, maintenance, and project management services to clients in the public and private sectors.
He has been in the Middle East for 10 years working in various industries spanning maritime, logistics, operations and construction. He is currently Secretary for the Middle East Branch of the Institution of Occupational Safety and Health (IOSH).
Published: 01st Feb 2008 in Health and Safety Middle East