Is Hearing Loss a Problem?

In 1993/94 in NSW there were 10,934 cases of hearing deafness reported to insurers (Source: Deafness, Statistical Profile 1993/94, WorkCover NSW). This figure represented about 68% of all occupational disease claims, as opposed to 65% in the previous year. A high incidence of deafness was noted in mining; manufacturing (other); manufacturing of metal products; non building construction; and forestry and logging. Over 80% of the claims were from persons aged 40 years or over. The average gross cost of deafness claims was $7,562 however, half the claims incurred a cost of $5,000 or less. The gross incurred cost of deafness was over $82m.

REGULATION OF HEARING CONSERVATION IN NSW

Hearing conservation legislation in NSW is governed by the Occupational Health and Safety Act 1993 (NSW). Under Part 3 of the Act an employer has a general duty to ensure “the health, safety and welfare at work of all his employees”. This duty is contravened if the employer fails to provide a workplace that is safe and without risk to the employees health. Under the Occupational Health and Safety (Noise) Regulation 1996, a workplace is unsafe and a risk to health if any person is exposed to noise levels that exceed an 8-hour noise level equivalent to 85 dB(A) (85 LAeq,8 hour) or the noise levels peak at more than 140 dB(lin). This Regulation refers to the WorkCover code of practice; Code of Practice: Noise Management and the Protection of Hearing at Work, and provides suggested practices for the reduction of noise in the workplace.

WorkCover’s Code of Practice outlines a number of issues which include: identification of noise problems, implementation of noise control policies and programs of action, education of employees, management awareness, and planning of future action in relation to existing and new noise sources.

The main three ways of compliance involve; noise controls, managing the noise, and usage of hearing protectors.

• Noise controls involves designs for new work areas, replacement of old equipment and modifications of old equipment.
• Managing the noise involves reducing workers exposure to noise levels by removing employees from noisy areas, including shorter shifts, or different ways of accessing work areas.
• Usage of hearing protectors could involve selection of correct hearing protectors, education of workers, and management, and sign posting of hearing protector areas.

When noise problems have been dealt with through management or hearing protection usage, audiometric testing of employees could also be considered.

CALCULATION OF NOISE EXPOSURE LEVELS

The noise exposure level that a worker is exposed to is calculated as a logarithmic average of A-weighted noise levels exposed to over a defined period (expressed as LAeq). An A-weighted noise in dB(A) is the noise level frequency weighted in a similar manner to the frequency weighting of the human ear for the same noise. The workplace noise limit in NSW as referred to above is 85 LAeq,8 hour and is also referred to as a noise dose of 1.0.

Diagram below, shows the worker noise exposure level as a portion of the total noise dose for one day (per NSW regulations). For example if a worker (without hearing protection) was exposed to 95 dB(A) for 10 minutes (20% daily allowable dosage) and 105 dB(A) for 4 minutes (80% daily allowable dosage), that person would have received the total allowable noise level for the day and would need to remain in quiet areas (less than 75 dB(A)) for the remainder of the day.

The diagram also shows the affect of workers, working double shifts (16 hours). The maximum continuous exposure to noise would be about 82 dB(A) for a person working straight 16 hour shift.

To calculate the maximum time that a person should be exposed to certain noise levels the following formula below can be used. The values of the partial noise dose can be calculated from Equation 1. The daily noise dose can be calculated by adding the partial noise doses.

Equation 1	PND = (T / 8) * 10 ^ (0.1 * (LAeq,T - 85)) PND = Partial Noise Dose T = duration of segment, in hours LAeq,T = equivalent continuous A-weighted sound pressure level to which the person is exposed, in decibels(A).

EFFECTS & CALCULATION OF NOISE EXPOSURE LEVELS

In 1996 NSW legislature changed the continuous noise exposure level over one day from 90 to 85 dB(A). The basis of this reduction was to reduce the level of noise induced hearing loss for workers throughout their working life. Estimations of noise-induced hearing loss provided in Australia Standard AS1269-1989 “Acoustics - Hearing Conservation”, are based upon equations in International Standard ISO 1999. These equations provide estimates of hearing losses in different frequencies, for certain percentages of the population, noise exposure levels and the number of years a person is exposed to this environment. Table 1 contains an extract from the Standard.

Table 1 - Estimated Prevalence of Noise-Induced Hearing Impairment in Noise-Exposed Populations (Extract from AS1269-1989)
		Hearing Impairment (Permanent Threshold Shift), dB
Exposure	Centile	Frequency, kHz
Duration		1			2			3			4
		Exposure Level (LAeq,8h, dB(A))
Years		80	85	90	80	85	90	80	85	90	80	85	90
5	95 50 5	0 0 0	0 0 0	0 0 0	0 0 0	0 0 2	0 2 6	0 0 1	0 2 4	0 6 11	0 1 1	1 4 6	3 8 13
10	95 50 5	0 0 0	0 0 0	0 0 0	0 0 0	0 1 2	0 2 7	0 0 1	1 3 6	3 8 15	1 1 2	3 5 7	6 11 16
20	95 50 5	0 0 0	0 0 0	0 0 0	0 0 0	0 1 2	1 4 9	0 1 1	2 4 7	6 10 18	1 1 2	4 6 9	8 13 19
45	95 50 5	0 0 0	0 0 0	0 0 0	0 0 0	1 2 3	4 6 11	0 1 1	3 5 8	9 12 22	1 2 2	5 7 10	10 15 22

When the hearing conservation limit was 90 dB(A), noise induced hearing loss could exceed 20 dB in at least one of the frequencies for an individual over their working life. At the current level of 85 dB(A) hearing loss should not exceed 10 dB. But if the maximum noise induced hearing loss was targeted at no greater than 2 dB for 95% of the population then 80 dB(A) would be an appropriate goal for the 8-hour noise level. Table 1 does not give the total permanent threshold shift to be expected in noise-exposed populations. This will be greater than the values given in Table 1 and is determined by a combination of noise-induced threshold shift, threshold shift associated with ageing, and threshold shift due to other conditions which have an adverse effect on hearing.

HEARING PROTECTOR USAGE

Hearing protectors are tested before release and this testing is carried out using a group of people. Each protector is assessed by each individual. From this data, the mean attenuation for each octave band, and the standard deviation for each octave band is determined. When calculating the attenuation of hearing protectors in a working environment, the standard deviation is subtracted from the mean octave band attenuation figures to provide an assumed hearing protection which should ensure that 80 percent of wearers obtain the stated degree of hearing protection. When persons are wearing earmuffs and safety glasses, the performance of the ear muff should be downgraded. The loss in performance is due to the arms of the glasses allowing noise to pass around the arm to the ear drum.

The SLC80 is derived from the octave band attenuation data for each hearing protector and incorporates the mean-minus-standard values. The SLC80 value of a hearing protector indicates the difference between the measured external C-weighted noise level and the A-weighted sound pressure level of the attenuated noise reaching the wearers ear drum.

The SLC80 method of assessment gives an approximate matching of hearing protection. A more precise method of assessment is to measure the workplace noise in octave band frequency spectrums and then match these workplace spectrums to the octave data supplied for each hearing protector.

Hearing protection carries the inherent risks of workers either using the hearing protection incorrectly and / or non-compliance of workers to instructions for wearing hearing protection. This non-compliance could simply be in the form of removing hearing protection to talk to fellow workers or a supervisor. For example if a worker in a noisy environment of 100 dB(A) removed their hearing protection for only 10 minutes to talk to others, this represents 50% of the allowed dosage of noise for the day. This would mean that they would have to be exposed to around 80 dB(A) for the rest of the day, to remain with a noise dosage under 85 dB(A).

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