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| Journal of the New Zealand Medical Association,
22-August-2008, Vol 121 No 1280
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Epidemiology of noise-induced hearing loss in New
Zealand
Peter R Thorne, Shanthi N Ameratunga, Joanna Stewart,
Nicolas Reid, Warwick Williams, Suzanne C Purdy, George Dodd, John
Wallaart
Noise-induced hearing loss (NIHL) is a form of hearing loss
caused by sustained and repeated exposure to excessive sound levels. While
commonly attributed to prolonged employment in high-noise industries, any form
of sound exposure can lead to NIHL provided there is sufficient intensity and
exposure time.1 The hearing loss occurs because
of damage to the hearing organ (cochlea) of the inner
ear.2,3
Exposure to sound above a level of approximately 85dBA
initially manifests as a temporary hearing loss or ‘dullness’ of
hearing (a temporary threshold shift) which recovers within 16–24
hours of the exposure. However, with repeated or sustained exposure, the hair
cells and associated nerve fibres degenerate and the threshold shift becomes
permanent (permanent threshold
shift).1
Hearing loss
is any change in hearing acuity in quiet or in the presence of background noise,
but can be quantified in an audiogram as an auditory threshold of greater than
15dB at any frequency.4 In New Zealand, hearing
loss is categorised as slight (15–25dB Hearing Level), mild
(26–40dBHL), moderate (41–55dBHL), moderately severe
(56–70dBHL) severe (71–90HL), or profound (91dBHL and
above).4,5
Generally a threshold of 25dBHL is regarded as the hearing
level at which a person will experience significant
problems.6 The hearing loss from noise exposure
is typically binaural; it increases in severity with exposure level and duration
and can result in a substantial hearing
disability.7,8
The loss of sensitivity and clarity of high pitched sounds
and inability to discriminate speech sounds particularly in the presence of
background noise result in major communication
difficulties.9 People may also become
intolerant of loud sounds and complain of
tinnitus.8,10,11 These difficulties result in
substantial physical and psychological distress for those affected by reducing
their quality of life through limiting communication, entertainment, and
employment opportunities, and place a substantial burden on their families and
friends.12 Not surprisingly, uncorrected
hearing loss can lead to social isolation and
depression.13,14
Internationally NIHL is recognised as a significant
occupational health problem,15 with estimates
of the prevalence ranging from about 7% of the population in Western countries
to 21% in developing nations.16 In Australia,
hearing loss is estimated to cost the country about $11.6 billion with NIHL
accounting for about one-third of this
cost.17
The epidemiology of NIHL in New Zealand is poorly understood
and more information is needed to identify effective strategies to prevent the
hearing loss and the resultant individual and societal burden.
This paper describes the burden of NIHL in New Zealand as
determined by data collated by national occupational and rehabilitation
compensation systems over the past decade. The predominant source of information
for this study is the number of claims to the Accident Compensation Corporation
(ACC). These describe the number and rate of successful claims for
rehabilitation from individuals with hearing loss from noise exposure. While
these do not necessarily reflect the overall incidence of NIHL in the
population, they serve as an index of the most readily available data on new
cases that can be used to inform trends over time, across industries, ages, and
ethnic groups.
Using these data, we examine related costs and high-risk
sociodemographic groups to inform a population health strategy to address this
problem.
MethodsA review of the published literature was undertaken to
identify articles in scientific journals estimating the prevalence of NIHL in
New Zealand. Medline and PsychInfo online databases were searched using relevant
MeSH terms and text words such as “noise-induced hearing loss”
“noise”, “sound”, “hearing loss”,
“deafness”, “occupational disease”, “hearing
conservation” and “hearing protectors”. Key journals in the
fields of audiology, hearing, noise and occupational medicine were hand-searched
and their reference lists checked.
Unpublished articles, reports, monographs, and
conference presentations in the ‘grey literature’ were identified
through websites and contacts with key researchers in New Zealand and Australia.
We also obtained and analysed de-identified data from ACC on new and active
claims relating to NIHL together with information on the age, gender, ethnicity,
and occupational group of claimants.
The data from each of these sources was summarised with
a particular focus on information that provided population-based estimates of
NIHL, and attention to the quality and reliability of these data, where this
information was available.
Poisson regression was used to investigate whether the
relationship of age with the probability of making an ACC claim had changed over
time. The outcome was the number of claims, with the population as an offset and
age, year and their interaction included as explanatory variables.
ResultsThere is very little published data on NIHL in New Zealand
and there are few reports that specifically address its prevalence in the
community.
Population surveys—In an overall
occupational health survey of 381 farmers in Southland, it was
reported that 11.6% (17.1% men; 1.5% women) had
NIHL.18,19 This high prevalence rate is
consistent with the noise levels experienced by agricultural
workers19 and ACC data discussed below.
Estimates of the prevalence of hearing loss, from any cause, in the New Zealand
population have been derived by extrapolating data from overseas
studies20,21 or from New Zealand Census data
collected in 1991,22 1996, and
2001.23,24
Overall, 10–13% of the population are estimated to
have significant hearing impairment with a higher prevalence in men. It is
estimated that in New Zealand between 30%23 and
50%20 of the prevalence of hearing loss in
adults can be attributed to noise exposure during a lifetime, which is
comparable with the estimates in Australia of
38%.17
Greville23 further suggests
that the reduction in the number of people working in potentially noisy
occupations (for example from 45% in 1981 to 36% in 1996) should result in a
decrease in the incidence of NIHL due to occupational noise.
Notifiable occupational disease system
(NODS)—Noise-induced hearing loss is a category within the NODS, a
voluntary register maintained by the Department of Labour Occupational Health
and Safety Service.25
From 1992 to 1998, 2411 cases (95% male) of NIHL were
reported to this register, with a further 709 notifications from 1998 to
2000.26 Collectively, NIHL accounted for
approximately one-third of the occupational disease notifications during the
period 1992–2000, a condition surpassed in frequency only by
‘occupational overuse syndrome/osteoarthritis’. However, as with
most other voluntary reporting systems, this database is unlikely to provide a
reliable indication of the prevalence of occupational NIHL at a population
level.
Accident Compensation Corporation of New Zealand
(ACC) Database: 1995–2006—The ACC maintains a database of
successful claims for NIHL within New Zealand—a routinely collected
administrative data source that has the potential to provide estimates of NIHL
in the community. An analysis of ACC data indicates a steady rise in new claims
from July 1995 to June 2006 (Figure 1). The numbers of new claims incurring a
cost to ACC in 2005–6 (5580 cases) were approximately double the figure in
1995–1996 (2823). The ongoing claims include provision of batteries as
well as replacement or new hearing aids. The detail of the ongoing claims was
not analysed further
Figure 1. The number and the cost of new and
ongoing ACC claims annually between July 1995 and June 2006.
 The ACC compensation costs associated with the 44,106 new
claims for NIHL over the study period was $89.94 million. With the ongoing NIHL
rehabilitation costs, ACC has paid $193.82 million since 1995. The cost of new
claims to ACC has increased by an average of 20% each year over the last decade.
The 5580 claims in the 2005–2006 financial year accounted for just over
$14 million representing an average first year cost of $2540 per claim (Figure
1). Over the same period, a further $38.8 million was spent on 17,871 ongoing
claims for compensation and rehabilitation costs, leading to an overall cost
directly related to NIHL in 2005–2006 of almost $53.06 million.
Agriculture and fisheries workers, plant and machine
operators, labourers, and trade workers were the most common groups of the
claimants based on the occupational category of individuals at the time of
lodging the claim (Figure 2).
Collectively, these groups accounted for over 50% of the
claims (excluding the proportion where occupational group is not recorded). This
finding is consistent with the recognised risk of exposure to high noise levels
in these industries. While these groups, in general, featured in similar
proportions across the analysis period, there appeared to be an increase in the
proportion of professional groups among claimants, largely due to an increase in
claims from engineering professionals. Of concern, however, the occupational
group was unknown or not recorded for over 30% of the ACC claimants.
Figure 2. Distribution of new ACC claims by
occupational categories for the period between July 1995 and June 2006. The
occupational categories have been collapsed into a smaller number representing
managerial, professional, clerical and trades for clarity
 Approximately 95% of claims were made by men. The vast
majority of new claims on the database related to people aged in their 50s and
beyond, with increasing numbers among those nearing retirement age (Figure 3).
Interestingly, the age distribution profile of new claims has shifted towards an
older age group over the past decade (Figure 4). The difference in the
probability of people in different age groups making a claim changed across time
(p<0.0001) with rates in the older age groups increasing more over the 11
years than in the younger age groups.
Figure 3. The distribution of new and ongoing
ACC claims across age for males and females for the period July 1995 to June
2006. The trendlines were calculated using a
6th-order polynomial
 Figure 4. The rate of new ACC claims (rate per
100,000 population) across age for 6 years (1996, 1998, 2000, 2002, 2004, 2006)
 The ethnic group was unknown or not recorded for 9% of
claimants. Of the remainder the ethnicity of claimants—as coded in this
database—was predominantly European/Pākehā (86%) with relatively
few identified as Māori (5%) and Pacific peoples (1%) (Table 1).
For males 45 years and older (as most of the claimants were
in this age bracket) approximately 7% of the population are Māori and 3%
Pacific peoples.27
Table 1. The percentage of new ACC claims
broken down by ethnic group according to ACC classification
*Mostly of Samoan, Tongan, Niuean, or Cook Islands
origin.
DiscussionThe most recent information from the ACC database indicates
that about 15 New Zealanders successfully claim compensation for a new case of
NIHL each day, and the costs associated with compensation and rehabilitation
have increased considerably over the past decade. For several
reasons—including the potential for inadequate screening and detection in
the community—selection biases in administrative databases, and changes in
reporting and coding practices, the apparent increase in the annual number of
new claims of NIHL in the ACC database over the past decade may not accurately
reflect trends in the population-based incidence of NIHL. The findings, however,
point to an indisputably high cost to ACC, government, and society as a result
of a theoretically preventable condition.
Despite the substantial burden, this review revealed a
remarkable lack of robust data on the epidemiology of NIHL in New Zealand. Based
on the census and linked disability surveys, the estimated prevalence of hearing
loss (overall) resulting in disability (10%) in New
Zealand23,24 is similar to the lower end of the
range of prevalence estimates in other high-income countries. For example, the
National Institute on Deafness and Communication
Disorders28 has estimated that the population
prevalence of hearing loss in the USA is about 10% with about 3–4% due to
noise exposure. This is compared with a prevalence of hearing loss of about 14%
(in the better hearing ear) in the United
Kingdom29 and of 17% in Australia, with
approximately 6% due to noise.17
No equivalent population-based information was available for
NIHL in New Zealand. Although the 2001/2 national disability survey sought
information on the causes of hearing loss (where present), noise was not
specified as a potential origin. The apparent inadequacies of available data are
mirrored internationally, and reflect difficulties in defining the cause of
hearing loss (if a single source was likely), differences in the criteria used
to define hearing loss in surveys, and changes over time in the exposure to
ambient noise, e.g. due to interventions within industry.
Nevertheless, the World Health Organization estimated that
at a global level occupational noise exposure was the cause of the hearing
impairment in one-sixth (16%) of those people with a moderate or greater hearing
loss.15 A US study assessed the contribution of
occupational noise exposure to total deafness rates as approximately 7% in the
most developed nations and 21% in developing
regions.(16)
Although the overall burden remains uncertain, the analysis
of ACC data revealed that a relatively high proportion of claims were lodged by
workers in the agricultural, building, and manufacturing sectors, as well as
plant and machinery operators. Overall this is consistent with the estimated
prevalence of hearing loss (from all causes) in different New Zealand
occupations (based on the 2001/2 Disability
Survey24), which is relatively high in those
employed in the construction (11.1%) and manufacturing (6.4%) industries.
In contrast, the prevalence of hearing loss (overall) among
those employed in the agriculture, forestry and fisheries group was lower
(3.8%), which may suggest a difference in motivation to claim for NIHL among the
different occupational groups. It should be noted, however, that the latter
survey did not seek information on the history of noise exposure or previous
occupations that may have been relevant to the loss of hearing. Furthermore, the
figures represented are likely to be influenced by the age distributions of the
labour force involved.
In general, a third or more of new claims for NIHL were made
by people older than the usual retirement age in New Zealand. Based on the data
available, we could not determine if this primarily reflected the latent period
between the onset of hearing loss and its appreciation by the claimant to a
point where assistance is sought, an interaction between NIHL and age-related
hearing loss, or both. The relative contributions of these factors and their
relationship to the age profile of NIHL claims require further investigation.
The low representation of claimants of Māori and
Pacific ethnicity raise particular concerns with regard to the potential biases
of these data. Notwithstanding the relatively younger age distribution of these
populations relative to New Zealand Europeans (and therefore the lower risks of
NIHL from an age perspective), both Māori and Pacific peoples are
over-represented in many industries where noise exposure is higher. Furthermore,
Māori have a higher prevalence of hearing loss (overall) across age
groups.20,22–24,30,31
Explanations for the lower than expected claims from these
ethnic groups could include the quality of ethnicity coding in the ACC database
(which does not currently employ the recommended New Zealand Ethnicity
Protocol), limited screening and diagnosis of NIHL of these groups in the
community, and barriers to accessing healthcare and ACC services. The 2001/2
national disability survey noted that despite their higher prevalence of hearing
loss resulting in disability, Māori adults were only half as likely as
non-Māori to use hearing aids or other assistive listening
devices.24
Without reliable information regarding changes in exposure
to ambient noise over time in major industries and given uncertainties regarding
the population-based incidence and prevalence estimates, it is difficult to
interpret the apparent increase in numbers of new compensation claims in New
Zealand. It is not possible to reliably determine if this reflects greater
exposure to hazardous levels of ambient noise at work and elsewhere over recent
decades, artefacts of surveillance (e.g. increasing awareness and screening for
hearing loss, diagnostic and reporting trends, and changes in ACC compensation
criteria), or other explanations such as the improvement in hearing technologies
and uptake of hearing aids.
In the absence of better knowledge regarding the underlying
determinants, it is also difficult to reliably project future trends in the
burden of NIHL. As noted by Greville,24 it
could be argued that as the number of people working in traditionally noisy
industries in New Zealand declines, the number of individuals developing hearing
loss from noise exposure should, at least theoretically, diminish. This
possibility requires more detailed examination, particularly as noise levels may
be increasing in “non-traditional” industries (e.g. hospitality and
education environments) and the decline in worker numbers may not be occurring
equally across industries with high NIHL incidence rates.
Furthermore, the apparent increase in the rate of new claims
in older age groups over the 1996–2006 period is not associated with a
decline in rate of new claimants in younger age groups. This is not consistent
with a decline in NIHL claims in the near future. In a context where increasing
numbers (and proportions) of claimants are of older age, distinguishing real
shifts in the age of onset of NIHL due to changes in exposure to noise from
age-related hearing loss is problematic. This is compounded by other temporal
changes in factors relating to claimants and providers such as increasing public
awareness of NIHL, increasing availability of audiology services and access to
more effective hearing aids. These issues require exploration using focused
studies that are not limited to claims data.
Given the many uncertainties but major public health
significance of NIHL, there is a critical need for dedicated surveys of workers
in different industries including audiometric assessments of hearing. These data
would provide more robust information to estimate the current burden and future
trends of occupationally-mediated NIHL. The impact of non-occupational noise
exposure (e.g. shooting and loud music) should also be considered, acknowledging
the value of blurring the distinction between on and off-work
injuries32,33 when considering the determinants
of NIHL and opportunities for prevention.
New Zealand and many other high-income countries have
implemented strategies of varying intensity to reduce the incidence of NIHL.
These efforts have been lauded as successful in some countries such as the UK
and Finland where the numbers of cases of NIHL and claims for compensation have
declined steadily.34 In contrast, this burden
is estimated to be increasing in other European countries such as The
Netherlands.34 Also the European Agency for
Safety and Health at Work reports an increase in the proportion of workers who
report hearing problems because of noise in the
workplace.
The European Agency for Safety and Health at Work reports an
increase in the proportion of workers who report hearing problems because of
noise in the workplace.34 The data underlying
these contradictory assertions must be examined with caution given differences
in the data collected, definitions of NIHL and criteria for compensation across
countries and over time. Regardless of these caveats, noise exposure is an
occupational hazard and interventions are required in industry to reduce its
effects on hearing.
To address the significant burden of NIHL in New Zealand, a
more rigorous population health approach to identify and monitor outcomes and
investigate and address antecedent causes is essential. In a recent review of
the best-practice approaches for NIHL
prevention35
we identified a shift internationally towards hearing loss prevention
and noise management approaches rather than the more passive
hearing conservation approach. Furthermore, it was clear that these
interventions can only be effective if there is strong management support and
commitment, consistent high quality noise and audiology monitoring and strict
adherence to the use of hearing protectors. Sole reliance on the use of hearing
protectors as the main means of protection against NIHL is unlikely to deliver
the expected outcome.
It is encouraging that in New Zealand, ACC, and the
Department of Labour along with the Health Research Council of New Zealand have
identified NIHL as a priority occupational condition and are working with
industry, researchers and the community to understand the epidemiology of the
problem in this country and develop better interventions.
Competing interests: One of the
authors, John Wallaart, is an employee of the Accident Compensation
Corporation.
Acknowledgement: This research was funded
in part by an Accident Compensation Corporation and Health Research Partnership
Project Grant.
Author information: Peter R Thorne,
Professor, Audiology, University of Auckland, Auckland; Shanthi N Ameratunga,
Associate Professor, Epidemiology and Biostatistics, University of Auckland,
Auckland; Joanna Stewart, Senior Research Fellow, Epidemiology and
Biostatistics, University of Auckland, Auckland; Nicholas Reid, Assistant
Research Fellow, Audiology , University of Auckland, Auckland; Warwick Williams,
Senior Research Scientist, National Acoustics Laboratory, Sydney, Australia;
Suzanne C Purdy, Associate Professor, Psychology (Speech Science), University of
Auckland, Auckland; George Dodd, Senior Lecturer, Acoustics Research Centre,
University of Auckland, Auckland; John Wallaart, Project Manager, Accident
Compensation Corporation, Wellington
Correspondence: Professor PR Thorne,
Section of Audiology, University of Auckland, Private Bag 92019, Auckland, New
Zealand. Fax: +64 (0)9 3737496; email: pr.thorne@auckland.ac.nz
References:
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