Abstract (summary)

TranslateAbstractNoise-induced hearing loss (NIHL) still remains a problem in developed countries, despite reduced occupationalnoise exposure, strict standards for hearing protection and extensive public health awareness campaigns. Therefore NIHL continues to be the focus ofnoiseresearch activities. This paper summarizes progress achieved recently in our knowledge of NIHL. It includes papers published between the years 2008-2011 (in English), which were identified by a literature search of accessible medical and other relevant databases. A substantial part of this research has been concerned with the risk of NIHL in the entertainment sector, particularly in professional, orchestral musicians. There are also constant concerns regardingnoise exposureand hearing risk in "hard to control" occupations, such as farming and construction work. Although occupationalnoisehas decreased since the early 1980s, the number of young people subject to socialnoise exposurehas tripled. If theexposurelimitsfromtheNoiseat Work Regulations are applied, discotheque music, rock concerts, as well as musicfrom personalmusic players are associated with the risk of hearing loss in teenagers and young adults. Several recent research studies have increased the understanding of the pathomechanisms of acoustic trauma, the genetics of NIHL, as well as possible dietary and pharmacologic otoprotection in acoustic trauma. The results of these studies are very promising and offer grounds to expect that targeted therapies might help prevent the loss of sensory hair cells and protect the hearing ofnoise-exposed individuals. These studies emphasize the need to launch an improvednoise exposurepolicy for hearing protection along with developing more efficient norms of NIHL risk assessment.

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Exposureto excessivenoiseis one major cause of hearing disorders. It has been estimated that worldwide as many as 500 million individuals might be at risk of developingnoise-induced hearing loss (NIHL). [1]

Prolongedexposuretonoiseat high intensity is associated with damage to the sensory hair cells of the inner ear and development of permanent hearing threshold shift, as well as poor speech innoiseintelligibility. There is also evidence thatnoise exposurefrequently leads to tinnitus which might be due to alterations in the central auditory function. [2] In the adult population it may significantly influence quality of life, and constitute a major limitation in relation to hearing-critical jobs, decreasing the potential worker's chance ofemployment. Thus, NIHL not only affects health, but is also a major social problem.

The aim of this review was to provide an overview of the studies and reportsfrominternational bodies published in English over the last four years (2008-2011) on NIHL. If appropriate, the results of the recent studies were then compared to previous findings. The papers were identified by a literature search of accessible medical and other databases (PubMed, Embase, Scopus, BioMed Central, Web of Science). This review summarizes some progress achieved over the recent years in our knowledge ofnoiseeffects on hearing loss. In particular the paper concerns the risk of hearing loss due to occupationalexposuresof musicians, farmers and construction workers, the advances in molecular genetics of NIHL and pharmacological/dietary otoprotection, and hearing conservation issues.

Hearing loss in professional musicians

According to several studies, including the most recent ones, professional orchestral musicians are often exposed to sounds (so-called "orchestralnoise") atlevelsexceeding the upperexposureaction values referred by the 2003/10/ECnoisedirective. [3],[4],[5]

Classical orchestral musicians are usually exposed to sounds at equivalent continuous A-weighted sound pressurelevelsof 81 - 90 dB (10 th-90 th percentiles), for 20-45 h (10 th-90 th percentiles) per week. According to the ISO 1999:1990 model, occupationalexposuresto such soundlevelsover 40 years ofemploymentmight cause hearing loss exceeding 35 dB (expressed as average of hearing thresholdlevelat 2, 3 and 4 kHz) in up to 26% of individuals. The highest risk (above 20%) is related to playing the horn, trumpet, tuba and percussion. [5]

Recently, bilateral soundexposureof classical symphony orchestra musicians was assessed bynoisedosimetry simultaneously in the left and right ear. [6] It confirmed that 'soundexposuredepends significantly on the specific instrument and the repertoire played by the exposed musician'. Concerts, group rehearsals and individual practice were all significant contributors to the soundexposure. The highest LAeq of 86-98 dB was found among the brass players. High string players were exposedfrom82 to 98 dBA and their left ear was exposed 4.6 dB more than the right ear. Percussionists were exposed to high sound peaks >115 dBC but less continuous soundexposurewas observed in this group. Musicians were exposed up to LAeq8h of 92 dB and a majority of musicians were exposed to soundlevelsexceeding LAeq8h of 85 dB. [6]

Because of insufficient audiometric evidence of hearing loss caused purely by musicexposure, there is still disagreement and speculation about the risk of hearing loss in professional musiciansfromtheir overexposure to music alone. [7],[8],[9],[10],[11] It has been shown that the distribution of hearing loss among musicians corresponded to that of the general population, but highly exposed musicians had somewhat greater hearing loss at frequencies above 3 kHz than less-exposed ones. Even so, music seems to damage hearing by less than what could be predicted based on the ISO 1999:1990 standard for occupationalnoise exposures. This might be explained by the relatively low number of individual susceptibility risk factors found in this group of professionals. [11],[12] Musicians are also less exposed to other contaminants generally found in industrial settings, like vibration or chemicals.

Despite a highlevelof soundexposureand a fairly large selection of earplugs available, musicians reported 'only seldomuse' ofpersonalhearing protectors. For better hearing conservation, it is important to identify and eliminate the reasons for low motivation in using hearing protection in this professional group. [13]

Hearing loss in farmers

Noise exposuresamong farming communities can exceed recommendedlevels. It was estimated that during planting, growing and harvesting seasons, an 8-h time-weighted average among adult farm workers rangedfrom46.1 to 89.6 dB using the OSHA criteria, andfrom62.6 to 92.1 dB using NIOSH/ACGIH actionlevel. Respective values for children werefrom15.4 to 81.2 andfrom42.4 to 85.5 dB. [14]

Farming is ranked among the top occupations with the highest risk for hearing loss, mainly because of non-useof hearing protection devices. [15] It has been shown that hearing loss is prevalent among adults in farming communities, with some evidence that it begins in childhood. [16] Baseline data including audiometric thresholds were collectedfromUS youths living on farms in 1994-1996 (n = 212) with follow-up in 2003-2004 (n = 132). Youths in this study had a higher prevalence of hearing loss when compared to nationally-representative data, and nearly 50% of them exhibited high-frequency hearing loss (mainly at 6 kHz). The prevalence ofnoise-induced threshold shifts, characterized by an audiometric notch, was nearly twice that of the national sample. [17] These data indicate that hearing loss is common not only in adult farmers, but also in teenagers living on farms. However, the age when NIHL begins among farmers remains unknown.

Hearing loss in construction workers

Construction workers are also at high risk of developing NIHL. Depending on the metric used, the measured full-shiftnoise exposureexceeded permissible and recommendedexposurelimits in one-third to three-quarters of 1310 construction workers. [18] Future studies are planned using different metrics ofnoise exposure(trade-mean-equivalent continuousexposure level; task-basedexposure level; and a hybrid combining task-based and subjective information) to evaluate theexposure-response relationship betweennoiseand NIHL in this group of workers.

Although theuseof hearing protector devices is much more common in construction workers than in agriculture, they often have no regular audiometric testing. It has been shown that among 169 construction employees examined for Hand-Arm Vibration Syndrome, 31 (18.3%) had hearing loss at or above thelevelat which a workers' compensation pension would be granted in Ontario (Canada). [19]

The very recent study performed in a much larger population of 29,644 Dutch construction workers has shown thatnoise-exposed subjects had greater hearing losses compared to their non-noise-exposed colleagues, as well as to the reference population reported in ISO-1999. When the dailynoise exposure levelrosefrom80 dB(A) towards 96 dB(A) only a minor increase in hearing loss was shown. Duration ofnoise exposurewas a better predictor thannoise exposure levels, probably because of the limitations in accuracy ofnoise exposureestimates. [20]

Hearing loss due topersonalmusic playersuseamong youths

It has been estimated that over 20 years, since the early 1980s to 2000, the number of young people with socialnoise exposurehas tripledfrom6.7% to 18.8%. [21] This emphasizes thatexposureto different types ofnoiseand sounds since early childhood should be recognized as having potential cumulative effects on hearing impairment in adulthood and in old age.

The major sources of sound/noise exposuresin teenagers and young adults are discotheque music, rock concerts, and musicfrom personalmusic players (PMP). Responding to increasing demandsfromthe public, the European Commission recently decided to check that sufficient preventive measures are in place to prevent hearing loss among children and adolescents exposed to musicfromdevices likepersonalmusic players (PMP). The opinion was delivered to the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) in 2008 by a group of experts (Chairman - Konrad Rydzynski, Nofer Institute of Occupational Medicine, Lodz, Poland, Rapporteur - Mariola Sliwinska-Kowalska, Nofer Institute of Occupaytiona Medicine, Lodz, Poland, members: Adrian Davis, Deepak Prasher, Hans Vershuure, Paolo Ravazzani, Yves Cazals, Staffan Hygge, Thomas Yung, James Bridges). [22] On the basis of the available literature on the subject, the expert group posed the question whether theexposureto music sounds (noise)fromPMP and other devices with music player function might cause a quantifiable health risk, in particular hearing loss. If the answer was yes, they were asked to identify thelevelofnoiseemission which would safeguard the hearing health of citizens, and identify priority issues for further research.

Based on a literature search it was estimated that 5 to 20% of young people have audiometric "notches" at 4-6 kHz, that may indicate excessivenoise exposure, but this rate seems to have remained constant over the last 30 years. [22],[23] The investigations were performed in German, Australian, Swedish and American populations where PMPs were commonly inusesince the 1980s. However, some studies point to an increasing overall prevalence of high-frequency hearing impairment in young people between 1987-2005, compared to that in the '70s and early '80s. [24] In addition, this increase in prevalence has been seen recently in young females. [25] However, the conclusionfromall these epidemiological studies should be viewed with caution, because notched audiogram prevalence varied greatly by definition; moreover, in up to 11% of adults it can occur in the absence of any positivenoisehistory, either occupational or leisure. [26]

The SCENIHR group of experts has agreed that theNoiseat Work Regulations can be used for calculatingexposureand risklevelrelated to PMPuse. Although this regulation and its limits apply to the workplace, the fact that they rely on theexposure leveland duration means that they can be successfully applied to other situations where sound can have a detrimental effect, whether used in workplace or leisure settings.

It was estimated that there are twelve million daily users of PMP in Europe. According to the literature data, thelevelsofexposureto soundsfromusing PMP on regular basis range widelyfrom60 to almost 120 dB(A) among the users, and weeklyexposuretime isfrom50%).

In relation to the priority issues for further research, the group underlined the lack of a proven causal relationship between temporary threshold shift afterexposureto music and subsequent permanent hearing damage, or of a changefrompost-exposuretinnitus to permanent tinnitus. They emphasized that long-term longitudinal cohort studies are needed to show whether theexposureto PMP music as teenagers can influence hearing loss in old age.

Since the SCENIHR report was produced in 2008, there has been a growing body of literature published on assessing the risk of hearing loss due to PMPusein teenagers and young adolescents. Williams' comparative study of Australian teenagers' listening habits with PMPS showed that average equivalent soundlevelsof music had decreased in 2009 compared to those in 2005. The mean decrease was 5 dB (from80 dB (A) in 2005 to 75 dB (A) in 2009); and the percentage of overexposed individuals droppedfrom25% to 17%. [27] Amongst 28 Canadian teenagers, the median soundlevelsat typical and "worst case" volume settings were 71 dBA and 79 dBA respectively. When typical soundlevelswere considered in combination with self-reported duration of dailyuse, none of the participants surpassed Leq(8h) 85 dBA. [28] The listening time was around or below 2 h a day, and overexposure focused on mostly young (college-age) people, mainly males. [29] The risk in people older than 30 was negligible. [27]

However, findingsfromsome recently published studies are alarming. Out of 1687 adolescents in Dutch secondary schools who underwent a questionnaire survey, 90% reported listening to music through earphones on MP3 players; amongst these, 28.6% were categorized as listeners at risk for hearing loss as a result of estimatedexposureof 89 dBA for >1 h per day. [30] As many as 5.5% of PMP teenage users were exposed tolevelsabove 100 dBA, whilst none of them were exposed to such highlevels fromdisco music. [31] Another study of 189 college students, 18-53 years of age, in a New York City college, found that 58.2% exceeded daily the 85 dB A-weighted 8-h equivalent soundlevels(LAeq ). The study concluded that the majority of PMP users are at increased risk for NIHL. [32] Also, in Canadian high school students, 42% of individuals were shown to be exposed to musicfromPMP at equivalent soundlevelsof 85 dB and more, raising concerns of hearing loss. Moreover, the authors showed fourfold significant increase of tinnitus prevalence in teenagers listening to music at thelevel>80 dB (A), as compared to those listening tolevels