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|Year : 2012 | Volume
| Issue : 61 | Page : 274--280
Noise-induced hearing loss
Mariola Sliwinska-Kowalska1, Adrian Davis2,
1 Department of Audiology and Phoniatrics, Nofer Institute of Occupational Medicine, 8 St Teresa Str, 91-348 Lodz, Poland
2 MRC Hearing and Communication Group, London, WC1X 8BP, United Kingdom
8 St Teresa Str, 91-348 Lodz
Noise-induced hearing loss (NIHL) still remains a problem in developed countries, despite reduced occupational noise exposure, strict standards for hearing protection and extensive public health awareness campaigns. Therefore NIHL continues to be the focus of noise research 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 regarding noise exposure and hearing risk in «DQ»hard to control«DQ» occupations, such as farming and construction work. Although occupational noise has decreased since the early 1980s, the number of young people subject to social noise exposure has tripled. If the exposure limits from the Noise at Work Regulations are applied, discotheque music, rock concerts, as well as music from personal music 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 of noise-exposed individuals. These studies emphasize the need to launch an improved noise exposure policy for hearing protection along with developing more efficient norms of NIHL risk assessment.
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Sliwinska-Kowalska M, Davis A. Noise-induced hearing loss.Noise Health 2012;14:274-280
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Sliwinska-Kowalska M, Davis A. Noise-induced hearing loss. Noise Health [serial online] 2012 [cited 2020 Jul 4 ];14:274-280
Available from: http://www.noiseandhealth.org/text.asp?2012/14/61/274/104893
Exposure to excessive noise is one major cause of hearing disorders. It has been estimated that worldwide as many as 500 million individuals might be at risk of developing noise-induced hearing loss (NIHL). 
Prolonged exposure to noise at 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 in noise intelligibility. There is also evidence that noise exposure frequently leads to tinnitus which might be due to alterations in the central auditory function.  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 of employment. 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 reports from international 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 of noise effects on hearing loss. In particular the paper concerns the risk of hearing loss due to occupational exposures of 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 "orchestral noise") at levels exceeding the upper exposure action values referred by the 2003/10/EC noise directive. ,,
Classical orchestral musicians are usually exposed to sounds at equivalent continuous A-weighted sound pressure levels of 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, occupational exposures to such sound levels over 40 years of employment might cause hearing loss exceeding 35 dB (expressed as average of hearing threshold level at 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. 
Recently, bilateral sound exposure of classical symphony orchestra musicians was assessed by noise dosimetry simultaneously in the left and right ear.  It confirmed that 'sound exposure depends 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 sound exposure. The highest LAeq of 86-98 dB was found among the brass players. High string players were exposed from 82 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 sound exposure was observed in this group. Musicians were exposed up to LAeq8h of 92 dB and a majority of musicians were exposed to sound levels exceeding LAeq8h of 85 dB. 
Because of insufficient audiometric evidence of hearing loss caused purely by music exposure, there is still disagreement and speculation about the risk of hearing loss in professional musicians from their overexposure to music alone. ,,,, 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 occupational noise exposures. This might be explained by the relatively low number of individual susceptibility risk factors found in this group of professionals. , Musicians are also less exposed to other contaminants generally found in industrial settings, like vibration or chemicals.
Despite a high level of sound exposure and a fairly large selection of earplugs available, musicians reported 'only seldom use' of personal hearing 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. 
Hearing loss in farmers
Noise exposures among farming communities can exceed recommended levels. It was estimated that during planting, growing and harvesting seasons, an 8-h time-weighted average among adult farm workers ranged from 46.1 to 89.6 dB using the OSHA criteria, and from 62.6 to 92.1 dB using NIOSH/ACGIH action level. Respective values for children were from 15.4 to 81.2 and from 42.4 to 85.5 dB. 
Farming is ranked among the top occupations with the highest risk for hearing loss, mainly because of non-use of hearing protection devices.  It has been shown that hearing loss is prevalent among adults in farming communities, with some evidence that it begins in childhood.  Baseline data including audiometric thresholds were collected from US 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 of noise-induced threshold shifts, characterized by an audiometric notch, was nearly twice that of the national sample.  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-shift noise exposure exceeded permissible and recommended exposure limits in one-third to three-quarters of 1310 construction workers.  Future studies are planned using different metrics of noise exposure (trade-mean-equivalent continuous exposure level; task-based exposure level; and a hybrid combining task-based and subjective information) to evaluate the exposure-response relationship between noise and NIHL in this group of workers.
Although the use of 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 the level at which a workers' compensation pension would be granted in Ontario (Canada). 
The very recent study performed in a much larger population of 29,644 Dutch construction workers has shown that noise-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 daily noise exposure level rose from 80 dB(A) towards 96 dB(A) only a minor increase in hearing loss was shown. Duration of noise exposure was a better predictor than noise exposure levels, probably because of the limitations in accuracy of noise exposure estimates. 
Hearing loss due to personal music players use among youths
It has been estimated that over 20 years, since the early 1980s to 2000, the number of young people with social noise exposure has tripled from 6.7% to 18.8%.  This emphasizes that exposure to different types of noise and 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 exposures in teenagers and young adults are discotheque music, rock concerts, and music from personal music players (PMP). Responding to increasing demands from the 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 music from devices like personal music 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).  On the basis of the available literature on the subject, the expert group posed the question whether the exposure to music sounds (noise) from PMP 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 the level of noise emission 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 excessive noise exposure, but this rate seems to have remained constant over the last 30 years. , The investigations were performed in German, Australian, Swedish and American populations where PMPs were commonly in use since 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.  In addition, this increase in prevalence has been seen recently in young females.  However, the conclusion from all 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 positive noise history, either occupational or leisure. 
The SCENIHR group of experts has agreed that the Noise at Work Regulations can be used for calculating exposure and risk level related to PMP use. Although this regulation and its limits apply to the workplace, the fact that they rely on the exposure level and 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, the levels of exposure to sounds from using PMP on regular basis range widely from 60 to almost 120 dB(A) among the users, and weekly exposure time is from <1 h-14 h. When transformed to A-weighted field equivalent sound pressure levels (SPLs), sound levels are on average from 75 to 85 dB, indicating that up to 25% of this population is at risk of developing hearing loss when listening to music at this level for 8 h, over a long period of time.
If the equivalent value of 80 dB (lower action level according to the 2003 / 10/EC noise directive) is converted using the time-intensity trade-off of 3 dB increase for halving the time, then the minimal action exposure level would be reached after listening to a PMP at 95 dB(A) for 15 min a day, or at 89 dB(A) for 1 h a day, assuming that this exposure is repeated over a long period. The results of the studies published up to 2008 indicated that equivalent SPL referred to 8-h time exposure (LAeq ,8h) ranging between 75 to 85 dB(A). Thus, in the majority of PMP users, a risk of hearing loss seems to be minimal. In conclusion, the group of experts concluded that 5-10% of young listeners (approx. 2.5 million teenagers in Europe) are at high risk of developing hearing loss after five or more years of exposure. These are the individuals listening to music for over 1 h a day at high volume control setting (>50%).
In relation to the priority issues for further research, the group underlined the lack of a proven causal relationship between temporary threshold shift after exposure to music and subsequent permanent hearing damage, or of a change from post-exposure tinnitus to permanent tinnitus. They emphasized that long-term longitudinal cohort studies are needed to show whether the exposure to 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 PMP use in teenagers and young adolescents. Williams' comparative study of Australian teenagers' listening habits with PMPS showed that average equivalent sound levels of music had decreased in 2009 compared to those in 2005. The mean decrease was 5 dB (from 80 dB (A) in 2005 to 75 dB (A) in 2009); and the percentage of overexposed individuals dropped from 25% to 17%.  Amongst 28 Canadian teenagers, the median sound levels at typical and "worst case" volume settings were 71 dBA and 79 dBA respectively. When typical sound levels were considered in combination with self-reported duration of daily use, none of the participants surpassed Leq(8h) 85 dBA.  The listening time was around or below 2 h a day, and overexposure focused on mostly young (college-age) people, mainly males.  The risk in people older than 30 was negligible. 
However, findings from some 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 estimated exposure of 89 dBA for ≥1 h per day.  As many as 5.5% of PMP teenage users were exposed to levels above 100 dBA, whilst none of them were exposed to such high levels from disco music.  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 sound levels (LAeq ). The study concluded that the majority of PMP users are at increased risk for NIHL.  Also, in Canadian high school students, 42% of individuals were shown to be exposed to music from PMP at equivalent sound levels of 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 the level >80 dB (A), as compared to those listening to levels ≤80 dB(A) (16% individuals vs. 4%). 
This review of evidence makes a good case that preventive measures are needed to reduce the risk of hearing loss in adolescents and young adults from PMP use. This could include encouraging manufacturers to produce safer products; launch public health campaigns to improve awareness of the risks to hearing from listening to high-volume music; discuss possible protective measures and safety guidelines and the consequences of hearing loss. The efficacy of this kind of a hearing conservation program has been recently assessed in elementary school (Grade 6) children. Differences between the intervention and control group responses to a behavioral questionnaire interview were measured at baseline, then at two weeks and six months after administration of a hearing conservation program (Sound Sense TM ). This intervention resulted in significant improvement of protective measures for hearing such as ear plug use at dances, rock concerts and other loud-noise events in the long and short term. There was also a tendency, although non-significant in statistical terms, to reduce the duration of use of personal music devices in the intervention group. 
Dietary and pharmacologic otoprotection in acoustic trauma
Noise-induced hearing loss is associated with the damage and loss of sensory outer hair cells in the cochlea. Recently, it has been shown that acoustic overexposure can also produce a rapid and irreversible loss of cochlear nerve terminals on inner hair cells and a slow degeneration of spiral ganglion cells, despite full recovery of cochlear thresholds, and no loss of inner and outer hair cells. , Alongside progress made over recent years in understanding molecular mechanisms involved in hair cell and nerve damage after noise overexposure and a large number of experimental studies in animals, some therapies are now becoming available for use in humans.
Since oxidative stress plays an important role in noise-induced cochlear injury, antioxidant compounds appear very promising for therapeutic use in humans. Many antioxidants, including these that directly influence the availability of antioxidant precursors, are obtained from dietary sources.  Permanent hearing threshold shift has been seen in CBA/J mice maintained on a diet supplemented with a combination of beta-caroten, vitamins C and E and magnesium.  This combination of nutrients, which produced significant increases in plasma concentrations of vitamins C and E, and magnesium, also effectively reduced temporary hearing loss in guinea pigs after acute exposure to noise.  This data suggests that free radical formation contributes to temporary threshold shift (TTS) as well as permanent threshold shift (PTS), and that antioxidative compounds could be effective in preventing both TTS and PTS. Long-term (one month) administration of magnesium has been shown to be an effective treatment in terms of hair cell preservation in guinea pigs exposed to impulse noise (three gunshots, 170 dB SPL), when compared with methylprednisolone and placebo.  Multiple mechanisms could be involved in this effect, such as calcium antagonism, anti-ischemic effect or NMDA (N-methyl-D-aspartate) - receptor channel blockage.
In a series of experiments, Kopke et al.,  showed that N-acetyl-L-cysteine (L-NAC), a precursor of glutathione, protected the hearing of chinchillas from the effects of a single exposure to noise. These effects were seen even at low doses and when given orally by gavage.  Although L-NAC efficacy was shown by other authors, , some studies did not confirm this finding in the same strain of animals  and in other species (C57BL/6J mice). 
Since L-NAC is approved for clinical use and is very safe, it seems to be a very promising drug candidate to be applied in humans (like military soldiers). However, a clinical open trial performed by Kramer et al.,  in 31 normal-hearing participants did not confirm that NAC can protect humans against noise-induced temporary threshold shifts after 2 h of exposure to live music in a nightclub. Recently, a double-blind, placebo-controlled crossover study was conducted on 53 male workers exposed to 88.4-89.4 dB noise  which showed that NAC significantly reduced TTS. This effect was more prominent in subjects with glutathione S transferases null genotypes (both GSTM1-null and GSTT1-null), indicating that the effectiveness of treatment might depend on gene polymorphisms.
The limitation to effective use of L-NAC in humans is that this drug does not readily cross the blood-brain barrier, and would need to be used in much higher doses than currently approved clinical prescriptions. An option for the future could be N-acetylcysteine amide (NACA), a novel antioxidant and potent heavy metal chelator which has a similar chemical structure to N-acetylcysteine (NAC), but seems to easily reach brain fluids. 
D-methionine (D-met), a component of cheese and yogurt, is an oral pharmacologic otoprotective agent that could soon become available to reduce, or for some exposures, eliminate noise-induced hearing loss. This compound has been proved to prevent NIHL in animal studies , even when first administered hours after a noise exposure. D-met has good oral bioavailability and appears to have a good safety profile. The drug is approaching clinical trials with the US Army.  With the current pace of development, oral drugs to protect against acoustic trauma should be available within the next 5-10 years. 
Genetics of noise-induced hearing loss
It is widely accepted that noise-induced hearing loss (NIHL) is a complex disease which results from the interaction of genetic and environmental factors. Inherited factors might explain up to 50% of the hearing loss variability after exposure to noise.
Over the last few years there has been a great increase in association studies trying to identify the susceptibility genes for NIHL in humans. Tens and hundreds of Single Nucleotide Polymorphisms (SNPs) of different genes known to play a functional and morphological role in the inner ear were screened. SNPs are common point mutations in the genome (occurring every 100 - 300bp), and their genotyping is believed to be a successful tool in analyzing the genetic background of complex diseases, like NIHL. In such studies, a disease susceptibility allele is expected to occur more often in the susceptible group than in a resistant one.
So far, the most promising results were obtained for genes involved in the inner ear potassium ion recycling , and heat shock protein genes (HSP70), , because they were replicated in the independent population, and were sufficient in size to yield high power for the detection of a causative allele. The other genes of interest are oxidative stress genes. Recently, the significance of genetic variation in NIHL development has been also shown for otocadherin 15 and myosin 14 genes. 
Up to the present time, association studies on susceptibility genes for NIHL have been conducted based on candidate gene approach. It has been shown that several gene polymorphisms are probably involved in determining susceptibility to NIHL. In establishing the role of some of them, it is, however, necessary to search for the interaction between gene variations and environmental factors.
Because of difficulties in replicating the results on the one hand, and the development of high-throughput genotyping methods along with the growing databases of SNPs on the other, a logical next step for research on the genetics of NIHL is Whole Genome Association Studies. Identification of susceptibility genes may lead to the development of genetic tests which would allow treatment to be personalized - gene therapy is a possible approach, but applying specific medications might also be advisable. Identification of susceptibility genes can also be helpful in identifying the population at high risk, as well as enabling better hearing protection in predisposed individuals. 
Recent studies indicate that hearing loss prevention programs (HLPPs) are still not satisfactory. The effectiveness of non-pharmaceutical interventions for preventing occupational noise exposure or occupational hearing loss was compared to no intervention or alternative methods.  The search comprised several databases, such as the Cochrane Ear, Nose and Throat Disorders Group Trials Register and the Cochrane Central Register of Controlled Trials. In total, 21 studies were included. Fourteen studies with 75,672 participants evaluated hearing loss prevention and six studies with 169 participants evaluated hearing protection. One study evaluated the effect of new legislation in reducing noise exposure. It found that the median noise level decreased by 27.7 dB(A), with a change in trend in time of -2.1 dB per year. The odds ratio for hearing loss was 3.0 (95% CI 1.1-8.0) despite hearing protection. In four studies, workers in hearing loss prevention programs had 0.5 dB HL greater hearing loss at 4 kHz than non-noise-exposed workers. Noise attenuation ratings of hearing protection under field conditions were consistently lower than the ratings provided by manufacturers. The authors have concluded that there is contradictory evidence that hearing loss prevention programs are effective for long periods, and better implementation and reinforcement, as well as technical interventions are needed. 
A promising intervention to increase the effectiveness of hearing conservation programs is the daily monitoring of at-ear exposure along with regular feedback on exposures from supervisors. Recently, annual rates of loss were compared between 78 intervention subjects and 234 controls in an aluminum smelter company. Individuals monitoring daily noise exposure experienced on average no further aggravation of high-frequency hearing loss, while matched controls showed decelerating hearing loss. 
In relation to environmental exposures, the results are also not satisfactory. In the 16-year follow-up study performed among students of 34 rural high schools, comprehensive educational intervention was shown to have very limited effectiveness in preventing NIHL. Although the intervention group reported significantly greater use of hearing protection, there was no significant difference between the intervention group and the group with objective measures of hearing loss. 
The necessity to launch an improved noise policy for hearing protection along with developing a good theoretical model of noise-induced hearing loss risk assessment has been emphasized in studies. The factors which are most effective in reducing the damage to the cochlea caused by noise seem to be encouraging the use of hearing protectors, limiting environmental exposures to loud noise, and change in lifestyle. 
As a result of the SCENIHR report conclusions, a draft European standard had been developed by the relevant working group in CENELEC European Committee for Electrotechnical Standardization.  In this document, a sound level of 85 dBA is considered to be safe under all conditions of PMP use. The sound level can be increased up to a maximum average of 100 dBA, but in that case the user has to be provided with warnings about the risks, which should be repeated after each 20 h of listening time.  CENELEC is expected to continue with the next step of mandated work comprising the development of "smart" methods of providing protection against excessive sound pressure levels from PMP based on the measurement of sound dose.
In summary, the research over the last three years underlines the fact that noise-induced hearing loss (NIHL) still remains a significant health and social problem. Nowadays NIHL is irreversible, necessitating as much effort as possible being put toward prevention. These activities should include identification of high-risk noise exposures, particularly those affecting young people, improvement of noise legislation and effectiveness of use of hearing protectors. Recently, much progress has been made in understanding the molecular mechanisms involved in hair cell and nerve damage after noise overexposure. Alongside a large number of experimental studies in animals, some therapies are becoming available which can be used in the human population.
This study was supported by Seventh Framework Program (Theme FP7-ENV-2008-1 Environment - including Climate Change) ENNAH European Network on Noise and Health (Project number: 226442).
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