Introduction
In can manufacturing factories and bottling plants, peak noise levels of 100 dBA have been recorded.1 Continual exposure to noise levels of this intensity damages and destroys hair cells within the inner ear, thereby causing noise-induced hearing loss. The risk of injury and hearing loss to the ear increases with the intensity of noise level, the duration and frequency of exposures, the individual's susceptibility, and the type of noise.Reference Alberti, Dafydd and Kerr2–Reference Akande and Ologe4 Individual susceptibility to noise-induced hearing loss varies greatly, but the reason for this difference in susceptibility is not well understood.Reference Goodhill and Goodhill5, Reference Clark and Bohne6 Although noise-induced hearing loss is an irreversible impairment, it is completely preventable.Reference Alberti, Dafydd and Kerr2, Reference Lusk7, Reference Lusk8
Many affected workers experience hearing loss considerably beyond 25 dB, and this can have significant effects on their employment, as well as on social and family interactions.Reference Hetu, Getty and Quoc9 Affected individuals suffer tinnitus, together with difficulty in detecting warning signals, comprehending speech, and localising and recognising sound sources in a setting of background noise. Everyday activities such as watching television, using a telephone or following group conversations can become difficult. This may lead to feelings of isolation and depression and, sometimes, to withdrawal from society, rather than individuals seeking help for their hearing loss.Reference Hetu, Getty and Quoc9–Reference McShane, Hyde and Alberti11
In Nigerian society, life in retirement can be challenging because some people withdraw from social life, largely because their retirement income cannot support their pre-retirement standard of living. There is also a tendency towards a perceived social irrelevance of older people. Significant hearing loss will worsen these effects and create greater isolation for older individuals. Hearing loss also reduces the likelihood of post-retirement employment and other contributions to society. Ultimately, the overall burden of the hearing loss inducing employment may become greater than its benefits.
In addition to audiometric testing, many authorities now conduct otoacoustic emission measurements, because the latter provide an earlier indication of noise-induced hearing damage than do pure tone thresholds alone.Reference Stevens, Dafydd and Kerr3, Reference Kowalska and Sulkowski12 Sliwinska-Kowalska and KotyloReference Sliwinska-Kowalska and Kotylo13 reported difficulty in distinguishing between noise-induced hearing loss and other diseases with cochlear hearing loss by means of conventional audiometric tests. Otoacoustic emission measurement is useful in this respect. In cases of noise-induced hearing loss, distortion product otoacoustic emission measurements demonstrate a very typical measurement profile, with a decrease (seen as a ‘notch’) primarily at the 3–4 kHz frequencies. Such a notch was not observed in cases of cochlear hearing loss caused by factors other than noise.Reference Sliwinska-Kowalska and Kotylo13
The aim of this study was to determine the prevalence of noise-induced hearing loss among selected workers exposed to excessive noise within a bottling factory, and also to determine any changes in the hearing thresholds of the selected workers after two years of further exposure to such noise.
Materials and method
Study population
The study took place within a bottling factory located in the central region of Nigeria. It had a staff of approximately 420, including depot staff.
The purpose of the study and the benefit to be derived were explained to the management staff, the head of units and departments, and all production workers (in groups). Production workers were asked for their consent to participate in the study, having been assured that their individual results were confidential and that their individual responses would not be disclosed to their administrator.
All the 116 workers in the production section who gave their consent were interviewed. Two separate sets of interviews and examinations were carried out, the first in December 2003 and the second in December 2005.
Ethical clearance for this study was obtained from the University of Ilorin Teaching Hospital ethics and research committee.
Data collection
A structured, self-administered questionnaire was used to collect the following information: biological and social demographic data; past medical, drug and occupational history; information regarding hearing protection devices; and the respondents' examination and hearing assessment results were thereafter recorded in the study protocol.
A Testo 815 sound level meter (Testo, Lenzkirch, Germany), calibrated with a Testo 0554.0009 sound level meter calibrator, was used to determine the noise level in each section or department of the factory. Four readings were taken for each department per day, over five days, and the average sound level for each department was recorded.Reference Ologe, Akande and Olajide14
The questionnaire was administered to the study population between 1 and 4 pm each day. This period covered the workers' break and change-over time, which gave the opportunity to interview and test workers coming for afternoon duty. Workers on morning and night duty were interviewed and assessed the week they were on afternoon duty. All the audiograms were undertaken before the start of the day's work, thus avoiding the temporary threshold shift effect. The workers were briefed about the procedure before examinations and audiometric tests were conducted. The following workers were excluded from the study: all non-consenting individuals; those with impacted wax that could not be removed at the study site; those with a history of chronic suppurative otitis media, previous ear surgery, head injury, measles, mumps or a family history of hearing loss; and those with abnormal tympanograms.
Otoscopy was carried out to assess the presence of wax, foreign bodies, ear discharge or tympanic membrane perforation. Retraction, scarring and dullness of the tympanic membrane were also assessed. Only subjects with clear external auditory canals and intact, shiny tympanic membranes proceeded further in the study. Impacted wax and foreign bodies were removed wherever possible.
An AT235 impedance tympanometer (Interacoustics, Assens, Denmark) was used to assess the middle-ear function of each participant, following a standard procedure.Reference Ologe, Akande and Olajide15, Reference Ologe, Okoro and Oyejola16 Subjects with an abnormal tympanogram were excluded from the study.
A clinical audiologist used a standard, dully calibrated Danplex AS 67 diagnostic audiometer (GN Otometrics A/S, Taastrup, Denmark) to determine subjects' hearing thresholds by pure tone audiometry (PTA). This test was performed in a quiet room in the factory clinic (sound level approximately 43.3 dB), far away from the production section and electricity-generating plant, so that ambient noise would not mask the acoustic stimuli. (For logistical reasons, subjects could not be transferred to our hospital for audiometry within a soundproof booth. Although this was less than optimal, it was acceptable given the measured sound level, and in light of a recent observation of significant agreement between hearing thresholds measured in non-soundproof working environments and in a soundproof booth.Reference Wong, Yu, Chen, Chiu, Wong and Wong17) Each ear was tested separately, taking approximately 10 minutes for each subject. Air conduction and bone conduction tests were performed. For air conduction, responses at 0.5, 1, 2, 3, 4 and 8 kHz were tested via well fitting earphones (TDH 39; BioMedical Technologies, Mandeville, USA) mounted in cushion. For bone conduction, responses at 0.5, 2, 3 and 4 kHz were tested via a bone vibrator applied to the mastoid bone, employing standard procedure.Reference Ologe, Akande and Olajide15, Reference Ologe, Okoro and Oyejola16, Reference Fitzgerald, Connor, Booth and Kerr18
Individuals' PTA results was determined by calculating the mean decibel loss on air conduction at 0.5, 1, 2 and 4 kHz on the audiogram.Reference Ologe, Akande and Olajide15, Reference Ologe, Okoro and Oyejola16 The results were classified as normal (−10 to 25 dB) or as mild (26–40 dB), moderate (41–55 dB), moderately severe (56–70 dB), severe (71–90 dB) or profound (>91 dB) hearing loss.Reference Ologe, Akande and Olajide15, Reference Ologe, Okoro and Oyejola16
Data analysis
Data were analysed using the Epi Info version 6.04 and the Statistical Package for the Social Sciences version 10.0 statistical software programs. Relationships between variables were assessed using chi-square and t-test. A p value of 0.05 or less was taken as statistically significant.
Results
One hundred and sixteen production workers were recruited into the study. However, 32 subjects could not complete the study due to abnormal otological findings such as suppurative otitis media (10), impacted wax (two) and abnormal tympanograms (eight). In addition, four workers had incomplete data sets and eight were not available for the second test in 2005; as a result, these subjects were also excluded from analysis. Therefore, a total of 84 participants was eligible for inclusion in the final analysis.
These subjects comprised 76 men (90.5 per cent) and 8 women (9.5 per cent). The male to female ratio was 9.5:1. The worker's mean age was 33.0 years (standard deviation (SD) ± 7.6) for the first test, with the majority being aged 20–29 years (37; 44.0 per cent) or 30–39 years (26; 31.0 per cent). The remaining participants were aged 40–49 years (19; 22.6 per cent) or 50 years and above (2; 2.4 per cent). At the second test, the workers' mean age was 35.0 years (SD ± 7.6), with the majority being aged 30–39 years (35; 41.6 per cent) or 20–29 years (23; 27.4 per cent); the remaining participants were aged 40–49 years (22; 26.2 per cent) or 50 years and above (4; 4.8 per cent).
The workers were exposed to noise levels above those specified by safety standards. In the first test (2003), 36 workers (42.9 per cent) in the bottling department were exposed to an average of 98.7 dB, 27 (32.1 per cent) in the maintenance department to 96.6 dB and 21 (25.0 per cent) in the quality assurance department to 91.5 dB. In the second test (2005), 36 workers (42.9 per cent) in the bottling department were exposed to an average of 98.1 dB, 27 (32.1 per cent) in the maintenance department to 96.2 dB, and 21 (25.0 per cent) in the quality assurance department to 91.6 dB.
The workers' mean duration of employment, in their current job, was 9.0 years (SD ± 6.2) and 11.0 years (SD ± 6.2) for tests one and two, respectively. In test one, the majority of workers (70.2 per cent) had held their current job for ≤10 years, while in test two, 57.1 per cent had held their job for ≤10 years.
Only five (6.0 per cent) of the workers had undergone a pre-employment hearing test. In test one of this study, the mean hearing thresholds at each frequency tested were elevated in both ears. The threshold worsened with increasing frequency, to a maximum at 4 kHz. The workers' mean hearing thresholds were significantly worse in test two compared with test one. The rate of deterioration of hearing loss between tests one and two was noted to be in the range of 1.0–3.2 dB for the right ear and 1.6–3.4 dB for the left ear This deterioration was at discrete frequencies (Table I, Figures 1 and 2). Table II shows subjects' mean hearing thresholds by department, at each frequency, for each ear, and for tests one and two. Results followed the same patterns as above, but differences between departments were not consistent.
Fig. 1 Average pure tone audiogram, right ear.
Fig. 2 Average pure tone audiogram, left ear.
Table I Subjects' mean hearing thresholds
SD = standard deviation; DF = degrees of freedom
Table II Subjects' mean hearing thresholds by department
Dept = department; SD = standard deviation; T1 = test one; T2 = test two
The results of subjects' audiometric testing showed mild sensorineural hearing loss (SNHL) in 63 (75 per cent) right ears and 42 (50 per cent) left ears in test one, compared with 68 (81 per cent) right ears and 67 (79.8 per cent) left ears in test two. Moderate SNHL was recorded in two (2.4 per cent) right ears and two (2.4 per cent) left ears in test one, compared with eight (9.5 per cent) right ears and three (3.6 per cent) left ears in test two. Normal hearing thresholds were recorded in 19 (22.6 per cent) right ears and 40 (47.6 per cent) left ears in test one, compared with only 8 (9.5 per cent) right ears and 14 (16.7 per cent) left ears in test two (Table III). Tables IV and V show the results according to subjects' duration of employment.
Table III Subjects' Degree of Hearing Loss
T1 = test one; T2 = test two
Table IV Subjects' Degree of Hearing Loss by Duration of Employment
*<26 dB; †26–40 dB; ‡41–55 dB. Yrs = years; T1 = test 1; T2 = test 2
Table V Subjects Degree of Hearing Loss by Duration of Employment: Statistics
T1 = test 1; T2 = test 2; DF = degrees of freedom
The prevalence of SNHL in workers in the various departments was: for the bottling department, 52.8 per cent in test one and 81.9 per cent in test two; for the maintenance department, 81.5 per cent in test one and 98.1 per cent in test two; and for the quality assurance department, 64.3 per cent in test one and 80.9 per cent in test two. The overall prevalence of hearing impairment among workers was 64.9 per cent in test one and 86.9 per cent in test two (Tables VI and VII).
Table VI Subjects' Degree of Hearing Loss by Department
*<26 dB; †26–40 dB; ‡41–55 dB. T1 = test 1; T2 = test 2
Table VII Subjects Degree of Hearing Loss by Department: Statistics
T1 = test 1; T2 = test 2; DF = degrees of freedom
More than half (53.6 per cent) of the workers did not have hearing protection devices. Of the 39 who had such devices, 36 (92.3 per cent) had earmuffs and three (7.7 per cent) had earplugs. However, only 15 (38.5 per cent) workers claimed to always use their device, and two (5.1 per cent) stated they never used it. These results were identical in test two (Table VIII).
Table VIII Subjects' Possession and use of Hearing Protection Devices
HPD = hearing protection device
Tables IX and X show that age significantly affected subjects' hearing thresholds, particularly in the left ear.
Table IX Subjects' Degree of Hearing Loss by Age
*<26 dB; †26–40 dB; ‡41–55 dB. T1 = test 1; T2 = test 2
Table X Subjects Degree of Hearing Loss by Age: Statistics
T1 = test 1; T2 = test 2; DF = degrees of freedom
Discussion
The average noise exposure level recorded within the production area of the bottling factory exceeded 90.0 dB A, which is the maximum or highest allowable level of noise that a worker should be exposed to for a period of eight hours per day, as determined by hearing conservation criteria.Reference May19–Reference Gosztonyi21 Exposure to noise above this level damages the hair cells and the blood supply in the cochlea, beginning at frequencies around 4 kHz. The damage is initially temporary, but with greater noise exposure it becomes permanent.Reference Alberti, Dafydd and Kerr2, Reference Akande and Ologe4 The immediate response to damaging noise is a transient blunting of hearing acuity, which shifts the subjects' threshold of barely audible sound up to a higher level for a period of several hours. Detection of such episodes of temporary threshold shift indicates exposure to a harmful level of noise. Tempoary threshold shift is usually reversible within 16 hours. It can be regarded as auditory fatigue, and most studies indicate that it is associated with either no sensory cell damage or minimal, reversible cell changes. Repeated exposure to excessive noise, sufficient to produce a temporary threshold shift, will eventually cause a permanent threshold shift, i.e. an irreversible increase in hearing thresholds. At this point, there is irreversible hair cell damage.Reference Alberti, Dafydd and Kerr2, Reference Goodhill and Goodhill5, Reference May19, Reference Morata, Dunn, Kretscner, Lemasters and Keitah22 In theory, the damage reflects both the intensity of the noise and the length or duration of exposure, in a fashion which is predictable (by the ‘equal energy principle’). Such hearing damage is not linear with respect to exposure; rather, the worker may experience a disproportionate loss in the early years of exposure.Reference May19
Sickle cell disease can be expected to increase susceptibility to cochlear ischaemia, particularly if the noise causes swelling of the capillary epithelial lining. However, there were no subjects with sickle cell anaemia in this study, as the factory production, maintenance and quality assurance sections were regarded as too strenuous for these individuals' usually fragile health. Affected individuals were conventionally assigned to administrative and similar sections of the factory.
In our study population, the prevalence rates of noise-induced hearing loss were 64.5 per cent in the first test and 86.9 per cent in the second test. In the literature, prevalence rates of 3–82 per cent have been reported.Reference May19, Reference Shakhatreh, Abdul-Baqi and Turk23–Reference Dobie, Bailey, Calhoon, Deskin, Johnson, Kohut, Pillsbury and Tardy26 Importantly, our study showed a significantly high rate of deterioration over a two-year period. It has been observed that deterioration in hearing threshold is more rapid in the first 10 to 15 years of exposure to excessive noise.Reference Morata, Dunn, Kretscner, Lemasters and Keitah22, Reference Dobie, Bailey, Calhoon, Deskin, Johnson, Kohut, Pillsbury and Tardy26 Our study population fell within this exposure category.
The implication of these findings is that continued exposure of these workers to their current noise level would lead to worsening noise-induced hearing loss. By the time they reach retirement age, they may possibly have moderate to severe noise-induced hearing loss. Ramazzini, in his De Morbis Artificum (translated from the Latin by WC Wright), reported that workers who hammered copper for a living had their ears so injured by the perpetual din that they became hard of hearing.Reference Ramazzini27 If they lived to old age and continued to work, they became completely deaf. This significantly affected the quality of their post-retirement life.
In our study, the maximum mean hearing loss for workers in all departments was recorded at 4 kHz. This observation has also been documented in previous studies.Reference Ahmed, Dennis, Badran, Ismail, Ballal and Ashoor25, Reference Osibogun, Igweze and Adeniran28–Reference Garcia and Garcia31 In typical noise-induced hearing loss, the hearing threshold shift is usually greatest at 4 kHz. However, studies in a Taiwanese oil refinery recorded the greatest hearing threshold shift in noise-exposed workers at 6 kHz.Reference Sulkowski, Kowalska and Lipowczan29, Reference Chen and Tsai32 Therefore, omitting measurements at 6 kHz, based on the assumption that the notch at 4 kHz is a well established clinical sign and the 6 kHz notch is variable and of limited importance,Reference McBride and Williams30 should be discouraged. It is also noteworthy that, although the maximum mean hearing loss was at 4 kHz, the rate of deterioration over the two-year study period was greater at frequencies lower than 4 kHz. This suggests that hearing at 4 kHz was worst affected in the earlier stages, but that hearing at lower frequencies progressively worsened as time elapsed. It is possible that, given another few years of further exposure, hearing at other frequencies would be worst affected. Thus, although noise-induced hearing loss appeared to be worst at 4–6 kHz, further research is needed to determine the differential effects of excessive noise at various frequencies over different stages and periods of exposure.
The mean duration of employment for the workers in this study was 9.0 and 11.0 years at tests one and two, respectively. The majority (70.2 per cent) of workers in test one and 57.1 per cent of those in test two had worked in the factory for between one and 10 years. Permanent hearing threshold shift was noted in workers exposed to excessive noise for less than 10 years.Reference Celik, Yakin and Ozturk33 Other studies have found that a longer duration of employment was required before workers developed a permanent threshold shift.Reference Mantysalo and Vuori34, Reference Chung, Willison and Gannon35 Thus, the duration of noise exposure required to cause permanent hearing threshold shift varies, possibly depending on the noise intensity and type and on individual susceptibility. It is therefore unwise to risk exposure to excessive noise, however brief. It should be noted that the hearing protection practice of our study population was poor.
Although noise-induced hearing loss is usually bilateral and symmetrical,Reference Celik, Yakin and Ozturk33 a study of industrial workers in two plants in Eastern Saudi ArabiaReference Ahmed, Dennis, Badran, Ismail, Ballal and Ashoor25 recorded asymmetrical SNHL in some workers; other studies have reported similar findings.Reference Garcia and Garcia31, Reference Mantysalo and Vuori34, Reference Chung, Willison and Gannon35 In one study, such asymmetry was suggested to be a manifestation of lateral difference in susceptibility to noise damage.Reference Chung, Willison and Gannon35
Workers' mean age was 33.0 years in test one and 35.0 years in test two. This is similar to other studies.Reference Ahmed, Dennis, Badran, Ismail, Ballal and Ashoor25, Reference Osibogun, Igweze and Adeniran28, Reference Thiery36 A high male to female ratio (9.5:1) was noted among the workers. This was expected, as the labour-intensive nature of tasks within the production section of most industries is most manageable by young, able-bodied men.Reference Osibogun, Igweze and Adeniran28 It is unfortunate that these young men pay a such heavy price for their daily wages, in terms of noise-induced hearing loss.
It is important to note that, in the general population, current research is inconclusive regarding specific patterns of gender differences in SNHL.Reference Pearson, Morrell, Gordon-Salant, Brant, Metter and Klein37–Reference Jerger, Chmiel, Stach and Spretnjak39 On the whole, the male sex may be associated with an increased incidence of hearing loss, after adjusting for age. One of the major reasons adduced is related to greater noise exposure, with a resultant increase in noise-induced hearing loss. The preponderance of men in this study may add to the body of evidence in this respect.
In our subjects, the effect of noise exposure outside the workplace on SNHL was difficult to determine. However, there is overwhelming evidence that occupational noise exposure is a major factor in our subjects. The noise level of the working environment, the prevalence and pattern of SNHL are evidences for this view point. In the present study, rigorous efforts were made to exclude other ear diseases which could cause SNHL; however, otoacoustic emission measurements would have been useful to help distinguish between noise-induced hearing loss and cochlear hearing loss of other origins.Reference Sliwinska-Kowalska and Kotylo13
• This study investigated the prevalence of noise-induced deafness among workers at a Nigerian bottling factory
• There was a high prevalence of sensorineural hearing loss and significant hearing deterioration among workers, due to exposure to excessive noise, over a two-year period
• The study demonstrates the practical importance of serial audiometry for noise-exposed workers as a means of monitoring hearing deterioration
• In developing countries becoming industrialised, the importance of hearing conservation programmes should be stressed
It is interesting to note that, despite being part of the study, those who had access to ear protectors did not increase their usage of them over the two-year study period. This shows that breaking the habit of poor compliance with hearing protection devices is not an easy task; merely supplying information is inadequate. In previous studies, we noted this dichotomy between, in one hand, knowledge of the hazards of excessive noise exposure and, on the other, the actual practice of hearing conservation.Reference Ologe, Akande and Olajide14, Reference Ologe, Okoro and Akande40 We surmise that such behaviour is akin to the practice of habitual smokers or those who engage in unprotected sex with multiple partners, despite being aware of the implications.Reference Ologe, Okoro and Akande40 Even in developed countries, this observation has been noted.Reference Daniell, Swan, McDaniel, Camp, Cohen and Stebbins41 These observations suggest that rigorous monitoring, with possible sanctions for default, may be necessary to complement appropriate health education, in order to ensure compliance with hearing conservation programmes.
Conclusion
We found a high prevalence of sensorineural hearing loss together with significant hearing deterioration among bottling factory workers, due to excessive noise exposure over a two-year period. The study demonstrates the practical importance of serial audiometry for noise-exposed workers as a means of monitoring deterioration in hearing.
It is recommended that noise levels within the various departments of such factories should be monitored regularly, perhaps monthly. Efforts to keep noise levels within permissible, safe standards should be rigorously pursued, especially regarding engineering and administrative protocols and maintenance. A hearing test should be included in the pre-employment medical examination of all workers, to serve as a baseline measurement. Periodic audiometric measurements should be carried out among workers, at least one per year. This will help to monitor the hearing thresholds of individual workers at risk. Workers with worsening hearing thresholds should be promptly reassigned to less noisy areas of the factory. Adequate counselling and health education regarding hearing conservation should be conducted annually for all factory staff. Regular and correct use of hearing protection devices should be enforced and properly monitored on the factory floor.
