Noise induced hearing loss (NIHL) is an irreversible sensorineural hearing loss associated with exposure to high levels of excessive noise. Prevention measures are not well established in developing countries. This comparative cross sectional study aims to determine the prevalence of hearing loss in both a group of high risk workers and a control group and to assess their knowledge of the effects of noise on hearing health. A total of 101 market mill workers and 103 controls employed within markets in the city of Accra, Ghana, were evaluated using a structured questionnaire and pure tone audiometry. The questionnaire assessed factors including self-reported hearing loss, tinnitus, knowledge on the effects of noise on hearing health and the use of hearing protective devices. Pure tone audiometric testing was conducted for both mill workers and controls. Noise levels at the work premises of the mill workers and controls were measured. Symptoms of hearing loss were reported by 24 (23.76%) and 8 (7.7%) mill workers and controls respectively. Fifty-five (54.5%) and fifty-four (52.37%) mill workers and controls exhibited knowledge of the effects of noise on hearing health. Five (5.0%) mill workers used hearing protective devices. There was significant sensorineural hearing loss and the presence of a 4 kHz audiometric notch among mill workers when compared with controls for the mean thresholds of 2 kHz, 3 kHz and 4 kHz (P = 0. 001). The prevalence of hearing loss in the better hearing ears of the mill workers and controls was 24.8% and 4.8% respectively (P < 0.5). The prevalence of hearing loss, which may be characteristic of NIHL in the better hearing ears of the mill workers and controls was 24.8% and 4.8% respectively. The majority of mill workers did not use hearing protection.
Keywords: Market mill workers, occupational hearing loss, noise induced hearing loss, audiometric assessment
|How to cite this article:|
Kitcher ED, Ocansey G, Abaidoo B, Atule A. Occupational hearing loss of market mill workers in the city of Accra, Ghana. Noise Health 2014;16:183-8
| Introduction|| |
Noise remains a common environmental pollutant in industrial work places and has been a constant issue since the industrial revolution. Noise induced hearing loss (NIHL) is an irreversible sensorineural hearing loss associated with excessive noise exposure. Noise in excess of 85 dBA in a work environment of an 8-h daily work regime predisposes workers to NIHL.  NIHL is usually bilateral and symmetrical, affecting higher frequencies (3 kHz, 4 kHz or 6 kHz) and subsequently lower frequencies (0.5 kHz, 1 kHz or 2 kHz).  Global estimates of the prevalence of disabling hearing loss from occupational exposure range from 7% to 21%. 
There is a large body of evidence demonstrating NIHL in workers following exposure to excessive noise in large-scale enterprises in the formal sector. ,,, A formal sector enterprise is an enterprise which is either government or privately owned, but is subject to government regulation. Within Ghana, Kitcher et al.  noted a high prevalence of early NIHL among workers in a formal enterprise of a large-scale stone crushing industry. Boateng and Amedofu  also reported NIHL among local saw mill, printing press and corn mill workers. Other local studies by the same authors and others  have documented the problem of occupational hearing loss among surface-level gold mining workers. Ologe et al.  noted deterioration of hearing among bottling factory workers and recommended serial audiometry as a means of monitoring workers' hearing. In their study McCullagh et al.,  observed a poor relationship between self-reported hearing ability and results of audiometry and advocated the need for hearing conservation programs even in a regulated work environment. However, local literature is sparse on occupational hazards in the informal, small-scale enterprise within local markets. The informal sector, small-scale enterprise is an enterprise which is privately-owned but unregulated by government. In this sector, there is a lack of workplace environmental noise regulation and other work safety regulations.
Common enterprises located within market places in the Ghanaian capital city, Accra, are informal, small-scale mills for food processing. A market mill consists of an electrical light-duty motor linked to a food processing unit. The food processing unit typically grinds grains such as maize, beans and rice and also vegetables such as tomatoes, onions and pepper. Such enterprises are privately-owned. Many of the workers are self-employed and work located in unregulated premises. The operation of a market mill is associated with high noise levels within the work place during working hours. Most of the workers may not be aware of the hazards of excessive noise levels on their hearing due to lack of knowledge and low levels of literacy.
Lack of regulation of permissible levels of noise exposure in this sector means that hearing protection will not be enforced and repeated exposure to high levels of noise from these machines is likely to lead to reduced hearing ability. The damage to the human ear by high levels of noise in the work place environment is dependent on the intensity of noise, duration of exposure  and spectrum. In the absence of local production of market mill machines with attenuated noise levels, the use of ear plugs and ear muffs during operations of these small-scale mills remains the only viable preventive measure for this occupational health hazard.
Available data from the sub-Saharan region of West Africa indicates that operation of market mills is associated with high noise levels within the workplace during working hours.  The noise levels within such work places and the subsequent degree of occupational hearing loss of market mill workers remains unknown. Evaluation of noise levels within market mill workplaces, together with the hearing assessment of small-scale mill workers, will help to shed light on the health risk of these environments. Information gained from such an evaluation will highlight the need for preventive measures. This study aimed to evaluate the prevalence of hearing loss among market mill workers and compare these findings with a control group.
Aims and objectives
This study was aimed to determine the pure tone hearing thresholds of market mill workers in comparison to those of a control group of petty traders; secondary aim is to assess the prevalence of hearing loss among market mill workers and to determine awareness of the effects of noise on health. The control group consisted of small-scale traders who sold vegetables, grains, meat and fish and other food stuffs in market stalls. Traditionally, this trade is undertaken by a majority of women in Ghana. The small-scale traders work within the same markets as the mill workers.
| Methods|| |
This was a comparative cross sectional study, conducted in the city of Accra, Ghana. The study group consisted of 101 market mill workers, (one female and 100 males) while the control group of small-scale traders was comprised of 68 females and 38 males. Mean ages of the market mill workers and controls were 33.14 ± 0.23 and 33.31 ± 2.09 years respectively.
The city of Accra has several markets within its suburbs. This study was carried out across the following markets within the suburbs of the Accra metropolitan area: "Tuesday market" Mamprobi, "London market" James Town, "Sraha market" Accra central, "Makola market" Accra central and "Kaneshie market" Kaneshie. The selected markets each have capacity ranging between 500 and 1500 stalls where large quantities of food stuffs including grains, vegetables, tubers and meat products were sold. The market mills were generally clustered together within an allocated zone in each of the markets. These mills typically grind a range of grains and vegetables. The working environment of the market mill workers during operation of the mills was characterized by excessive unregulated noise generated by the mills. There were two types of market mills, namely grain processing mills, also called "dry mills" and vegetable processing mills, also called "wet mills." The work environments of the small-scale traders were located remotely from the market mills and were characterized by acceptable environmental noise from human traffic. In selecting these market sites the investigators took into consideration their proximity to the hearing test site to enable easy access for all volunteers to return quickly to the market after the test.
A total of 101 market mill workers were recruited and gave informed consent. Similar numbers of small-scale traders were recruited from each market, with a total of 103 small-scale traders as controls. The study group and control group had similar socioeconomic characteristics. Exclusion criteria included a history of chronic ear disease and history of non-occupational noise exposure.
Sample size determination
The sample size of 101 was found to be adequate using the formula below: 
n = z2p (1-p)/d2
n = required sample size
z = confidence level, i.e., 1.96
p = prevalence rate of the condition
d = degree of accuracy desired, i.e., 0.05.
Global prevalence of the NIHL is estimated to be about 7%.
We desire a 0.05 degree of accuracy and a confidence level of 1.96.
Therefore, n = 1.96 2 × 0.07 (1-0.07)/0.05 2 = 100.04 ≈ 100 mill market workers respectively.
Each market mill worker completed a structured questionnaire, in either written or oral format. The questionnaire probed for self-reported hearing loss in normal listening situations and in the presence of background noise, tinnitus, knowledge on the health hazards associated with work in a noisy environment, the use of hearing protective device and the duration of service at the market mill enterprise [Figure 1]. Demographic data, such as educational background, monthly income, history of non-occupational noise exposure, hobbies and family history of hearing loss were also collected.
Measurement of environmental noise was conducted using an RS-232 sound level meter (SLM) 72-860A SLM manufactured in Taiwan, Republic of Korea by Tenma Test Equipment Springboro, Ohio), set for "A" weighting. The response of the SLM was set for "slow" response and the microphone was placed in the free field. The measurement of both maximum and minimum noise levels for each market mill work station was conducted by an experienced audiologist. Measurement of noise levels in each market mill premises was conducted both with the mill in full operation and when mill was turned off. Measurements of a total of ten noise levels were taken every 30 s over a period of 5 min. Average minimum and maximum values were determined for each market mill premises. Measurement of noise levels was also similarly conducted on the premises of small-scale traders. The permissible level of noise exposure for these workers should be an 8-h time-weighted average of 85 dBA or less.
The pure tone audiometric assessment of the mill workers was carried out in groups of 10 workers per day and at least 14 h after their last exposure to noise. The audiometric evaluation of these workers was carried out at the Hearing Assessment Centre, Korle Bu Teaching Hospital in the city of Accra. The center is equipped with a noise-treated audiology booth with an ambient noise level of 34-40 dBA. Before the audiometric test, each worker's outer ear was inspected by the principal author to rule out any outer or middle ear diseases. A qualified audiologist (second author) conducted a conventional eight-frequency pure tone audiometric behavioral threshold test in 5 dB HL steps at frequencies of 250, 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz in both ears using a GSI 16 Clinical Audiometer (manufactured by Grason-Stadler in Littleton, Massachusetts). Pure tone audiometry was also conducted on the control group.
Hearing impairment was calculated using the pure tone average frequencies of 500, 1000, 2000 and 4000 Hz. Hearing impairment was classified according to the World Health Organization definition;  normal hearing < 25 dB hearing threshold level (HL), mild hearing loss = 26-40 dB HL, moderate hearing impairment = 41-60 dB HL and severe hearing impairment = 61-80 dB HL.
The mean thresholds of hearing for the frequencies of 2000 Hz, 3000 Hz and 4000 Hz were also computed as the frequencies of 500 Hz and 1000 Hz are typically resistant to NIHLL. The authors examined the configuration of the audiograms of mill workers and the control group and noted the presence or absence of an audiometric notch at 4 kHz, 3 kHz and 6 kHz.
The main outcome of hearing evaluation was the mean threshold hearing level using the World Health Organization classification, the mean threshold hearing levels for the frequencies of 2000 Hz, 3000 Hz and 4000 Hz and the presence of significant audiometric notch at 4 kHz. A 4 kHz notch was defined when the 4 kHz threshold minus the 2 kHz threshold and the 4 kHz threshold minus the 8 kHz threshold were both ≥10 dB.
The data were analyzed using SPSS Statistics for Windows Version 17.0, Chicago: SPSS Inc. An Independent sample t-test was used to compare the mean HLs of subjects and controls for 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz frequencies and also for 2000 Hz, 3000 Hz and 4000 Hz frequencies. The authors also compared the presence or absence of a 4 kHz notch in the audiometric configuration of the audiograms for mill workers and controls. The level of significance was set at P < 0.05.
The protocol for this study was approved by the Ethical Protocol Review Committee of the University of Ghana Medical School (Protocol Identification number MS-Et/M.2-P 3.1/2012-13). Each participant signed an informed consent form after investigators explained the purpose of the study and procedure of the study.
| Results|| |
Nearly 60% of market mill workers had at least 9 years of formal education up to the junior secondary school level whereas 58% of petty traders had the same level of education. The mean monthly income of mill workers and petty traders was equivalent to 200 and 250 US dollars respectively. The service duration of mill workers is outlined in [Table 1]. The mean duration of service of the mill workers was 10.78 ± 1.84 years with a range of service duration of 0.5-42 years.
Self-reported hearing loss occurred in 23.76% (24) market mill workers while hearing loss, as determined audiometrically, occurred in 43.56% (44) market mill workers. In the control group, self-reported hearing loss occurred in 7.77% (8) of the controls while on audiometric evaluation 5.83%, (6) of controls had a hearing loss. Tinnitus was reported by 38 (37.6%) mill workers and 22 (21.36%) controls. Adequate knowledge on the effects of noise on health was recorded from 55 (54.5%) mill workers and 56 (54.37%) controls. Most of the controls and mill workers had some degree of knowledge on the effect of noise on health obtained from radio and television. Only 5 (5.0%) of the market mill workers reported using hearing protective devices.
The mean work place environment noise level when the market mill was switched off was 54.5 dBA-57.5 dBA. The noise levels of the market mill work premises when the machines were in full operation is outlined in [Table 2]. The mean work place environment noise level of small-scale traders was 61.5 dBA-69.4 dBA.
The mill workers worked daily for an average period of 9 h/day. Sixty-one (60.4%) market mill workers had normal hearing in both ears and 40 (39.6%) market mill workers had a sensorineural hearing loss in one or both ears. In comparison, 99 (96.1%) controls had normal hearing in both ears and four controls (3.9%) had a sensorineural hearing loss in one or both ears.
Audiometric evaluation outcomes for the better hearing ear is outlined in [Table 3]a.
The hearing thresholds of the worse hearing ear are also outlined in [Table 3]b. The audiometric configuration with respect to an audiometric notch for the mill workers and controls is provided in [Table 4]. The distribution of 4 kHz audiometric notches among the study population, which is characteristic for early NIHL, is also outlined in [Table 5]. There was a statistical significance for the presence of sensorineural hearing loss in both the better and worse hearing ears of the market mill workers compared with the controls (P < 0.05). There was also a statistical significance for the presence of 4 kHz notch among the mill workers compared to the controls.
|Table 4: Configuration of audiograms in respect of audiometric notch present in market mill workers and control|
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|Table 5: Distribution of 4 kHz notch in the audiogram of market mill workers and controls|
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The prevalence of sensorineural hearing loss in the worse ear was 44% and 6% for the mill workers and controls respectively. The prevalence of sensorineural hearing loss for the better ear for the mill workers and the controls was 24.8% and 4.8% respectively. The mean HLs for 2 kHz, 3 kHz and 4 kHz are displayed in [Table 6]. There was significant sensorineural hearing impairment of mill workers when compared to the controls, using mean HLs at the same frequencies (P = 0.001).
|Table 6: Mean threshold of hearing at frequencies of 2 kHz, 3 kHz and 4 kHz|
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This study has demonstrated statistical significant sensorineural hearing loss and the presence of a 4 kHz notch in the audiometric configuration of mill workers compared with controls. Furthermore this study also revealed that mill workers were exposed to excessive work placenoise and have suffered NIHL.
| Conclusion|| |
The prevalence of possible NIHL in the better hearing ear was 24.8% for the market mill workers and 4.8% for the controls. Even though roughly 50% of market mill workers had some knowledge of the effects excessive noise on health, only 5% used hearing protective devices for prevention of NIHL.
| Discussion|| |
The results of this study suggest that market mill workers have significantly more sensorineural hearing loss when compared to controls. This pattern of sensorineural hearing loss, coupled with the significant presence of 4 kHz audiometric notch among the mill workers strengthened our assertion that the hearing loss of the mill workers was likely to be attributable to NIHL. Our study noted that only 5% of mill workers used hearing protection devices, despite more than half (54.5%) of these workers having some knowledge of the harmful effects of noise on hearing.
The findings from this study are similar to observations of other researchers  who have noted that possessing knowledge of the detrimental effect of noise on hearing does not automatically translate into regular use of hearing protective devices.
Our study demonstrated that market mill workers were exposed to environmental noise levels ranging from 85.9 to 110.8 dBA which far exceeds the permissible levels of 85 dBA. The level of noise in this unregulated industry is likely to have contributed to significant sensorineural hearing impairment of mill workers (P = 0.001). Our findings suggest a prevalence of possible NIHL of 25% in the better hearing ear for the mill workers, which is lower than the prevalence of 83% reported by Osibogun et al.  in a study in the textile industry. Such differences may reflect variation in factors including the sources and levels of noise exposure, duration of work in the industry and other exposure and socio-demographic factors.
Importantly, self-reported hearing loss under-represented rates of actual hearing loss on audiometric testing, indicating that individuals may not accurately recognize hearing loss. Further examination of this issue, particularly on levels at which self-reported hearing loss appeared to become more consistent, would be a useful adjunct to this research.
These findings are consistent with the observation of other researchers.  who confirmed a poor relationship between perceived and actual hearing loss in noise exposed workers and suggest that the best practice requires inclusion of audiometric screening as part of a complete hearing loss prevention program.
Lack of government regulation on issues of occupational hearing protection for market mill workers may at least in part contribute to the high prevalence of occupational hearing loss.
This study now provides a baseline from which preventive measures can be initiated within small scale industries. Collective efforts at prevention should include advocacy for regulation of work practices within small scale enterprises such as market mills. Regulation, coupled with comprehensive health education and hearing conservation programs should contribute to necessary impacts in reducing the prevalence of NIHL in this sector.
Our study population was predominantly male, attributable to the local socio-culture practice of having men operate high powered machines, with more females traditionally engaged in small scale trading in the markets. This observation in our study contrasted the findings by Omokhodion et al.  who reported that equal numbers of men and women were engaged in the market mill industry. The socio-cultural difference in the study populations may account for the difference in gender ratio.
One noted weakness of this study was our inability to control for gender between subjects and controls due to socio-cultural factors. Ghanaian society does not encourage women to engage in operation of machines. There are relatively fewer men who sell in the markets.
| Conclusion|| |
We conclude from this study that the market mill workers had possible a NIHL with a prevalence of 25% in the better hearing ear. A large majority of mill workers did not use any hearing protective device even though 54.5% of them exhibited solid knowledge on the effects of noise on health. Self-reporting of hearing loss was unreliable.
Further research in this field should focus on impact of preventive measures on prevalence of sensorineural hearing loss among small scale traders working in other self-employed industry with unregulated work place noise.
| Acknowledgment|| |
We wish to thank Professor Joe-Nat Clegg-Lamptey for his useful advice in the preparation of the manuscript.
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Dr. Emmanuel D Kitcher
Department of Surgery, University of Ghana Medical School, P. O. Box 4236, Accra
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]