Journal of Otorhinolaryngology Noise induced hearing loss and its determinants in workers of an Automobile manufacturing unit in Karachi, Pakistan

Introduction: Excessive noise is an important health hazard at the present time. It can lead to auditory and extra-auditory effects which affect the welfare of individuals and groups exposed. We carried out this study to understand the prevalance of Noise Induced Hearing-Loss (NIHL) in workers of an automobile manufacturing unit in Karachi, Pakistan. Method: The study consisted of the following: work. Two hundred and seventy two workers participated in the study and out of them 206 underwent audiometric evaluation. Results: The basic demographic, professional data and smoking habits of 272 workers participating in the study is as follows: 34.92% of workers belong to 41-50 years of age, 58.8% belong to Mohajir ethnic group, 19.1% were smoker/ex-smokers, 62% earned less than Rs 10,000/month, 87.5% received education for 0-10 years and 40% had work experience of 0-10 years. The majority of the workers were exposed to noise level higher than the safe limit of 85 dBA. According to the WHO criteria [1] 61% of the workers were suffering from hearing loss. Those who were suffering from hearing loss were further evaluated according to Cole’s criteria for NIHL [2]. Of the total number of workers who underwent audiometry 25% fulfilled the Coles’ criteria [2] for NIHL. The age, duration of exposure and severity of exposure were important determinants of NIHL. The workers had insufficient knowledge regarding prevention of NIHL. The personal protective equipments were used inadequately. There was no clinical follow up or monitoring of hearing among the workers. Conclusion: There is an urgent need for the government to take practical steps for making a policy and implement it so that occupational hearing loss can be prevented.


Introduction
Hearing is important for a successful life. Loss of hearing affects life as well as employment, education and well being, and is therefore a challenge for an individual during their regular as well as his/her social life. At the work-place hearing loss decreases not only efficiency but also puts a questionmark on the individual's safety as well.
Adult-onset hearing loss is the fifteenth most serious health problem which leads to social isolation and to serious economic burden [5]. 16% of the disabling hearing loss in adults worldwide is due to occupational noise, while in different sub-regions ranges from 7% to 21% [5]. Noise induced hearing loss (NIHL) is more common in less developed countries [5]. It is getting worse over the years with continued occupational noise exposure [6] and is irreversible [7], but could be prevented [8]. It is the second most common form of acquired hearing loss [9].
In the USA, 9 million workers are exposed to sound levels of 85 dBA and above 10 million have NIHL. In European Union, 28% of workers reported that at least 25% of the time at work they are exposed to loud noise [5].
The majority of adults suffering from NIHL belong to Asia, where NIHL is a serious health problem. Asian countries are developing/less developed and lack preventive and curative health services. There is lack of awareness of NIHL among workers, employers and health providers. This is the main barrier in prevention of NIHL in these countries [16].
In Pakistan, there are large numbers of people who are not aware of the adverse effects of noise. Noise is the major cause of preventable hearing loss globally [9]. The economic burden due to NIHL is tremendous and has been estimated to be billions of dollars. The condition further aggravates the dependence of these socially handicapped people.
The aim of the present study was to assess the prevalence and characteristics of the hearing loss in people working in noisy environment in an automobile manufacturing company. Our intention was to get evidence that will serve to inform the policy makers in the Government and the industry to institute control and safety measures, in order to raise the level of awareness of the workers regarding the health risks of their work.

Methods
A cross sectional descriptive study was carried out in December 2011 in an automobile manufacturing unit in Karachi, Pakistan. Written consent was obtained from Chief Executive Officer of the company. Verbal consent was taken from subjects who were workers in different departments. The study consisted of noise monitoring, questionnaire, otoscopic examination, audiometry of the workers and health/safety survey. We used random method of sampling. 272 workers participated in questionnaire study. 206 workers i.e.76% of the sample underwent audiometry.
The noise survey was carried out by a trained health and safety coordinator with a sound level meter TES 1351 made in Taiwan. Calibration was done by the instrument department of the company. Noise level meter was placed at shoulder level of the worker. Nine readings were measured at three different times on the same day. The minimum and maximum were noted and average of 9 readings was calculated.
In the questionnaire study: A face to face interview was conducted by the trained interviewer. The workers were asked questions regarding hearing loss, tinnitus, knowledge, attitude and behavior regarding NIHL. Demographic data and smoking habits were recorded.
Otoscopy was carried out by a trained auditory technician. Both ears were examined. Workers were examined for wax and perforation of the ear drum. Those workers who were having cerumen or perforation of the tympanic membrane were excluded from the study.
Pure tone audiometry examination was offered to all the workers who participated in questionnaire study and 206 workers responded positively. A pure tone clinical audiometer GSI 38, Westain Hearing Aid Company, USA was used. Each worker was explained the procedure and its importance. Air and bone conductions were tested at frequency of 0.25, 0.5, 1, 2, 4, 6 and 8kHz. All the records were entered manually. Hearing impairment was defined as an average air conduction threshold in the better hearing ear at 0.5, 1, 2 and 4kHz more than 25 dBA. The hearing loss was classified into mild (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40), moderate (41-60 dBA), severe (61-80 dBA) or profound (more than 80 dBA). The better hearing ear was taken for calculation of the prevalence of hearing loss. In those workers who fulfilled the Coles' criteria of NIHL i.e. bilateral sensorineural hearing loss at high frequency with characteristic notch at 4 kHz was considered as NIHL and the rest as non-NIHL hearing loss.
Occupational health and safety survey: We carried out walk through survey of the factory and all findings were noted.
Monaural and binaural hearing impairment: Monaural and binaural hearing impairment is expressed in terms of % by using threshold average of frequencies 1, 2, 3 and 4kHz. Hearing impairment was defined as a threshold average greater than 25 dBA hearing level [17].

Statistical Analysis
Age, job duration, minimum noise exposure, maximum noise exposure and average noise exposure level were compared between NIHL and non-NIHL subjects with Wilcoxon rank sum test. P-values less than 0.05 (two-tailed) were considered as statistically significant.
Average noise and minimum noise did not differ statistically significantly between NIHL and non-NIHL subjects (p=0.64, p=0.17). However, there was significant median difference between maximum noise level where median value for non-NIHL was 101 and for NIHL subjects 96 (p=0.017). Thus there was no statistically significant relationship between average and minimum noise exposure. But maximum noise exposure has statistically significant relationship with NIHL. NIHL subjects were significantly older than non-NIHL subjects (Median=45 vs Median=41, p=0.011). Therefore there was statistically significant relationship between age and NIHL.
No statistically significant difference was noted between job duration and NIHL/non-NIHL subjects (p=0.39). Therefore duration of job has no statistically significant relationship with NIHL.

Questionnaire for Hearing Evaluation
We obtained following results: 22% of the 272 workers complained that they can't hear in noisy background, 11% found problem hearing over telephone, 7.7% hear better from one ear than other, 5.9% turned TV volume too high, 9.6% had their ears examined. 9.6% suffered from ringing of the ears (Table 3).  Summary of the result is as follows. 26.9% of workers consider that noise level at work site is very high. 86% know that loud noise causes NIHL. 89.6% consider that NIHL can be prevented by using PPE. 72.8% were using PPE. 46% were always using PPE when exposed to excessive noise (Table 4).

Noise Induced Hearing Loss
Those workers who were suffering from hearing loss, their audiometry reports were evaluated. Those who were suffering from bilateral sensorineural hearing loss at high frequency and having a characteristic notch at 4kHz were classified as suffering from noise related hearing loss. We found that 25% of the workers were suffering from noise related hearing loss (Charts 5-12).
53% of the workers suffering from NIHL belong to 41-50 years followed by more than 50 years, 20-30 years, and 31-40 years respectively. The 45% workers suffering from NIHL were welders, followed by fitters, Dye makers, machine operators, electricians, painter, press operator, and coordinator respectively. 29.41% of workers belong to bus body line, followed by component, trim, tools and dye, press shop, protoshop, and axle shop, PDI respectively. 54.90% had an exposure of more than 90 dBA followed by 85 to 90 dBA and less than 85 dBA respectively. Chart 9: NIHL versus average noise exposure. 47.05% had exposure of less than 85 dBA followed by 85-90 dBA and more than 90 dBA respectively.

Health and Safety Survey
We carried out a walk through survey of the company. A department of health and safety was present which was being run by qualified personnel. Health and safety coordinators, and safety engineers were present. Health and safety policy was signed by the chief executive. A clinic run by an occupational health nurse was present. 100% of the workers were provided with PPE by the company , but few of them were useingit. Fire fighting system was adequate. First aid boxes were present in all shops. Labor canteen was neat and clean, food was served without cost.
Training programs for the workers were carried out on regular basis. We found excessive noise in certain departments of the factory. The different shops were not segregated therefore noise produced by one shop was added to the noise produced by other shops thus there was an additive effect of noise. We observed that workers in the body line shop were bolting while lying in awkward position and creating excessive noise and vibration. In the paint shop, there was poor housekeeping, excessive paint dust and high temperature. Excessive noise was being produced via the hooter used by the loader. All grinded residues were sprinkled on the floor. The accidents were registered and investigated. MSDS were available. Yearly audit was carried out by ISO certifying organization. Air sampling and water sampling was carried out regularly. All industrial waste was collected safely and disposed off.
There was no record of occupational disease and injuries. All the workers were entitled to medical treatment at social security hospitals as well as at authorized private medical centers (Figure 1). The workers were entitled to paid sick leave. Biological monitoring was carried out for selected workers with pulmonary function and audiometry.

Discussion
Automobile manufacturing particularly of heavy vehicle is a complicated and lengthy process. In Pakistan most of the parts, including the engine and chassis, are imported. The chassis is built up and engine is fitted over it and body of the vehicle is manufactured and all the components are installed. Most of the work is done manually. There are about 120,000 workers who are engaged by the automobile industry in Pakistan [19].
Automobile manufacturing industries have highest injury rates over all. There is high levels of chemical hazards and solvent exposure among production service workers during routine cleaning, spills management and confined spaces. There is excessive noise in all segments of industry leading to hearing loss. Most of the workers suffered from psychosocial stress. Increased rate of lung tumors and respiratory disorders found in foundry workers could be due to exposure to silica. There is increased incidence of tumors of GIT, Pancreas and larynx among machine operation workers. Those machine workers who are exposed to synthetic fluids suffer from skin disorders. Cumulative trauma disorders are common among assembly workers [18].
According to WHO classification, we found hearing impairment among 62% of the automobile manufacturing workers. Our result has been verified by a study in Indonesia where they found prevalence of hearing loss as 55.9% at a Power station, Sarawak Indonesia [20]. We classified hearing loss into mild, moderate, severe and profound. We found in our study that 50.5% were suffering from mild, 9.7% moderate, 0.5% severe, 0.5% profound hearing loss. Thus mild hearing loss was most common. We found hearing loss slightly more common in right ear as compared to left ear. Same result was found in a study on Nigerian traders in Nigeria [21]. The percentage of binaural hearing loss in 58.35% workers was ranging from 1-20%, in 13.1% was 21-40% and in 2.4% more than 40%. Thus most common binaural hearing loss is between 1-20%. We found that those who were suffering from hearing loss maximum 42 In an automobile manufacturing unit rotors, gears, turbulent fluid flow, impact processes, electrical machines, internal combustion engines, pneumatic equipment, drilling, crushing, blasting, pumps, compressors are used [22]. These machines produce excessive noise which affect ears and causes hearing impairment, if exposure continues it leads to total hearing loss.
During our study we found that majority of the workers 78.7% were exposed to noise more than safe limit of 85dBA. The occupational risk of NIHL in industries that expose workers to continuous high levels of noise is well established [23]. Hearing may be affected by one time exposure to high noise or to continuous high noise level [24]. When an individual is exposed to excessive noise there is a change in hearing threshold which is temporary if he is removed from that environment his hearing threshold improves but if exposure is continuous then it leads to permanent change in hearing threshold and leads to permanent hearing loss. Noise induced hearing loss is one of the most common occupational health hazard [20] not curable [20,25] it causes worker's morbidity as well as loss to the industry.
According to Coles' criteria for making a diagnosis of NIHL there should be high frequency sensorineural hearing impairment, excessive noise exposure and a typical audiometric notch at 4000Hz. In our study we found that 24.75% of the workers fulfilled the criteria for noise induced hearing loss. Thus noise induced hearing loss among automobile manufacturing workers was 25%.This could be probably due to chronic exposure to noise level above 85dBA. In a study at an automobile manufacturing unit at Chennai, India [22] they found NIHL prevalence 18% which is less than our study. It could be due to low level of upper limit of noise exposure. We found upper limit as 118dBA whereas the other study in India found it to be at 110dBA. In addition there are multiple factors which are responsible for final outcome like race, genetics, individual susceptibility and use of personal protective equipment etc.
The difference in prevalence of hearing loss could be due to individual susceptibility or several other factors which includes age, previous sensorineural hearing loss, smoking, ototoxic medication, type2 diabetes and hypertension etc [23].
Age is positively related to hearing loss [20,21,25,[32][33][34]38]. The relationship between age related hearing loss and noise induced hearing loss is complex and controversial but it could be additive [39]. We found in our study that those workers who were suffering from NIHL 15% belongs to less than 40 years, 29.16% 40-50 years, 36.66% more than 50 years of age. We found that as age advances % of workers suffering from noise related hearing loss increases. Thus there is an upward trend in NIHL with age. The findings of our study have been verified by studies at a Lock factory, India [40] and a Power station at Sarawak, Indonesia [20]. We found a statistically significant relationship between age and NIHL p=0.011.
NIHL is a slow and gradual process [20]. Duration of exposure is an important determinant of noise induced hearing loss [20,23,24,33]. Sri H et al., [26] Nirmalya M et al. [33] found that as duration of exposure increases hearing loss increases. We found in our study that workers suffering from noise related hearing loss 47% had exposure of 11-20 years, 31.72% 0-10 years, 28.20% 21-30 years respectively. Thus we found that maximum number of workers had exposure of 11-20 years. Our findings have been verified by a study at Sarawak, Indonesia [20]. Though excessive noise affects hearing early but usually manifests after long period therefore only 13.72% who were having 0-10 years exposure had NIHL. The number of workers with NIHL after 20 years of exposure is less, it could be due to healthy workers effect i.e those workers who suffer from hearing difficulty changed their job to a less noisy factory or changed their profession [41]. We did not found statisticcally significant relationship duration of work and NIHL p=0.39.
NIHL is caused by repeated exposure to severe noise [21], it increases as severity of noise increases [25,33]. If a worker is exposed to a daily average noise level that is above 85dBA he might get permanent hearing loss over a period of time [7,42]. We found in our study as average noise exposure increases prevalence of noise related hearing loss increases. Those workers who had noise related hearing loss their average noise exposure in 15.68% of the cases was less than 85 dBA, 29.41% 85-90 dBA, 54.98% more than 90 dBA respectively. We did not found statistically significant relationship between average noise exposure and NIHL p=0.64.
We found that maximum number of workers suffering from NIHL had exposure of more than 90 dBA. This confirms that NIHL is related with severity of noise. Osibogun A et al. found that those who had noise exposure less than 90 dBA NIHL was found in only 11.3% as compared to 79.8% in those who are exposed to more than 90dBA [43]. The result of above mentioned study verifies our results. Thus in order to prevent noise induced hearing loss we should reduce level of noise at source. We found statistically significant relationship between maximum noise exposure and NIHLp=0.017.
Use of Personal Protective Equipment (PPE) is important for the prevention of NIHL. If PPE is used properly, exposure to noise can be reduced [26] which can lead to reduction in Madridge J Otorhinolaryngol. ISSN: 2640-5148 NIHL. In our study we found that those who were suffering from NIHL 78.43% were using PPE for different durations at work. 50.98% were using PPE when exposed to excessive noise and only 3.92% were using PPE for 8 hours. We found that use of PPE was not satisfactory among workers. Similar results were found by Maisarah SZ et al. 5.1% [44] Emmanuel DK et al.5.5% [35], Razman MR et al. 11.9% [45]. Hafiz Omer Ahmed13.2% [46], Foluwasayo EO et al. 28% [47]. These studies verifies our finding that PPE use was not satisfactory among industrial workers.
All workers who are exposed to excessive noise should be given training regarding NIHL. We found that majority of the workers knew about excessive noise and its relationship with NIHL but only 1.96% knew that noise reduction is the most important step in prevention of NIHL. It shows that workers knowledge regarding prevention of NIHL was deficient. Hafiz OA [46], Razman MR et al. [45] found in their studies that workers' knowledge regarding NIHL was lacking. It verifies our results. There should be a structured training program for the workers regarding NIHL.
NIHL is a slowly progressive disease that is untreatable but preventable. The mainstay of management is early diagnosis. Early diagnosis needs pre-employment audiometry as well as annual audiometry of all workers who were exposed to excessive noise. We found in our study that 9.3% had undergone ear examination out of total subjects.Those who had NIHL only 5.88% had ear examination and none of them had audiometric evaluation. It shows severe deficiency in audiometry of the workers. Razman MR et al. found at saw mill, Malaysia that 3.6% had audiometry [46]. In saw mill, Malaysia clinical monitoring was slightly better than our results but still deficient. Atmaca I. Peker et al. found at a concrete factory, Turkey, 44.92 % of the workers were undergoing audiometry every year and 19.7% at Iron and steel factory every 2 years [48]. They have better results than our study which could be due to better health and safety implementation.
An audiometric health surveillance program can be considered as a costly exercise and employers in developing countries might therefore be reluctant to undertake audiometry. A questionnaire study might be a cheaper alternative. We found in our study that 17.64% had trouble in hearing in noisy background, 11.76% mentioned that family members and co-workers observed missing words, 7.84% accepted that when they talked to different people it seems to them that they are mumbling. We found "trouble in hearing in noisy background" as most common symptoms among those suffering from noise related hearing loss (17.64%). This shows that a questionnaire study will give low prevalence rate of NIHL in our setting. The findings of our study has been verified by M. Rosso, R. Agius, N. Calleja, mentioned that a questionnaire for early detection of NIHL was having a 32% sensitivity and 79% specificity and this cannot be a valid substitute for audiometry [49]. Thus pure tone audiometry should be used more widely in developing countries and audiometry will remain main choice for early diagnosis of NIHL.
Smoking has been characterized as a risk factor for hearing loss [38,50]. According to Mizoue T et al. the combined effect of exposure to noise and smoking may be additive [38]. We found in our study that 19.60% of those suffering from NIHL were smokers. This shows a weak relationship between smoking and NIHL. Joo HS et al., [51] Mohammadi S et al. [52] found in their study that there was increased frequency of hearing loss in smokers as compared to non-smokers who were exposed to excessive noise. Their findings is different from our finding it could be due to selection bias or deliberately hiding of smoking by the workers due to fear of losing their jobs.
Tinnitus is described as the hearing of sound in the absence of any external noise and it can be very distressing for a person. Neghab M, et al. mentioned that tinnitus is found among those who are suffering from hearing loss and is more common when there is exposure to excessive noise [7]. According to MH Azizi Tinnitus is a NIHL symptom and it should be considered in those suffering from NIHL [25]. We found tinnitus in 9.55% of the total subjects. The result of our study has been verified by various studies, which have reported that the prevalence of tinnitus in populations exposed to noise at work are 5.9% to 87.5% [53]. The variability could be due to factors such as the type of participant, the characteristics of the noise exposure and the definition of tinnitus used [53]. Out of 26(9.55%) who had tinnitus, 17(65.38%) participated in audiometry. Out of 17(65.38%) who had tinnitus and participated in audiometry, 12(70.58%) had hearing loss, 75% of them had non noise related hearing loss and 25% had noise related hearing loss. Thus we found a strong relationship between tinnitus and hearing loss.
Those workers who had tinnitus, 70% of them had hearing loss. We did not find strong relationship between noise induced hearing loss and tinnitus because only 3(5.88%) of those who had noise induced hearing loss had tinnitus also. Our observation is different from various studies carried out which shows that noise induced hearing loss is associated with tinnitus [53]. This could be due to the reason that 35% of those who had tinnitus did not participate in audiometry, it could be a deliberate attempt to hide hearing deficit due to fear of losing their job. Tinnitus is related to severity of noise and duration of exposure [53]. We have found same result as mentioned above i.e. 93.30% had exposure of more than 85 dBA and 38.46% had exposure of more than 20 years. We found tinnitus a good screening test for hearing loss and is cost effective.

Conclusion
Occupational noise is a significant cause of adult-onset hearing loss. The burden of hearing loss caused by excessive noise at work has many consequences for individual and society. Multiple factors contribute to occupational NIHL but major contributor is lack of prevention.
Occupational exposure to noise can be reduced by reducing the noise at the source. Hearing loss prevention programs should include: noise control, noise assessments, Madridge J Otorhinolaryngol. ISSN: 2640-5148 Volume 1 • Issue 1 • 1000101 audiometric monitoring, appropriate use of PPE, worker's education, record keeping and program evaluation. Hearing loss prevention programs needs commitment and resources. If there is a will there is a way.
In light of our research we recommend to all stake holders that they should make policies regarding prevention of noise induced hearing loss and implement it so that human sufferings and economic loss can be prevented.

Conflicts of Interest:
The author(s) report(s) no conflicts of interest.