Occupational noise exposure and noise-induced hearing loss (NIHL) have been recognized as a problem among workers in Indian industries. The major industries in India are based on manufacturing. There are appreciable numbers of casting and forging units spread across the country. The objective of this study is to determine the prevalence of permanent hearing threshold shift among the workers engaged in Indian iron and steel small and medium enterprises (SMEs) and compared with control group subjects. As a part of hearing protection intervention, audiometric tests were conducted at low (250-1000 Hz), medium (1500-3000 Hz), and high (4000-8000 Hz) frequencies. The occurrence of hearing loss was determined based on hearing threshold levels with a low fence of 25 dB. Comparisons were made for hearing threshold at different frequencies between the exposed and control groups using Student's t test. ANOVA was used for the comparison of hearing threshold dB at different frequencies among occupation and year of experience. A P value <0.05 was considered as statistically significant. All data were presented as mean value (SD). Over 90% of workers engaged in various processes of casting and forging industry showed hearing loss in the noise-sensitive medium and higher frequencies. Occupation was significantly associated with NIHL, and hearing loss was particularly high among the workers of forging section. The analyses revealed a higher prevalence of significant hearing loss among the forging workers compared to the workers associated with other activities. The study shows alarming signals of NIHL, especially in forging workers. The occupational exposure to noise could be minimized by efficient control measures through engineering controls, administrative controls, and the use of personal protective devices. Applications of engineering and/or administrative controls are frequently not feasible in the developing countries for technical and financial reasons. A complete hearing conservation programme, including training, audiometry, job rotation, and the use of hearing protection devices, is the most feasible method for the protection of industrial workers from prevailing noise in workplace environments in the developing countries.
Keywords: Hearing conservation, NIHL, occupational hearing loss
|How to cite this article:|
Singh LP, Bhardwaj A, Kumar DK. Prevalence of permanent hearing threshold shift among workers of Indian iron and steel small and medium enterprises: A study. Noise Health 2012;14:119-28
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Singh LP, Bhardwaj A, Kumar DK. Prevalence of permanent hearing threshold shift among workers of Indian iron and steel small and medium enterprises: A study. Noise Health [serial online] 2012 [cited 2014 Oct 26];14:119-28. Available from: http://www.noiseandhealth.org/text.asp?2012/14/58/119/97256
| Introduction and literature survey|| |
Occupational noise is an integral part of the job in all types of industries especially in iron and steel industries, and high noise exposure causes severe hearing loss.  Noise is the hazardous industrial pollutant causing severe hearing loss in workers of every country in the world. The workers in industries such as mining, construction, printing, crushers, drop forging, iron, and steel companies are at high risk. The workers engaged in different processes of casting and forging industry are exposed to continuous, intermittent, and impulse/impact noise.  A continuous noise is a noise whose maxima occur more often than once per second. Thus, impulsive noise is assumed to have peaks occurring less often than once a second and is limited to peak sound pressure levels of 140 dB. All continuous, intermittent, and impulsive noise between the levels of 80 and 130 dB (A) must be included in the exposure assessment. Hearing protection must be provided at no cost to employees and must be worn by all workers exposed to a time-weighted average (TWA) of 90 dB (A) and above.  The different types of noise causes different affect on hearing and other physiological parameters, and the effects of noise can be auditory and non-auditory, with the non-auditory mainly including the nervous system, cardiovascular system, endocrine system, and psychology.  The majority of the workers in the small-scale hand tool forging units are highly exposed to high noise levels [>90 dB (A)], 60-72 hr/week without proper ear protection, which is very high when compared to OSHA norms.  Exposure to excessive noise is the major avoidable cause of hearing impairment. Worldwide, about 16% of the hearing disability is caused by occupational noise exposure.  Due to exposure to the higher noise intensity, prevalence of hearing loss was greater in dal mill workers. These workers were more susceptible to NIHL compared with the Dana bazaar workers.  Small and medium enterprise (SME) workers were under high levels of occupational noise exposure and heat stress.  Noise-induced hearing impairment was present in 100% of the workers exposed for a period of 14 years. By 4-8 years, 100% of sawmill workers had developed hearing impairment.  Hearing loss was significantly associated with working experience of more than 10 years and overtime. The prevalence of hearing impairment was significantly more in continuously exposed group when compared with intermittently exposed group. Occurrence of hearing impairment was also directly proportional to the duration of exposure.  The correlation between level of sound exposure and hearing impairment was found to be significant (r0 = 0.98; P < 0.05).  The prevalence of hearing loss associated with occupational noise exposure and other risk factors, involving 269 exposed and 99 non-exposed subjects (non-industrial noise-exposed subjects) randomly selected in Saudi Arabia was determined. Gross occupational exposure to noise was demonstrated as a cause of hearing loss.  The prevalence of sensorineural hearing loss among the workers in a steel rolling mill in Nigeria was also determined. The prevalence of sensorineural hearing loss among the study population was high. Pre-employment and regular audiometry while on the job is highly recommended.  The casting and forging sectors of the country constitute a considerable proportion of employment. The workers engaged in forging and grinding sections were more prone to noise-induced hearing loss (NIHL). The study recommended that there is a strong need to implement the standard of working hours as well as heat stress and noise control measures. The severity of hearing impairment is affected by the type and intensity of noise.  Certain insights on the biological and audiological effects of exposures to impulse noise were revealed. First, impulse noise may damage the cochlea by direct mechanical processes. Second, after exposure to impulse noise, hearing may recover in an erratic, non-monotonic pattern. Third, exposure to certain blast and impact noise of different parameters, i.e., level, duration, and temporal pattern, produces conditions of TTS and PTS that are frequently in consistent with the equal energy hypothesis. Impulse noise can interact with background continuous noise to produce great hearing loss than would have been predicted by the simple sum of the individual noises.  Temporary threshold shift (TTS) recovery time is dependent on the magnitude of the initial hearing loss. Also, TTS driven by noise exposure is enhanced by heat and workload. , An exposure of chin chills to broadband high-level impact noise on an interrupted 6 hours daily schedule over 20 days has shown that pure-tone thresholds measured immediately following each daily exposure improve as much as 30 dB despite the continuing noise exposure.  A-weighted L eq continuous noise levels of 108 dB for hammer operators and 99 dB for press operators in a drop-forging industry were determined. For long-term exposures of 10 years or more, hearing losses resulting from impact noise in the drop-forging industry are as great as or greater than those resulting from equivalent continuous noise.  Industrial impulse noise causes in general about 5-12 dB more severe hearing loss than steady-state noise. The exposure to impulsive noise is composed of very rapid sound bursts that have short duration and low energy content. Consequently, the audibility is lower than the actual level of the impulses. The hearing protective devices attenuate industrial impulse noise effectively but do not prevent from advanced hearing loss among workers.  It was reported that impulse noise seemed to produce permanent threshold shifts at 4000 and 6000 Hz after a shorter duration of exposure than continuous steady-state noise. Exposure to high levels of impulse noise (despite the use of ear protectors) is more detrimental to hearing than are high levels of continuous noise (even continuous with slightly impulsive features). The frequencies most sensitive to impulse noise are 4000 and 6000 Hz. Therefore, it is reasonable to assume that the qualities necessary for ear protectors to protect hearing from impulsive noise differ from those necessary to protect from continuous noise. In future studies, it would be more realistic to study not only the effects of exposure to noise but also the attention paid to subjective characteristics and individual factors of work conditions.  Majority of the forging workers are exposed to high noise in addition to musculoskeletal strain. The authors emphasized that there is a strong need to implement the ergonomics interventions and noise control measures. It is recommended that hearing conservation programme should be implemented under strict legal control.  The effects of noise exposure on hearing varied across age groups were studied and they highlighted the importance of applying age and gender corrections prior to determining the relative contribution of occupational noise exposure in patients with Sensori Neural Hearing Loss. There were significant effects of age on noise-induced permanent threshold shifts (NIPTS) but no significant gender or ear differences in terms of NIPTS. The NIPTS at 2000 Hz was found to be significantly greater than NIPTS at frequencies 500, 1,000, 4,000, and 8,000 Hz. In addition to the NIHL, other authors reviewed a variety of studies on history of concerns about long working hours and the current scientific evidence regarding their effects on workers' health.
Scope of the study
The manufacturing industry especially casting and forging (iron and steel) industry comprises a major part of the occupation in India. The casting industry gives employment to more than 5 lacs people directly and three times indirectly. At the same time, forging industry provides direct employment to about 2 lacs people, contributing directly to the livelihood of more than three quarter of a million people. Due to unemployment, the workers have accepted the jobs with low resource settings including health and safety consideration. This has made them more prone to the hazards of technology. Industrialization in India is primarily focused on production, whereas health and safety have a very low priority according to Jaiswal et al. Except a few major, reputed, public and private industries, other industrialists are insensitive toward the importance of occupational health and safety. The employer of small-scale units are totally lagging behind in providing occupational health and safety to the workers, and therefore the manpower employed in small-scale casting and forging units are more exposed to occupational noise, heat stress, musculoskeletal strain, dust etc. The workers are exposed to high noise throughout their life time of work, but there are very few NIHL studies in India to show its prevalence.
| Methods|| |
This study included randomly selected SME casting and forging units (each three) located in northern India. The approachability to the management of the organization and their willingness to participate in the study were more of a concern than randomization as work conditions of most casting and forging units in this region are almost similar. Hence, convenience of approaching the management was the basis of selecting units for the study. The study has been conducted involving randomly selected 572 workers out of 1500 workers of 12 units. The questionnaire included noise exposure, working hours, use of protective equipments, and awareness about benefits of PPEs. The questionnaire was pretested before it was used to assess the information. As workers of these units are mostly illiterate or less educated, statements of the questionnaire were translated to both local language of the state, i.e., Punjabi and Hindi. The interview was conducted by the authors in the local language and responses were entered in questionnaire.
Noise and dose assessment
A weighted (Leq) ambient noise was assessed by using a quest sound level meter "model SOUNDPRO SP-DL-1-1/3." OSHA norms for hearing conservation were incorporated including an exchange rate of 5 dB(A), criterion level at 90 dB(A), criterion time of eight hours, threshold level equal to 80 dB(A), upper limit equal to 140 dB(A), and with F/S response rate. In addition, the noise exposure was measured using Noise Pro DLX-1 ANSI SI.25-1991-Personal Noise Dosimeter. Long-term recording was done for 8 hours and TWA dose was calculated. Five observations in each type of operation were taken. Similarly, the sound pressure was recorded for 15 minutes each time on each work station and one long term recording for 8 hours was done. At each section, sound pressure was recorded at least four to five times at different locations where the movement of the workers was most frequent.
Pure tone audiometry
A sample of 165 exposed subjects were deliberately selected from sections like punching/blanking, forging, molding, grinding, welding, and tool room. A control group of 57 subjects (faculty, staff and students) were selected from the institute. The exposed group was selected from 572 workers who answered the questionnaire; however, the control group was not assessed for questionnaire. Mean age of exposed group was statistically matched (at P value of less than 0.05) with the control group. Subjects of both the groups were invited to conduct pure tone audiometry at the institute's ergonomics laboratory. The subjects were thoroughly instructed about the test. Threshold level of both ears was checked. The hearing threshold was measured in an audiometric room. Audiometric tests were conducted at low (0.25-1.0 KHz), medium (1.5-3.0 KHz), and high (4.0-8.0 KHz) frequencies. Portable audiometer Arphi 500 km III was used which works on 220/230 V AC mains supply as well as on 15 V DC supply. The audiometer was calibrated and tested before conducting the test. The subjects were explained about the audiometry test and the same was conducted. The prevalence of hearing loss was determined based on hearing threshold levels (HTLs) with a low fence of 25 dB.
All data were presented as mean value (SD). Comparisons were made for hearing threshold (dB (A)) at different frequencies between the exposed and control groups' characteristics such as smoking, alcohol consumption, and tobacco use using Student's t test. ANOVA was used for the comparison of hearing threshold (dB (A)) at different frequencies among occupation and year of experience. A P value <0.05 was considered as statistically significant.
| Results|| |
The sound pressure was recorded for short and long duration. There was hardly a difference of 0.5-1.0 dB (A) between long-term recoding and short-term recording. In the molding section at different locations, noise was impulsive or intermittent; in grinding and melting sections, there was almost steady noise. Most of the work locations have an almost steady noise except the drop forge hammers, punching, blanking, trimming, and molding sections. Noise level at various sections like drop hammer, cutting/blanking presses, punching press, grinding, barrelling, and machine molding sections was found greater than 90 dB (A) permissible limits. The workers engaged in different processes of casting and forging industry are exposed to continuous, intermittent, and impulse/impact noise. The ambient noise levels L eq (A) dB and 8 hours percentage dose are shown in [Table 1]. The noise levels in punching/blanking, forging, molding, grinding, except gas cutting/welding, and tool room sections were more than the prescribed limits, i.e., 90 dB (A). Therefore, the percentage dose was also more than 100%. The workers engaged in different activities in forging section were found with very high dose % [like punching blanking 367%, trimming 630% Forger 881%, furnace job taker 635%, and roap puller (hammer operator) 749%]. The mean values of hearing threshold of exposed group (workers engaged in various activities of casting and forging firms) and control group are shown in [Table 2] for both left and right ears. Workers engaged in forging sections were found to have higher loss of hearing when compared with workers engaged in other sections. Process wise audiograms for left and right ear threshold are shown in [Figure 1]. The overall audiograms for exposed group v/s control group are shown in [Figure 2]. The "t" statistics for overall hearing thresholds for both the groups is shown in [Table 3]. There is a significant hearing loss among exposed group subjects when compared with control group subjects. ANOVA was applied for the comparison of hearing threshold for characteristics like occupation and years of experience. Type of occupation significantly influenced all frequencies except for left and right ears at 500, 4000, 6000, and 8000 Hz [Table 4]. The multiple comparisons among various processes and control were performed using ANOVA Tukey's analysis; the results of one way are shown in [Table 5] and [Table 6]. Similarly, occupational experience significantly influenced the hearing threshold of left ear but except in right ear frequencies at 250, 500, 3000, 4000, 6000, and 8000 Hz [Table 7].
|Figure 1: Audiogram of mean hearing threshold (left and right ear) for exposed group subjects engaged in various sections and control group subjects|
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|Figure 2: Audiogram of mean hearing threshold (left and right ear) of exposed group and control group|
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|Table 2: Hearing threshold "dB (A)" for left and right ear at different frequency bands|
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|Table 3: T test statics for comparison between exposed group (N=165) and control group (N=57)|
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|Table 4: Effect of occupation on hearing threshold "dB (A)" at different frequency: ANOVA results|
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|Table 7: Effect of occupational experience (in years) on hearing threshold (dB (A)) at different frequency|
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Use of personal protective equipments
Although majority of workers reported that they are aware of the benefits of using PPE, a big proportion of workers do not use PPE in both casting and forging units. The subjective responses regarding the use of PPEs are revealed in [Table 8]. Only 25% of the workers wear dungarees, 41% use gloves, 35% report using eye protection (goggles), and 37% workers wear gum shoes or boots. and nose mask was used by 25% workers. The ear protection was found to be the least preferred or ever used PPE, and only 17% workers used it. The reasons stated for not using PPE: 40% did not feel uncomfortable, 10% are not used to wearing the same, 30% admitted to negligence, and around 25% said management did not provide PPE at work place.
|Table 8: Details of score at five point scale of use of PPEs by the workers|
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| Discussion|| |
There was a significant hearing loss, i.e., permanent hearing threshold shift (PTS) in the left ear among the industry subjects at 1-8 KHz. There was also significant loss of hearing in the right ear at all frequencies except 0.25 and 1.5 KHz among industry workers. About 90% of workers engaged in various processes of casting and forging industry showed hearing loss in the noise-sensitive medium (1.5-3.0 KHz) and higher frequencies (4.0-8.0 KHz). The analysis demonstrates a higher prevalence of significant hearing loss among the workers doing forging tasks when compared with the workers associated with the other activities. Therefore, it reveals that the prolonged exposure (more than 10 years) to impulsive/impact (101.2-105.3 dB A) noise is more hazardous to hearing when compared with continuous or intermittent (90-98 dB A) noise. It results in permanent shift in the hearing threshold among exposed subjects at almost all frequencies. These findings are also matching with the findings of  that impulse noise seemed to produce permanent threshold shifts at 4000 and 6000 Hz after a shorter duration of exposure than continuous steady-state noise. Exposure to high levels of impulse noise (despite the use of ear protectors) is more detrimental to hearing than are high levels of continuous noise (even continuous with slightly impulsive features). Thus, impulsive noise produces permanent threshold shifts at certain frequencies after an exposure of one decade when compared with continuous noise. The frequencies most sensitive to impulse noise are 4000 and 6000 Hz. Therefore, it is reasonable to assume that the qualities necessary for ear protectors to protect hearing from impulsive noise differ from those necessary to protect from continuous noise. There was no significant difference between hearing threshold at frequencies 250, 500, and 1000 Hz for occupation like punching/blanking, furnace operator, hammer operator, molding, grinder, gas cutter and welder, tool room, and normal subjects for both the ears except forger. However, there was significant difference between HTL at 1000, 1500, 2000, 3000, 4000, 6000, and 8000 Hz for left ear of forgers and subjects engaged in other job. In addition, there was also significant difference of hearing threshold of workers engaged in forging process at 250 and 500Hz. At the same time, hearing threshold of right ear of forgers is significantly different from control group at all frequencies and from workers of tool room at 2000, 3000, 4000, 6000, and 8000 Hz. Therefore, PTS is significantly influenced by occupation. The characteristic like smoking was not found to be associated with hearing thresholds shift for both the ears were insignificant. The reason for the same is that the exposed group subjects were used to smoking (1 to 3 biddies or cigarette/day) not chain smokers. Some of the subjects even smoke rarely. Our results are also consistent with some other studies which were not able to show a relationship between smoking and NIHL. , However, in contrast, a few of the studies reported the relationship between smoking and NIHL; most of them in different parts of the world have shown this relationship (although with different significances). , The subjective responses collected through the interviews of industry workers revealed that majority of the workers are not wearing ear protections. The workers also revealed that management did not enforce use of PPE; in most small-scale units management did not even bother about it. Reason, management concentrates on executing the orders and shipments. Another significant factor is that workers in small-scale casting and forging units also work under contractors who ignore this aspect. It is also true that workers do not expect health and safety care from the management; rather they have accepted hazard conditions as a part of their job and life. Whatever the reason may be, workers do not use PPE; thus it is obvious that majority of the workers are directly exposed to noise. Around 85% of workers work more than 8 hr/day with additional over time of 2-4 hr/ day (12-24 hr/week) which is a major factor contributing toward very high noise exposure (more than the OSHA norms).  Such long working hours increase the risk of NIHL since the workers are rarely using ear protection. The ethical considerations regarding long working hours are to be thought of as questions about the type of society we want to create.  Thus, ethical implications of unconventional shift work and long work-hour schedules should be considered. A just and fair society will take actions to ensure that sui- jobs are available for as many people who want to work as possible, and that the jobs are safe and properly compensated, allowing not only for a beneficial work life but also for a life that has time for rest, health, family, leisure activities, and the attainment of one's personal values. This risk can be reduced by ensuring the proper use of ear protections and if in case the workers are not getting accustomed with the use of PPE the job can be rotated with the low noise jobs. Moreover, the prevailing work schedules in SMEs generally do not include sufficient rest allowances, thus there is also a strong need to set reasonable performance standards for various activities with appropriate rest allowances.
| Conclusions|| |
The study shows higher prevalence of PTS, especially among forging workers when compared with molders, grinders, and tool room operator. Occupation and exposure are significantly associated with the permanent threshold shift where as habits like alcohol consumption, smoking, and tobacco intake were not associated with the hearing threshold shift. Exposure to high levels of impulse noise (despite the use of ear protectors) is more detrimental to hearing than are high levels of continuous noise (even continuous with slightly impulsive features). The frequencies most sensitive to impulse noise are 4000 and 6000 Hz. Therefore, it is more vital to assure that the ear protectors of necessary quality are provided to the workers to protect hearing from impulsive noise when compared with those necessary for protection from continuous noise. At the same time, there is a need to implement the job rotation so that the overall noise dose can be reduced. The ethical consideration regarding the long working hours are required to be considered so that the jobs are safe and properly compensated, allowing a life that has time for rest, health, family, leisure activities, and the attainment of one's personal values.
| Limitations|| |
This study has certain limitations regarding the randomization selection of exposed group (industrial workers) as the approachability to the management of the organization and their willingness to participate in the study was more of a concern than randomization. However, work conditions of most casting and forging units in this region are almost similar. The study also lacks in measuring the impulse content of the noise.
| Acknowledgment|| |
We hereby acknowledge the co-operation extended by the managements of SMEs for giving us permission to conduct the survey. We also acknowledge the efforts of the subjects of both the groups who volunteered for the study and for sparing their valuable time.
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Lakhwinder Pal Singh
Department of Industrial and Production Engineering, Dr. B R Ambedkar National Institute of Technology, Jalandhar-144 011
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]