|Year : 2012 | Volume
| Issue : 3 | Page : 125-128
Occupational noise induced hearing loss and hearing threshold profile at high frequencies
Jayesh D Solanki, Hemant B Mehta, Chinmay J Shah, Pradyna A Gokhale
Department of Physiology, Government Medical College, Bhavnagar, Gujarat, India
|Date of Web Publication||12-Nov-2012|
Jayesh D Solanki
Department of Physiology, 4th Floor, Behind S T Bus Stand, Jail Road, Bhavnagar, Gujarat
Source of Support: None, Conflict of Interest: None
Background: The textile workers exposed to industrial noise are prone to develop hearing loss concentrated mainly at high audible frequencies. It is affected by duration of exposure and type of noise. Aims: Present study was conducted to (a) investigate hearing thresholds of textile workers by pure tone audiometry, (b) correlate thresholds at 4 kHz, 6 kHz and 8 kHz between noise exposed and unexposed group and (c) study effect of duration of exposure on hearing profile. Materials and Methods: A cross-sectional study was carried out on 50 weavers of plastic industries exposed to high levels of constant noise. Objectively, hearing defects were inquired by self-administered questionnaires. Subjectively, pure tone audiometry was carried out to measure hearing thresholds at various frequencies. It was simultaneously compared with the similar data of matched control group. Results: Prevalence of hearing loss symptom and hearing thresholds were more in exposed group than in control group. There was significant hearing loss at high frequency as compared to lower ones. Audiometry revealed notch at 4 kHz with less effect at 6 or 8 kHz, cumulative effect of duration of exposure. Conclusion: Textile weavers are more at risk to develop Occupational Noise Induced Hearing Loss (ONIHL) mainly affecting higher frequencies concentrated at 4 kHz. Despite least effects on hearing, this disease silently progresses and there is a definite scope of prevention by appropriate protective measures.
Keywords: Audiometry, Hearing threshold, High frequency, Noise, Occupational noise induced hearing loss
|How to cite this article:|
Solanki JD, Mehta HB, Shah CJ, Gokhale PA. Occupational noise induced hearing loss and hearing threshold profile at high frequencies. Indian J Otol 2012;18:125-8
|How to cite this URL:|
Solanki JD, Mehta HB, Shah CJ, Gokhale PA. Occupational noise induced hearing loss and hearing threshold profile at high frequencies. Indian J Otol [serial online] 2012 [cited 2019 Jul 17];18:125-8. Available from: http://www.indianjotol.org/text.asp?2012/18/3/125/103438
| Introduction|| |
Exposure to environmental noise is a common and preventable cause of hearing loss in many industries including the textile industry. Noise Induced Hearing Loss (NIHL) is the second most leading cause of hearing loss after age-related hearing loss. Various studies have shown that people exposed to noise level above 85 dB suffered from NIHL. 
Occupational Noise Induced Hearing Loss (ONIHL) is still one of the most prevalent occupational diseases despite various preventive measures taken in vogue to reduce effects of noise.  ONIHL causes problems not only for the individuals concerned  but also for their families  and co-workers. , As per Dobie's criteria,  ONIHL is sensorineural, bilateral, and progressive with loss always being more at 3000-6000 Hz than 500-2000 Hz. Traditionally loud noise produces an audiometric notch at 4 kHz known as "Aviator's Notch." , However few studies showed notch at 6 kHz. ,, Notch at 8 kHz is well-known in cases of presbycusis (age-induced hearing loss).
Plastic weaving is the main occupation of many workers of Kumbharwada in Bhavnagar, Gujarat. Workers are exposed to a noise level of 105 dB of weaving power loom machines. They work for 8-12 hours a day for 6 days a week continuously without the use of any hearing protective devices. The present study tried to evaluate the prevalence of audiometric notch at higher frequencies of 4000 Hz, 6000 Hz, and 8000 Hz, and to look for its significance by comparison with matched controls. It also tried to find the frequency that affected the most by impact type of noise of weaving industry. We also tested the changing pattern of hearing with regard to increase in duration of exposure.
| Materials and Methods|| |
After being approved by the Institutional Review Board of our college, the study was conducted in Bhavnagar city of Gujarat, India from March 2009 to March 2010. Sample size was determined by using software Raosoft. Out of 64 subjects screened initially by personal history, past history, family history, occupational history and medical history, 14 were excluded due to presence of some other cause of hearing loss apart from noise. Total 50 workers (33 males and 17 females) with at least 5 years of exposure to noise without any interruption in job were recruited randomly from 5 different plastic weaving textile factories. For comparison, 50 age- and sex-matched controls not exposed to any kind of noise were evaluated.
Assessment of hearing
Subjective hearing assessment was carried out by pre-designed, validated, structured questionnaires related to presence or absence and type of hearing loss. The problem was quantified by pure tone audiometry using ALPS manual audiometer done in a silent room away from the workplace at weekends in order to avoid temporary threshold shift by allowing a minimum of 16 hours rest after last exposure to noise. Pure tones were presented by air conduction at various frequencies to test the hearing threshold of both ears with sensitivity of 2.5 dB. Presence of audiometric notch was determined as per criteria given by Sataloff et al. 
Data entry and analysis were accomplished using software Sigmastat 2.0 under Windows. Statistical significance of the results was analyzed using Chi-square test, Z test and t test. Any observed difference was considered statistically significant with P value less than 0.001 for Chi-square test and 0.05 for Z test.
| Results|| |
The present study tested the hearing profile of noise exposed symptomatically and audiometrically.
Subjective assessment of symptoms of hearing loss was more prevalent in the study group as compared to the control group [Table 1].
|Table 1: Prevalence of subjective symptom of hearing loss in study and control groups|
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There is an increased hearing threshold for high frequencies as compared to the lower (speech) frequencies [Table 2].
|Table 2: Average hearing thresholds at different frequencies among exposed and unexposed groups|
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In the study group, just 16% had no hearing loss with majority having mild to moderate degree of hearing loss while in the control group just 16% had hearing loss mainly of mild degree [Table 3].
Nearly half of the individuals had the highest threshold at 8 kHz frequency that is normal while 4 kHz notch was more prevalent in the study group and 6 kHz notch was more prevalent in the control group [Table 4].
|Table 4: Prevalence of notch at various frequencies in audiogram in study and control groups|
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| Discussion|| |
Assessment of hearing of the textile workers exposed to a constant type of noise done by symptomatic and audiometric testing showed a typical picture of ONIHL and the results were significant when compared with properly matched controls. The level of noise in textile industries is 105 dB.  This is far beyond the maximum permissible exposure limit for 8 hours time weighted average (TWA) in India that is 90 dB. 
Prevalence of symptoms of hearing loss was more in the noise exposed group as compared to the control; however, speech frequencies were least affected. When hearing loss is limited to the high frequencies, individuals are unlikely to have difficulty in quiet conversational situations. The first difficulty patients usually notice is trouble understanding speech when a high level of ambient background noise is present. As NIHL progresses, individuals may have difficulty understanding high-pitched voices (e.g., women's, children's) even in quiet conversational situations. Symptoms like trouble in normal and telephonic conversation, turning up TV and radio volume, and tinnitus are early to occur.  In textile industries, weaving section has the highest prevalence of hearing loss of 84.5% as documented by a study conducted on textile weavers of Lagos metropolis; our result, which is 84%, is also in line with this study. 
Various studies have shown that ONIHL affects higher frequencies more than the lower ones and hearing loss is mainly concentrated at 4 kHz or 6 kHz. ,, Our study confirmed the presence of 4 kHz notch that is a classical sign of ONIHL; however, 6 kHz is less affected. The presence of 4 kHz notch can be attributed to: (a) the fact that human hearing is more sensitive at 1-5 kHz (b) the fact that tympanic reflex attenuates loud noise below 2 kHz,  (c) non-linear middle ear function as a result of increased intensity, (d) high amplitude of traveling waves at the region of cochlea,  (e) resonance characteristics of ear canal to loud sound,  (f) weak attachment of basilar membrane at that region of cochlea, (g) paraossicular conduction to basal turn of cochlea, etc.
There was not much effect at 6 kHz in our studies because it is seen more commonly with acute acoustic trauma like among air force personnel ,, or in musicians.  Few studies have shown it to be present even in normal healthy adult's population  just like the controls of the present study. Hence its presence or absence is not much significant.
A study done  in 1992 evaluated the development of ONIHL under the influence of pulse noise. Workers engaged with weaving and blacksmithing were the 2 study groups, both being exposed to noise level of 109 dB. The result showed a reliable difference between the study groups both in the quality of cochlear neuritis and the grade of hearing loss. Mild hearing failure dominated in the weavers' group whereas moderate and severe form appeared in blacksmiths. The impulsive noise appeared to be more aggressive than the constant noise. Due to the constant noise of power loom machines comparatively less damage occurs and as a result hearing loss was mild to moderate in the present study. Impact noise is more likely to be seen in the context of occupational noise exposure. It is frequency superimposed on a background of more sustained noise. Boettcher  has shown that when impact noise is superimposed on continuous noise, the injurious potential is synergistically enhanced. So workers exposed to constant noise have less damage and the use of protective devices can be very effective if properly used.
As it is verified in several studies, ,, the correlation between the age and time of exposure are factors that have influenced the worsening of these workers' auditory threshold because according to the increase of age and time, the audiometry presented with some altered frequencies.
The present study shows that hearing loss is more at higher frequencies as compared to the lower ones and with increase in the duration of exposure this magnitude increases. A transmission electron microscopic study showed that there is edema and swelling of afferent nerve endings below the inner hair cells on exposure to noise.  Increases in duration of exposure to high intensity sound eventually lead to breakdown of organ of Corti, elimination of sensory structures and finally gets replaced by a single flat cell. 
| Conclusion|| |
Weavers of textile industries exposed to high intensity constant noise without protection showed higher prevalence of subjective symptom of hearing loss but not much affecting normal day-to-day activities. Audiometry revealed presence of 4 kHz notch with normal hearing at 6 kHz and age-induced changes at 8 kHz in nearly half of the subjects. Hearing loss was of mild to moderate degree that progresses with time. Use of hearing protective devices can bring the noise exposure below the upper permissible limit and the disease can be prevented theoretically if not practically.
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[Table 1], [Table 2], [Table 3], [Table 4]