Occupational hearing loss of the workmen of an open cast chromite mines.

BACKGROUND: The present work aimed at to describe hearing threshold based on audiometry data of the mine workers based on their age, work station and years of working of an open cast chromite mine in Odisha, India at high fence.
MATERIALS AND METHODS: A cross-sectional study of hearing threshold of the subjects of the chromite mine was carried out. Audiometric data of 500 subjects were taken from the hospital of the mines of Sukinda Valley, Jajpur, Odisha, India. The latest audiometry data available during the period 2002 to 2008 was used in the statistical analysis.
RESULTS: The age group 50-60 years is found to be the most influential age group suffering significant hearing loss on both the ears. Also, the Work Zone area is found to be most significant area affecting hearing loss on both the ears. However, the subjects having experience of 30-35 and 25-30 years have the most significant hearing loss on the left and right ears, respectively.
CONCLUSION: The hearing loss is found to be at 6 kHz, thus the working areas of the subjects working at work zone should be regularly rotated in less noisy areas to reduce the exposure duration. High frequency noise protective device should be advocated among all the subjects in general and HEMMs operators in particulars. Regular audiometry test of all the subjects should be performed to identify the hearing loss of the subjects occurring at 6 kHz. It is essential to perform periodic maintenance of all the HEMMs to keep all the vehicles in good condition those are generating noise at dominating frequency of 4 and 6 kHz.

INTRODUCTION

Auditory effects or any other health effects may develop to any person at any frequency level depending upon the individual exposure to noise situation. Low frequency noise,[1] up to 100 Hz may cause non-aural physiological and psychological effects below the individual hearing threshold. Leventhal et al,[2] also emphasized that 10-200 Hz frequency noise is an environmental noise sensitive to people in their homes and offices. This is the group which generates many complaints and is generally seen to the people of middle age. It may also occur to the subjects working in industry, but generally found at levels well above threshold. Similarly, low frequency noise[3] produced by the armoured vehicle also causes negative effects on reaction time to the normal hearing subjects. Pal et al,[4] have investigated that dozer, dumper, screen plant and pit exhaust fan generate noise in the alarming frequency range of 250-500 Hz. Similarly, haulage machine generates noise at dominating frequency of 1.6 kHz. When spectral noise[5] level of 50 dB SPL is applied at the mid-frequencies, viz., 1 and 3 kHz to the exposed subjects, auditory sensitivity loss increases with decrease of loss of frequency selectivity linearly. Mahmood et al,[6] studied that there is an increase in systolic and diastolic blood pressure to the young adults exposed to noise level of 90 dBA at 4 kHz. Subjects working in a Hydroelectric Power Plant[7] generally exposed to the noise levels in the range of 95-110 dBA at 2-8 kHz and hearing loss occurs within the first 10 years of exposure to noise and a dip found at 4 kHz. The subjects exposed to the high level of noise produced by aircraft and heavy machineries[8] exhibited notch at 6 kHz. Similarly, road traffic noise [9] contributes high frequency noise level of 70 dBA which causes listening interference equivalent to 3 dB. McBridge et al,[10] investigated that intense low frequency noise may cause maximal loss over the 0.5-2 kHz range and intense high frequency noise loss at 6 or 8 kHz. Therefore, the present work aimed at to describe hearing threshold based on audiometry data of the mine workers at different working areas of an Open Cast Chromite mines. The speech frequencies and the respective sub-groups were determined using different statistical methods where the noise induced hearing loss was found to be the maximum.

MATERIALS AND METHODS

Audiometric data of 500 subjects (481 males and 19 females) were taken from the hospital of an open cast chromite mine of Sukinda Valley area, Odisha, India for the period 2002-2008. The data were used to evaluate a possible dip or notch at high fence i.e., at frequencies 4, 6 and 8 kHz due to exposure to different levels of noise by the subjects. The subjects were divided into four age groups, eight experience groups and five work stations as depicted in the Table 1. Data were entered and cleaned using SPSS version 16.0 for Windows. We describe data using means, standard deviations, frequencies and percentages where applicable. The line plot was used to depict the hearing thresholds in different frequencies for both left and right ears. The independence between workers’ characteristic and hearing loss were assessed using Generalized Linear Model ANOVA, Post hoc analysis and Gabriel Multiple comparison for mean difference. Similarly, the Student's t-test and Paired t-test were used to evaluate any significant hearing loss difference between both the ears. All the results were deemed significant if P-values were less than 0.01.

Table 1

Descriptive statistics of the subjects (500) in three demographic categories and Mean HTL

RESULTS

As shown in Table 2, assuming the null hypothesis that there is no significant difference between the left and right ears with regard to hearing loss, a two tailed t-test was conducted for the whole subjects and found that both the ears do not exhibit any significant difference at 4, 6 and 8 kHz as P>0.01. Thus, asymmetric hearing loss may be attributed either in the left or the right ear at 4 kHz, 6 kHz or 8 kHz.

Table 2

Descriptive statistics of hearing loss (dB HL) of the whole subjects

ANOVA was performed to test any significant difference among the different age groups, experience groups and work stations with regard to hearing loss on both the ears. The most influential age group, experience group and work station were determined by Gabriel multiple comparison of mean difference.

Hypothesis: Age groups, years of experience and work stations are independent for hearing loss at high fence.

The hypothesis is rejected at 1% level of significance as P<0.01. Hence, concluded that the hearing loss is not homogeneous at high fence with respect to age, experience and work station.

Figure 1 exhibits the variation of mean hearing loss of the whole subjects with respect to all the test frequencies (0.5, 1, 2, 4, 6 and 8 kHz). The audiogram indicates a bilateral, no hearing loss below low fence (0.5, 1 and 2 kHz) and then moderate flat sloping mild hearing loss from 2 to 6 kHz, a notch at 6 kHz and then slight recovery at 8 kHz. The audiogram does not exhibit any “notch” at 4 kHz that is generally found due to noise induced hearing loss but shows little “dip” at 6 kHz. Thus, asymmetric hearing loss may be attributed in either of the ear.

Figure 1

Noise Induced hearing loss, dip or notch, high fence, low fence, asymmetry hearing loss

As exhibited in Table 3, the Post hoc analysis of multiple comparisons of means by Gabriel method reveals that the test frequencies 4 and 6 kHz are found to be the most significant test frequencies for right and left ears, respectively, indicative of maximum noise induced hearing loss at 1% level of significance with respect to age and work station. Similarly, the Post hoc analysis of multiple comparisons of means by Gabriel method reveals that the test frequency 4 kHz is found to be the most influential for both the ears, indicative of maximum noise induced hearing loss in other words a deep at 1% level of significance with respect to experience. The post hoc analysis also reveals that the age group 50-60 years is the most influential age group suffering significant hearing loss on both the ears. Also, the Work Zone area is found to be most significant area affecting hearing loss on both the ears. However, the subjects having experience of 30-35 and 25-30 years have the most significant loss on the left and right ears, respectively.

Table 3

The most influential test frequency and the subgroups

DISCUSSION

From Table 3, the Post hoc analysis of multiple comparisons of means by Gabriel method reveals that the age group of 50-60 years is the most significant age group affected hearing loss at high fence on both left and right ears separately. Ertem et al,[11] identified the first hearing loss at the age of 45 years or more to the textile workers. However, Kim et al,[12] found that the hearing loss attributed to age starts after 65 years of age or more. But, the present work, it has been found the age group 50-60 year as the most significant group at 4 and 6 kHz and may be assigned these values as the occurrence of a dip or notch and attributed to the possibility of the sensitivity of both the ears at the earlier stage. This asymmetry[13] may be caused due to the presence of a subgroup (operators of the HEMMs) who generally exposed to higher noise level i.e. more acoustic energy of the sound reaching in the right ear[91415] than to the left ear. Frank[16] investigated that 90% of workers attained hearing loss by the age of 50 years as confirmed with the present study. Similarly, Johansson et al,[17] found that hearing threshold levels starts more rapidly in the 50 year age group for frequencies more than 3 kHz.

It is found that working experience of 25-30 years is the most significant for the right ear and that of 30-35 years of experience is the most significant for the left ear affected by the hearing loss at different level of frequencies. Yildirim et al,[18] found in their study that hearing loss begins to all the textile workers within the first eight years of noise exposure and more evident could be ascertained between 5-8 years and then the hearing loss showed a slight increase in latter period of service. In the present study, the most significant group was found to be the subjects having working experience of 25-30 and 30-35 years at the test frequency of 4 kHz for right ear and left ear, respectively. This asymmetric behaviour may be attributed to the acoustic energy of the sound reaching both the ears is not equal.[1920] It may be inferred that there could be variation between the two ears with regard to response and recovery.[19] Like in earlier studies,[131421-24] the worse ear is not equated to the better ear in the present study and considered separately so that respond to sound is not happening in the same manner. Therefore, both ears have assumed equally sensitive[25] to any loudness strength and offer the same level of speech recognisation on that particular bandwidth.[26-28]

Similarly, there exists a significant difference among the various work stations and the work zone is the most significant work station to have hearing loss at different frequency levels at 1% level of significance.

Since, the most significant frequency is 4 kHz for both right and left ears, paired t-test was conducted to evaluate whether there is any significant difference exists between the right and left ears of the subjects with reference to experience. With Paired t-test, P-value is found to be 0.982 (P>0.01) and it may be said that there is no significant difference in hearing loss for both the ears at 4 kHz at 1% level of significance.

From the above findings, the hearing loss is found to be dependant on age, experience and working stations. It has been found that 6 kHz is the frequency of influence to the left ear for the older subjects i.e. the age group 50-60 years (7.8% subjects having hearing loss of >25 dB HL with mean hearing loss of 35 dB HL) and also for the subjects working in the Work Zone area (9.8% subjects having hearing loss of >25 dB HL with mean hearing loss of 34.90 dB HL). The frequency of influence to the right ear is 4 kHz and is the most sensitive to the age group 50-60 years, the experience group 25-30 years and the group working in the Work Zone area. Thus, it is to infer that any subjects begin their service at the age of 30 years in the work zone area of an open cast chromite mines than the noise-induced hearing loss may be the most significant after working for more than 25 years. So, the following are outlined for the chromite mining complex to abate noise impairment of the workmen:

The hearing loss is found to be at 6 kHz, thus the working areas of the subjects working at work zone should be regularly rotated in less noisy areas to reduce the exposure duration.

High frequency noise protective device should be advocated among all the subjects in general and HEMMs operators in particulars.

Regular audiometry test of all the subjects should be performed to identify the hearing loss of the subjects occurring at 6 kHz.

It is essential to perform periodic maintenance of all the HEMMs to keep all the vehicles in good condition those are generating noise at dominating frequency of 4 and 6 kHz.