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| Year : 2009
| Volume
: 11 | Issue : 45 | Page
: 185-188 |
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| Effects of shift work on noise-induced hearing loss |
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Yu-Fung Chou1, Jim-Shoung Lai2, Hsien-Wen Kuo3
1 Institute of Environmental Health, Taiwan
2 Institute of Environmental Health; Deparment of Occupational Safety and Health, School of Public Health, China Medical University, Taiwan
3 Institute of Environmental Health; Deparment of Occupational Safety and Health, School of Public Health, China Medical University; Institute of Environmental and Occupational Health Sciences, National Yang-Ming University, Taipei, Taiwan
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| Date of Web Publication | 2-Oct-2009 |
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Evidence has accumulated concerning the adverse effects of noise on hearing acuity, but it is not clear whether working shifts may decelerate the effects of hearing loss. The objective of this study is to assess the effects of shift work on hearing loss in a noisy work environment. A sample of 218 male workers recruited at a semiconductor factory with no known occupational hazards that affected hearing acuity other than noise was chosen. The subjects worked either in an eight-hour or 12-hour shift. A standardized audiometric procedure was performed by a qualified audiologist to measure pure-tone hearing thresholds at 0.5kHz, 1kHz, 2kHz, 3kHz, 4kHz, 6kHz and 8kHz in both ears. Using multiple linear regression adjusted for age, smoking habits, and work duration, the results showed that the severity of hearing loss in both ears was significantly lower in subjects who worked a 12-hour shift. In conclusion, working a 12-hour shift followed by a day off is best for workers and hearing protection should be provided in high noise areas. Keywords: : Hearing loss, hearing protection, shift work
How to cite this article:
Chou YF, Lai JS, Kuo HW. Effects of shift work on noise-induced hearing loss. Noise Health 2009;11:185-8 |
| Introduction | |  |
According to otolaryngologists, occupational noise is a common hazard in Taiwan which leads to noise induced hearing loss (NIHL). [1] The majority of chronic NIHL cases are due to exposure to noise at work. Previous studies [2] reported hearing loss is entirely preventable. In addition to engineering controls, employers should provide their workers with at least two means of protection - personal protection equipment (PPE), which is the most common, and administrative controls such as periodic shift rotation and limiting exposure to noise when noise levels exceed 85dB. [3],[4]
Because exposure to noise at the workplace is continuous, it is important to determine the long-term effects exposure to continuous noise has on hair cell loss. The small amount of hair cell damage caused by exposure to intermittent noise could be related to the recovery periods between noise phases. [5] Several studies [6],[7] have been published regarding threshold shifts produced after repeated exposure to noise for a long period of time; however, only a few studies have addressed the issue of threshold shifts produced after repeated exposure to noise for a short period of time. It is necessary to determine whether exposure to a pulsating noise is sufficient to initiate the recovery process. As a consequence of its effects on reduction of continuous exposure to noise, noise-induced free radical formation may be a significant factor in decreased cochlear blood flow, free radical formation, and oxidative stress. [8],[9],[10] Several articles concerning the adverse effects noise has on hearing acuity have been published, but it is not clear whether working shifts decrease the effects of hearing loss. A few studies have been conducted on the relationship between working shifts and noise-induced hearing loss. The objective of this study is to investigate this relationship.
| Materials and Methods | | |
Study population
A cross-sectional study was conducted in a large semi-conductor factory in Taiwan. About 218 male workers who had not been exposed to known occupational hazards (i.e. organic solvents) voluntarily participated and filled out informed consent forms. The 34 female workers at the factory were excluded from this study due to their low exposure to noise hazards. Based on the findings of the health examinations and questionnaires, workers with histories of ototoxic drug use, diabetes mellitus, hyperthyroidism, a history of ear infections, ear surgery, or exposure to nonoccupational noise (such as amplified music, participation in war or hunting etc.) were excluded from the study. Even though the work environment was listed as a noise hazard, the turnover rate of workers was low due to the high wages and good benefits offered by the company. Because the company requires that the machines operate 24 hours a day, workers were given the option of either working an eight-hour swing shift or working 12 hours a day for two consecutive days and then have two days off.
| Noise Monitoring and Hearing Examination | | |
In the annual noise monitoring program, a sampling strategy is designed to identify employees for inclusion in the hearing conservation program under typical operating conditions. An integrated sound level meter must meet ANSI S1.43-1997, type 2 requirements, and be set at an A-weighted slow response. All instruments used to measure employee noise exposure are calibrated to ensure accurate measurements.
In this study, a qualified audiologist assessed hearing acuity in the subjects by using standardized audiometric procedures after an assured period of noise avoidance for at least 14 hours. The hearing examination was done in an isolated acoustic room meeting ANSI S3.1-1991 standards with a diagnostic audiometer (Model AD 229e, interacoustic Denmark Co. Ltd). Pure-tone audiometry was completed in a soundproof booth with special treatment to the walls, ceiling, and floor to ensure that background noise did not affect test results. Only those sounds that the audiologist introduced into the room, either through earphones or through speakers located in the room, were heard. Pure-tone hearing thresholds at 0.5k, 1 k, 2 k, 3 k, 4 k, 6 k and 8 kHz were measured for air and bone conduction, in both ears. Three different definitions of noise-induced hearing impairment were defined as the binaural pure-tone average (PTA) hearing threshold level greater than 25 dB for the following frequencies: (a) low frequency hearing (LFH, (0.5 kHz +1 kHz +2 kHz)/3), (b) high frequency hearing (HFH, (3 kHz +4 kHz +6 kHz)/3), (c) 4 kHz. Hearing loss was defined as a hearing threshold level greater than 25 dB at 4K, LFH, and HFH. An audiologist, otolaryngologist, or physician reviewed problematic audiograms and determined whether there was a need for further evaluation.
Statistical analysis
Data was analyzed using SPSS version 10. The following statistical tests were used to analyze the data. The two-sample t-test was used to test differences between the study groups for quantitative parameters with hearing loss. The χ2 test was used to examine differences between the study groups for demographic data. Using binary logistic regression adjusted for age, exposure duration, smoking and the frequent use of a personal stereo device with a headset, we measured the difference in hearing loss in the two groups.
| Results | | |
The demographic information of the workers in the study is shown in [Table 1]. The average age was approximately 36 years old. Roughly 50% of the subjects were machine operators and only three percent were managerial staff or assistant. Approximately 56% of the subjects had senior high school-level education. 47% were non-smokers and 11% were ex-smokers. About 66% of the subjects claimed that they did not consume alcohol on a regular basis and 23% claimed to drink alcohol regularly; 36% of workers did not regularly wear ear-plugs in workplace, 67% worked regular work on eight-hour schedules and 33% were on 12-hour work schedules.
A comparison between the type of shift and severity of hearing loss is shown in [Table 2]. The 12-hour work schedule of workers showed a significantly lower level of hearing loss (LFH, HFH and 4 kHz) in both ears than those who worked regular eight hours. The severity of hearing loss in the right ear was slightly higher than that in the left ear.
[Table 3] illustrates shift work affect hearing loss using multiple logistic regressions adjusted for age, smoking and work duration. The odds ratio (1.88, 1.84 and 1.47) on hearing loss was higher in the three outcomes (LFH, HFH and 4 kHz) for those who worked regularly on eight-hour shifts compared with those who worked the 12-hour work schedule, but not significantly.
| Discussion | | |
Long-term exposure to noise at work causes hearing loss. Although countermeasures have successfully reduced noise levels in many industries, noise is still a common occupational hazard, and noise induced hearing loss is one of the major occupational diseases worldwide. [11],[12],[13],[14] Our study found that the employees who worked the 12-hours work schedule suffered a lower degree of hearing loss than those on the eight-hour shift. Since the group that worked five consecutive days suffered greater hearing loss, we can infer that continuous exposure to high noise levels as opposed to intermittent exposure increases the risk of hearing loss . Evidence has accumulated concerning the adverse effects of noise on hearing acuity, but it is not clear whether 12-hour work schedule may decelerate the effects of hearing loss. Free radical insults that build for a continuous period would potentially explain observation of hair cell death that accelerates with time after exposure for a period of up to 14 days. [15] Due to its effects on the reduction of continuous exposure to noise, we hypothesized noise-induced free radical formation resulted in a significant factor in decreased cochlear blood flow and free radical formation to alleviate the progressive degree of hearing loss. [9],[16] Continuous noise causes more damage to the cochlea than intermittent noise of the same intensity. At the intensities tested, damage to the cochlea is not proportional to the total noise energy. [17] Our findings indicate that the 12-hour work schedule may reduce cochlear damage caused by continuous exposure. Results indicated that the second work schedule produced smaller TTS and PTS, and less cochlear damage than the eight-hour work schedule or continuous exposure to noise for five consecutive days. A brief interruption in noise exposure clearly resulted in significantly smaller PTS and less hair cell damage than continuous noise exposure. [17] However, the mechanism(s) by which a brief interruption in continuous noise induces the recovery process have still to be clarified. It may be related to noise-induced metabolic stress which results in a reduction of blood flow, formation of reactive oxygen species, and/or the (Ca 2+ ) changes induced by high noise levels.
The prevalence of sensorineural hearing loss with the average thresholds at 4 kHz among 218 semi-conductor workers is approximately 20% under the exposure to noise ranged from 80 dBA to 90 dBA. While noise is recognized as an occupational hazard, initiatives are required to increase use of effective preventative measures. Simultaneous exposure to hazardous noises and certain chemical pollutants may have an added effect on hearing loss. Toluene, carbon monoxide and carbon disulphide in combination with noise are known to cause more severe high frequency hearing loss than noise alone. [18]
Other agents such as lead, mercury, xylene and trimethyltin are suspected to either worsen NIHL or alter susceptibility to NIHL. [19] Since the manufacturing process at the factory in this study is similar with those of other semi-conductor factories in Taiwan, the study participants were not exposed to chemical pollutants. A synergistic effect of smoking, noise exposure and age on hearing loss was found. [20] This was consistent with a report by Mizoue et al. [21] which said smoking could be a probable risk for high frequency hearing loss, and its combined effects on hearing with exposure to occupational noise is additive. However, our finding was not consistent with previous studies because of the promotion of a smoke-free policy and the prohibition of smoking in the workplace. The prevalence of smoking by the subjects in this study was 42% as opposed to the general population which is reported by Taiwan's Department of Health to be 51%; this results in underestimation because of the healthy worker effect in the study population.
There are certain limitations in this study. This was a cross-sectional study which makes it difficult to evaluate the effects of an alternating shift schedule; the turn-over of workers in this factory is low.
Well-fitted earplugs seem to be able to protect the ear from continuous exposure to high-level noise (greater than 85 dB). This was consistent with a report by Ologe et al. [22] which showed that pure-tone average and the average hearing thresholds at 4 kHz significantly increased with an increasing noise exposure level and correlated with the prevalence of sensorineural hearing loss. In addition, authors suggest that occupational noise regulation appears to have a strong positive impact on hearing conservation by reducing exposure and increasing use of hearing protection devices and medical surveillance, even exposure to noise lower than 85dB. Technological advances will improve monitoring, allow better noise engineering controls, and lead to more effective hearing protection. [23],[24]
In conclusion, we believe that working a 12-hour schedule followed by a day off is best for workers, because they have enough time to recover from exposure to noise at the workplace. Additional research must be conducted to understand the mechanisms of shift schedules on hearing loss, and workers in areas where noise levels were previously considered nonhazardous should be encouraged to use PPE.
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Correspondence Address:
Hsien-Wen Kuo
No.155, Sec.2, Linong Street, Taipei, 112 Taiwan (ROC), Institute of Environmental and Occupational Health Sciences, National Yang-Ming University, Taipei
Taiwan
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/1463-1741.56210
[Table 1], [Table 2], [Table 3] |
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