Abstract: The development of temporary threshold shifts among employees due to noise may lead to noise-induced hearing loss over time. The aim of this study is to explore occurrence of temporary standard threshold shifts on adopting different permissible exposure limits, 90 and 85 dBA. Countries such as Malaysia and US are adopting 90 dBA as permissible exposure limit. In this intervention study, there were 203 participants from two factories. They were exposed to noise levels above action level which is 85 dBA in one factory and 80 dBA in another factory where permissible exposure limits were 90 and 85 dBA, respectively. The sample size required was 52 in each factory. Noise level was measured using personal exposure noise dosimeter and sound level meter. Data on standard threshold shifts were measured using manual audiometer. Hearing protection devices with appropriate noise reduction rate were used to reduce noise exposure among participants. According to National Institute of Occupational Safety and Health recommended standard, there was statistically significant association between participants from both factories and standard threshold shift at 1000 Hz. This change was noted on left ear at 6th month, χ2(1) = 3.93, φ = -0.211, p = 0.047 as per-protocol analysis. There were more threshold shifts on subjects adopting 90 dBA compared with 85 dBA. These temporary threshold shifts may progress to permanent threshold shifts over time. The prevalence of noise-induced hearing loss may be reduced on adoption of 85 dBA as permissible exposure limit.
Balachandar S. Sayapathi and David KOH, 2015. Comparing Standard Threshold Shift among Employees Adopting 85 and 90 dBA as Permissible Exposure Limits over Six Months. Research Journal of Biological Sciences, 10: 29-36.