Introduction For Surveillance of Noise-induced hearing Loss (NIHL), Thai Workers in Hearing Conservative Programs (HCPs) must have hearing test (audiograms) annually. Occupational Medicine Physicians (OMPs) use Notch Criteria for early diagnosis of NIHL. Naturally, notch would be deeper and wider after additional exposure to loud noise. The purpose of this study was to describe nature of Notch and Notch Area in target population.

Method In 2015, a descriptive cross-sectional study was performed by collecting audiograms from 1122 employees at chemical industries in Rayong province, Thailand. The investigators used criteria concluded from previous study to identify Notch at 3,000 4,000 6,000 Hertz, V-shape Notch, U-shape Notch and calculate Notch Area.

Results The most common Notch was 6,000 Hertz 4,000 Hertz and 3,000 Hertz respectively. V-shape notch is more common than U-shape Notch. The means of notch area (square unit) in right ear and left ear were 17.7 and 21 accordingly. For workers with bilateral notch, mean difference of notch area between right and left ear was 19.4.

Discussion and Conclusion Occupational Medicine Physician may use Notch Area to make diagnosis of ONIHL. Notch Area was useful for identifying symmetrical hearing loss. Longitudinal study should be conducted to show how Notch Area may change gradually which would be useful for identifying gradual onset of hearing loss. Another descriptive study of Notch and Notch Area should be conducted in population with diagnosed Occupational NIHL and other work-related hearing loss. 6,000 Hertz Notch and V-shape notch were common