Audiometric notch as a sign of noise induced hearing loss (NIHL) among the rice and market flour mill workers in Tamil Nadu, South India

Abstract

Objective This study aims to determine the prevalence of NIHL among rice and flour mill workers exposed to high decibel noise levels at the workplace. Methods One-hundred and seven workers exposed to noise from rice and flour mill machinery and 82 age-matched controls not exposed to noise at the workplace were recruited for the study from the south Indian state of Tamil Nadu. All the mill workers and the controls were evaluated for their hearing thresholds by pure-tone audiometry. Results The characteristic bilateral notch in 4000 Hz was detected in 25% of the mill workers, while the prevalence of the unilateral or bilateral notch was observed in 51%. The high-frequency slope (HFS) involving HL at 4000 and 8000 Hz was observed in 23.36 and 29.92% among the left and right ears of the mill workers, respectively. Years of experience and exposure to noise were strongly associated with a bilateral notch in 4000 Hz (the odds of having the notch increased with increased years of noise exposure). Self-reported HL under-represented the rates of actual HL, indicating perception of the HL was absent till audiometric evaluation revealed the loss. Conclusion Our study demonstrates exposure to high noise levels in this unorganized sector of rice and flour mill workers. Awareness education of NIHL, early detection, and the use of personal hearing protection devices can prevent HL from extending to speech frequencies.