A noise-induced hearing loss (NIHL) survey is conducted in a textile mill. Workers are found to have Threshold Shift (TTS) in the 4000 Hz frequency range. Which of the following statements about NIHL is CORRECT?
- A NIHL preferentially affects the low-frequency range (250-500 Hz) first
- B NIHL is typically asymmetric, affecting the right ear more
- C The 4000 Hz notch (C5 dip) is the earliest and most characteristic audiometric finding ✓
- D Permissible noise exposure limit in India is 90 dB(A) for 8 hours, and doubles for each 3 dB increase
Correct answer: C. The 4000 Hz notch (C5 dip) is the earliest and most characteristic audiometric finding
Explanation
The earliest audiometric sign of NIHL is a dip (notch) at 4000 Hz, known as the 'C5 dip' or '4 kHz notch', corresponding to damage to the outer hair cells in the basal turn of the cochlea. NIHL is bilateral and symmetric (unlike asymmetric hearing loss from acoustic trauma). The permissible noise exposure limit under Indian Factories Act is 90 dB(A) for 8 hours, with a 5 dB exchange rate (halving time of 5 dB).
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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