Abstract:
Noise induced hearing loss (NIHL) is one of the most common occupational disease in the United States with greater than 9 million workers exposed to noise levels that could induce hearing loss. Greater than 95% of U.S. Army aviators fly helicopters, placing them at risk of NIHL due to the generally open environment of the aircraft, with cockpit noise levels reaching 105 dB. Chronic exposure to noise, even at levels as low as 85 dB can result in a permanent loss of hearing due to sensory hair cell damage in the cochlea. Aviators, like most noise-exposed workers, are exposed to a broad frequency spectrum of noise; yet hearing loss secondary to noise exposure typically is first seen in the higher frequencies in the 4-6 kHz range. This is because the human auditory canal enhances sound waves in these frequencies. The pattern of NIHL differs from that as a result of the normal aging process, presbycusis, with the latter resulting in threshold shift that is typically greatest in the 8 kHz range.
The time frame for developing hearing loss can vary depending on the magnitude of the exposure, but typically will be manifested within 3-5 years of chronic noise exposure above 85 dB. The increase in hearing threshold at 4-6 kHz, often referred to as a “notch”, will deepen and widen spreading to lower and higher frequencies with continued exposure to excessive noise. Factors that affect NIHL are noise intensity, exposure time, and individual susceptibility.
Taken from the beginning of thesis.