The following factors have been implicated: 

missing acoustic reflex
previous sensorineural hearing loss
a bad general health state: bad cardiovascular function, insufficient intake of oxygen, a high platelet aggregation rate; and most importantly, a high viscosity of the blood
cigarette smoking
exposure to ototoxic chemicals (medication or environmental chemicals that can damage the ear), including certain solvents and heavy metals
type 2 diabetes

Diagnosis
Both NIHL caused by acoustic trauma and gradually-developed-NIHL can often be characterized by a specific pattern presented in audiological findings. NIHL is generally observed to decrease hearing sensitivity in the higher frequencies, also called an audiometric notch, especially at 4000 Hz, but sometimes at 3000 or 6000 Hz. The symptoms of NIHL are usually presented equally in both ears.This typical 4000 Hz notch is due to the transfer function of the ear. As does any object facing a sound, the ear acts as a passive filter, although the inner ear is not an absolute passive filter because the outer hair cells provide active mechanisms. A passive filter is a low pass: the high frequencies are more absorbed by the object because high frequencies impose a higher pace of compression-decompression to the object. The high frequency harmonics of a sound are more harmful to the inner-ear.However, not all audiological results from people with NIHL match this typical notch. Often a decline in hearing sensitivity will occur at frequencies other than at the typical 3000–6000 Hz range. Variations arise from differences in peoples ear canal resonance, the frequency of the harmful acoustic signal, and the length of exposure.
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