Hearing impairment or deafness refers to conditions in which individuals are fully or partially unable to detect or perceive at least some frequencies of sound which can typically be heard by members of their species.[1] Use of the term impaired implies that deafness presents an inherent disadvantage to an animal, a view that is rejected within the Deaf culture movement, where the terms Deaf and hard of hearing are preferred.
Hearing impairments are categorized by their type (conductive, sensorineural, or both), by their severity, and by the age of onset. Furthermore, a hearing impairment may exist in only one ear (unilateral) or in both ears (bilateral).
The severity of a hearing impairment is ranked according to the loudness (measured in decibels (dB)) a sound must be before being detected by an individual. Hearing impairment may be ranked as mild, moderate, severe or profound as defined below:

for adults: between 25 and 40 dB
for children: between 20 and 40 dB
Moderate: between 41 and 55 dB
Moderately severe: between 56 and 70 dB
Severe: between 71 and 90 dB
Profound: 90 dB or greater

Hearing sensitivity varies according to the frequency of sounds. To take this into account, hearing sensitivity can be measured for a range of frequencies and plotted on an audiogram.

The following are some of the major causes of hearing loss.
Long-term exposure to environmental noise
Disease or illness
Exposure to Ototoxic Chemicals
Physical trauma