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Profound hearing loss is loosely defined as the inability to hear sounds quieter than 95 decibels (dB). This definition is used by most health care professionals but may vary slightly in some parts of the world. Most legal systems use a formula by which degrees of hearing loss are defined as a percentage loss of total hearing ability. These standards may vary from one jurisdiction to another, but profound hearing loss is most commonly defined as a loss of at least 90%. People with profound hearing loss have difficulty understanding speech and are likely to rely on sign language, lip-reading or both.
In the health care field, hearing loss is categorized as mild, moderate, severe, and profound. Mild or moderate hearing loss can make following a conversation difficult, especially when other background noises are present. Someone with moderate hearing loss is likely to use a hearing aid. Severe hearing loss is characterized by an inability to hear sounds outside the 70 to 95 dB range. Many people with severe hearing loss will need a powerful hearing aid in addition to using sign language or lip-reading.
The most extreme degree of hearing loss that does not qualify as complete deafness is profound hearing loss. Someone with this level of hearing impairment has difficulty hearing all but the loudest sounds, and most everyday sounds are completely inaudible. The 95 dB threshold for someone with profound hearing loss is the equivalent of being unable to hear a noise quieter than that of a subway train at a distance of 200 feet (61 m).
Several factors can contribute to a loss of hearing, up to and including total deafness. Exposure to very loud noises can damage hearing. This damage can result from prolonged exposure to very loud noises, or from very brief exposure to extremely loud noises. Sounds such as those generated by gunshots, power construction equipment and jet engines can cause a profound hearing loss after an exposure of just a few minutes. Disease, congenital birth defects, advanced age, physical injury, nerve damage, certain medications and exposure to some chemicals, metals and solvents can all cause hearing loss.
Hearing loss may occur in one or both ears. It is common for someone with a hearing impairment to have differing degrees of hearing loss in each ear. Some types of hearing loss may be treatable, resulting in the restoration of some or all of a patient's hearing. Some people may gradually recover full hearing capability after a hearing loss due to exposure to loud noise, whereas some other causes of hearing loss can result in permanent hearing impairment.