Hearing loss can be classified as conductive or sensorineural. Conductive hearing loss occurs when sound is not normally conducted through the outer or middle ear or both. Since sound can be picked up by a normally sensitive inner ear even if the ear canal, ear drum, and ear ossicles are not working, conductive hearing loss is often only mild and is never worse than a moderate impairment. Hearing thresholds will not rise above 55-60 dB from outer or middle ear problems alone. Generally, with pure conductive hearing loss, the quality of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard.
A conductive loss can be caused by any of the following: Ear canal obstruction Middle ear abnormalities: Tympanic membrane Ossicles Inner ear abnormalities: Superior canal dehiscence syndrome

A sensorineural hearing loss is due to insensitivity of the inner ear, the cochlea, or to impairment of function in the auditory nervous system. It can be mild, moderate, severe, or profound, to the point of total deafness. This is classified as a disability under the ADA and if unable to work is eligible for disability payments.
The great majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve, the Vestibulocochlear nerve or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood.
Most sensory hearing loss is due to poor hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and intrinsic abnormalities, like deafness genes.
Sensorineural hearing loss that results from abnormalities of the central auditory system in the brain is called Central Hearing Impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from a central cause is unusual.


Causes

Long-term exposure to environmental noise

Populations of people living near airports or freeways are exposed to levels of noise typically in the 65 to 75 dB(A) range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing. The U.S. EPA and various states have set noise standards to protect people from these adverse health risks. The EPA has identified the level of 70 dB(A) for 24 hour exposure as the level necessary to protect the public from hearing loss and other disruptive effects from noise, such as sleep disturbance, stress-related problems, learning detriment, etc. (EPA, 1974).
Noise-Induced Hearing Loss (NIHL) typically is centered at 3000, 4000, or 6000 Hz. As noise damage progresses, damage starts affecting lower and higher frequencies. On an audiogram, the resulting configuration has a distinctive notch, sometimes referred to as a "noise notch." As aging and other effects contribute to higher frequency loss (6-8 kHz on an audiogram), this notch may be obscured and entirely disappear. Louder sounds cause damage in a shorter period of time. Estimation of a "safe" duration of exposure is possible using an exchange rate of 3 dB. As 3 dB represents a doubling of intensity of sound, duration of exposure must be cut in half to maintain the same energy dose. For example, the "safe" daily exposure amount at 85 dB A, known as an exposure action value, is 8 hours, while the "safe" exposure at 91 dB(A) is only 2 hours (National Institute for Occupational Safety and Health, 1998). Note that for some people, sound may be damaging at even lower levels than 85 dB A. Exposures to other ototoxins (such as pesticides, some medications including chemotherapy, solvents, etc.) can lead to greater susceptibility to noise damage, as well as causing their own damage. This is called a synergistic interaction.
Some American health and safety agencies (such as OSHA and MSHA), use an exchange rate of 5 dB. While this exchange rate is simpler to use, it drastically underestimates the damage caused by very loud noise. For example, at 115 dB, a 3 dB exchange rate would limit exposure to about half a minute; the 5 dB exchange rate allows 15 minutes.
While OSHA, MSHA, and FRA provide guidelines to limit noise exposure on the job, there is essentially no regulation or enforcement of sound output for recreational sources and environments, such as sports arenas, musical venues, bars, etc. This lack of regulation resulted from the defunding of ONAC, the EPA's Office of Noise Abatement and Control, in the early 1980s. ONAC was established in 1972 by the Noise Control Act and charged with working to assess and reduce environmental noise. Although the Office still exists, it has not been assigned new funding.
Most people in the United States are unaware of the presence of environmental sound at damaging levels, or of the level at which sound becomes harmful. Common sources of damaging noise levels include car stereos, children's toys, transportation, crowds, lawn and maintenance equipment, power tools, gun use, and even hair dryers. Noise damage is cumulative; all sources of damage must be considered to assess risk. If one is exposed to loud sound (including music) at high levels or for extended durations (85 dB A or greater), then hearing impairment will occur. Sound levels increase with proximity; as the source is brought closer to the ear, the sound level increases. This is why music is more likely to cause damage at the same output when listened to through headphones, as the headphones are in closer proximity to the ear drum than a loudspeaker. With the invention of in-ear headphones, these dangers are increased.

Genetic

Hearing loss can be inherited. Both dominant gene and recessive genes exist which can cause mild to profound impairment. If a family has a dominant gene for deafness it will persist across generations because it will manifest itself in the offspring even if it is inherited from only one parent. If a family had genetic hearing impairment caused by a recessive gene it will not always be apparent as it will have to be passed onto offspring from both parents. Dominant and recessive hearing impairment can be syndromic or nonsyndromic. Recent gene mapping has identified dozens of nonsyndromic dominant (DFNA#) and recessive (DFNB#) forms of deafness.
- The most common type of congenital hearing impairment in developed countries is DFNB1, also known as Connexin 26 deafness or GJB2-related deafness.
- The most common dominant syndromic forms of hearing impairment include Stickler syndrome and Waardenburg syndrome.
- The most common recessive syndromic forms of hearing impairment are Pendred syndrome, Large vestibular aqueduct syndrome and Usher syndrome.
- The congenital defect microtia can cause full or partial deafness depending upon the severity of the deformity and whether or not certain parts of the inner or middle ear are corrected.

Disease or illness

- Measles may result in auditory nerve damage
- Meningitis may damage the auditory nerve or the cochlea
- Autoimmune disease has only recently been recognized as a potential cause for cochlear damage. Although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs.Wegener's granulomatosis is one of the autoimmune conditions that may precipitate hearing loss.
- Mumps (Epidemic parotitis) may result in profound sensorineural hearing loss(90 Decibel|dB or more), unilateral (one ear) or bilateral (both ears).
- Presbycusis is a progressive hearing impairment accompanying age, typically affecting sensitivity to higher frequencies (above about 2 kHz).
- Adenoids that do not disappear by adolescence may continue to grow and may obstruct the Eustachian tube, causing conductive hearing impairment and nasal infections that can spread to the middle ear.
- AIDS and AIDS-related complex|ARC patients frequently experience auditory system anomalies.
- HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.
- Chlamydia may cause hearing loss in newborns to whom the disease has been passed at birth.
- Fetal alcohol syndrome is reported to cause hearing loss in up to 64% of infants born to alcoholism|alcoholic mothers, from the ototoxic effect on the developing fetus plus malnutrition during pregnancy from the excess ethanol|alcohol intake.
- Premature birth results in sensorineural hearing loss approximately 5% of the time.
- Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
- Otosclerosis is a hardening of the stapes (or stirrup) in the middle ear and causes conductive hearing loss.
- Superior canal dehiscence, a gap in the bone cover above the inner ear, can lead to low-frequency conductive hearing loss, autophony and vertigo

Medications

Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the aminoglycosides (main member gentamicin).
Various other medications may reversibly affect hearing. This includes some diuretics, aspirin and NSAIDs, and macrolide antibiotics.
Extremely heavy hydrocodone (Vicodin or Lorcet) abuse is known to cause hearing impairment. Commentators have speculated that radio talk show host Rush Limbaugh's hearing loss was at least in part caused by his admitted addiction to narcotic pain killers, in particular Vicodin and OxyContin.

Physical trauma

- There can be damage either to the ear itself or to the brain centers that process the aural information conveyed by the ears.
- People who sustain head injury are especially vulnerable to hearing loss or tinnitus, either temporary or permanent.
- Exposure to very loud noise (90 Decibel|dB or more, such as jet engines at close range) can cause progressive hearing loss. Exposure to a single event of extremely loud noise (such as explosions) can also cause temporary or permanent hearing loss. A typical source of acoustic trauma is an excessively loud music concert.


Quantification of hearing loss

The severity of hearing loss is measured by the degree of loudness, as measured in decibels, a sound must attain before being detected by an individual. Hearing loss may be ranked as mild, moderate, severe or profound. It is quite common for someone to have more than one degree of hearing loss (i.e. mild sloping to severe). The following list shows the rankings and their corresponding decibel ranges:

Mild: 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

The quietest sound one can hear at different frequencies is plotted on an audiogram to reflect one's ability to hear at different frequencies. The range of normal human hearing (from the softest audible sound to the loudest comfortable sound) is so great that the audiogram must be plotted using a logarithmic scale. This large normal range, and the different amounts of hearing loss at different frequencies, make it virtually impossible to accurately describe the amount of hearing loss in simple terms such as percentages or the rankings above.
Measuring hearing loss in terms of a percentage is debatable in terms of effectiveness, and has been compared to measuring weight in inches. Though in specific legal situations, where decibels of loss are converted via a recognized legal formula, one can infer a standardized "percentage of hearing loss" which is suitable for legal purposes only.
Another method for determining hearing loss, is the Hearing in Noise Test (HINT). HINT technology was developed by the House Ear Institute, and is intended to measure an ability to understand speech in quiet and noisy environments. Unlike pure-tone tests, where only one ear is tested at a time, HINT evaluates hearing using both ears simultaneously (binaural), as binaural hearing is essential for communication in noisy environments, and for sound localization.


Treatment

Some types of hearing impairment can be treated with custom hearing aids. In addition to hearing aids there exist cochlear implants of increasing complexity and effectiveness. These are useful in treating the mild to profound hearing impairment when the onset follows the acquisitions of language and in some cases in children whose hearing loss came before language was acquired. Recent research shows variations in efficacy but some promising studies show that if implanted at a very young age, some profoundly impaired children can acquire effective hearing and speech..

Future strategies include hair cell regeneration. A 2005 study, achieved successful regrowth of cochlea cells in guinea pigs. It is important to note however, that the regrowth of cochlear hair cells does not imply the restoration of hearing sensitivity as the sensory cells may or may not make connections with neurons that carry the signals from hair cells to the brain. A 2008 study has shown in mice, that gene therapy targeting Atoh1 can cause hair cell regrowth and attract neuronal processes. It is expected that a similar treatment will ameliorate hearing in humans.