Hearing loss, also known as hearing impairment, is a partial or total inability to hear.^[5] A deaf person has little to no hearing.^[2] Hearing loss may occur in one or both ears.^[2] In children, hearing problems can affect the ability to learn spoken language and in adults it can create difficulties with social interaction and at work.^[6] In some people, particularly older people, hearing loss can result in loneliness.^[2] Hearing loss can be temporary or permanent.

Hearing loss may be caused by a number of factors, including: genetics, ageing, exposure to noise, some infections, birth complications, trauma to the ear, and certain medications or toxins.^[2] A common condition that results in hearing loss is chronic ear infections.^[2] Certain infections during pregnancy, such as cytomegalovirus, syphilis and rubella, may also cause hearing loss in the child.^[2][7] Hearing loss is diagnosed when hearing testing finds that a person is unable to hear 25 decibels in at least one ear.^[2] Testing for poor hearing is recommended for all newborns.^[6] Hearing loss can be categorized as mild (25 to 40 dB), moderate (41 to 55 dB), moderate-severe (56 to 70 dB), severe (71 to 90 dB), or profound (greater than 90 dB).^[2] There are three main types of hearing loss: conductive hearing loss, sensorineural hearing loss, and mixed hearing loss.^[3]

About half of hearing loss globally is preventable through public health measures.^[2] Such practices include immunization, proper care around pregnancy, avoiding loud noise, and avoiding certain medications.^[2] The World Health Organization recommends that young people limit the use of personal audio players to an hour a day in an effort to limit exposure to noise.^[8] Early identification and support are particularly important in children.^[2] For many hearing aids, sign language, cochlear implants and subtitles are useful.^[2] Lip reading is another useful skill some develop.^[2] Access to hearing aids, however, is limited in many areas of the world.^[2]

As of 2013 hearing loss affects about 1.1 billion people to some degree.^[9] It causes disability in 5% (360 to 538 million) and moderate to severe disability in 124 million people.^[2][10][11] Of those with moderate to severe disability 108 million live in low and middle income countries.^[10] Of those with hearing loss, it began during childhood for 65 million.^[12] Those who use sign language and are members of Deaf culture see themselves as having a difference rather than an illness.^[13] Most members of Deaf culture oppose attempts to cure deafness^[14][15][16] and some within this community view cochlear implants with concern as they have the potential to eliminate their culture.^[17] The term hearing impairment is often viewed negatively as it emphasises what people cannot do.^[13]

Use of the terms "hearing impaired", "deaf-mute", or "deaf and dumb" to describe deaf and hard of hearing people is discouraged by advocacy organizations as they are offensive to many deaf and hard of hearing people.^[13]

Human hearing extends in frequency from 20–20,000 Hz, and in intensity from 0 dB to 120 dB HL or more.

0 dB does not represent absence of sound, but rather the softest sound an average unimpaired human ear can hear; some people can hear down to −5 or even −10 dB.

Sound is generally uncomfortably loud above 90 dB and 115 dB represents the threshold of pain. The ear does not hear all frequencies equally well; hearing sensitivity peaks around 3000 Hz. There are many qualities of human hearing besides frequency range and intensity that cannot easily be measured quantitatively. But for many practical purposes, normal hearing is defined by a frequency versus intensity graph, or audiogram, charting sensitivity thresholds of hearing at defined frequencies. Because of the cumulative impact of age and exposure to noise and other acoustic insults, 'typical' hearing may not be normal.^[20][21]

Hearing loss is sensory, but may have accompanying symptoms:

There may also be accompanying secondary symptoms:

Hearing loss is associated with Alzheimer's disease and dementia. The risk increases with the hearing loss degree. There are several hypotheses including cognitive resources being redistributed to hearing and social isolation from hearing loss having a negative effect.^[22] According to preliminary data, hearing aid using can slow down the decline in cognitive functions.^[23]

Hearing loss has multiple causes, including ageing, genetics, perinatal problems and acquired causes like noise and disease.

For some kinds of hearing loss the cause may be classified as of unknown cause.

There is a progressive loss of ability to hear high frequencies with aging known as presbycusis. For men, this can start as early as 25 and women at 30. Although genetically variable it is a normal concomitant of ageing and is distinct from hearing losses caused by noise exposure, toxins or disease agents.^[24] Common conditions that can increase the risk of hearing loss in elderly people are high blood pressure, diabetes or the use of certain medications harmful to the ear.^[25][26] While everyone loses hearing with age, the amount and type of hearing loss is variable.^[27]

Noise exposure is the cause of approximately half of all cases of hearing loss, causing some degree of problems in 5% of the population globally.^[28] The National Institute for Occupational Safety and Health (NIOSH) recognizes that the majority of hearing loss is not due to age, but due to noise exposure. By correcting for age in assessing hearing, one tends to overestimate the hearing loss due to noise for some and underestimate it for others.^[29]

Hearing loss due to noise may be temporary, called a 'temporary threshold shift', a reduced sensitivity to sound over a wide frequency range resulting from exposure to a brief but very loud noise like a gunshot, firecracker, jet engine, jackhammer, etc. or to exposure to loud sound over a few hours such as during a pop concert or nightclub session.^[30] Recovery of hearing is usually within 24 hours, but may take up to a week.^[31] Both constant exposure to loud sounds (85 dB(A) or above) and one-time exposure to extremely loud sounds (120 dB(A) or above) may cause permanent hearing loss.^[32]

Noise-induced hearing loss (NIHL) typically manifests as elevated hearing thresholds (i.e. less sensitivity or muting) between 3000 and 6000 Hz, centred at 4000 Hz. As noise damage progresses, damage spreads to affect lower and higher frequencies. On an audiogram, the resulting configuration has a distinctive notch, called a 'noise' notch. As ageing and other effects contribute to higher frequency loss (6–8 kHz on an audiogram), this notch may be obscured and entirely disappear.

Various governmental, industry and standards organizations set noise standards.^[33]

The U.S. Environmental Protection Agency has identified the level of 70 dB(A) (40% louder to twice as loud as normal conversation; typical level of TV, radio, stereo; city street noise) 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.^[34] Noise levels are typically in the 65 to 75 dB (A) range for those living near airports or freeways and may result in hearing damage if sufficient time is spent outdoors.^[35]

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 the intensity of sound, duration of exposure must be cut in half to maintain the same energy dose. For workplace noise regulation, 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.^[36] Different standards use exposure action values between 80dBA and 90dBA.

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 agents, solvents, etc.) can lead to greater susceptibility to noise damage, as well as causing its own damage.

This is called a synergistic interaction. Since noise damage is cumulative over long periods of time, persons who are exposed to non-workplace noise, like recreational activities or environmental noise, may have compounding damage from all sources.

Some national and international organizations and agencies use an exchange rate of 4 dB or 5 dB.^[37] While these exchange rates may indicate a wider zone of comfort or safety, they can significantly underestimate the damage caused by loud noise.

For example, at 100 dB (nightclub music level), a 3 dB exchange rate would limit exposure to 15 minutes; the 5 dB exchange rate allows an hour.

Many people 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, motor vehicles, crowds, lawn and maintenance equipment, power tools, gun use, musical instruments, 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 loss will occur.

Sound intensity (sound energy, or propensity to cause damage to the ears) increases dramatically with proximity according to an inverse square law: halving the distance to the sound quadruples the sound intensity.

In the USA, 12.5% of children aged 6–19 years have permanent hearing damage from excessive noise exposure.^[38] The World Health Organization estimates that half of those between 12 and 35 are at risk from using personal audio devices that are too loud.^[8]

Hearing loss due to noise has been described as primarily a condition of modern society.^[39] In preindustrial times, humans had far less exposure to loud sounds.

Studies of primitive peoples indicate that much of what has been attributed to age-related hearing loss may be long term cumulative damage from all sources, especially noise.

People living in preindustrial societies have considerably less hearing loss than similar populations living in modern society.

Among primitive people who have migrated into modern society, hearing loss is proportional to the number of years spent in modern society.^[40][41][42] Military service in World War II, the Korean War, and the Vietnam War, has likely also caused hearing loss in large numbers of men from those generations, though proving that hearing loss was a direct result of military service is problematic without entry and exit audiograms.^[43]

Hearing loss in adolescents may be caused by loud noise from toys, music by headphones, and concerts or events.^[44] In 2017, the Centers for Disease Control and Prevention brought their researchers together with experts from the World Health Organization and academia to examine the risk of hearing loss from excessive noise exposure in and outside the workplace in different age groups, as well as actions being taken to reduce the burden of the condition. A summary report was published in 2018.^[45]

Hearing loss can be inherited.

Around 75–80% of all these cases are inherited by recessive genes, 20–25% are inherited by dominant genes, 1–2% are inherited by X-linked patterns, and fewer than 1% are inherited by mitochondrial inheritance.^[46]

When looking at the genetics of deafness, there are 2 different forms, syndromic and nonsyndromic. Syndromic deafness occurs when there are other signs or medical problems aside from deafness in an individual. This accounts for around 30% of deaf individuals who are deaf from a genetic standpoint.^[46] Nonsyndromic deafness occurs when there are no other signs or medical problems associated with an individual other than deafness. From a genetic standpoint, this accounts for the other 70% of cases, and represents the majority of hereditary hearing loss.^[46] Syndromic cases occur with disorders such as Usher syndrome, Stickler syndrome, Waardenburg syndrome, Alport's syndrome, and neurofibromatosis type 2. These are diseases that have deafness as one of the symptoms or as a common feature associated with it. Many of the genetic mutations giving rise to syndromic deafness have been identified. In nonsyndromic cases, where deafness is the only finding, it is more difficult to identify the genetic mutation although some have been discovered.

Some medications may reversibly affect hearing.

These medications are considered ototoxic. This includes loop diuretics such as furosemide and bumetanide, non-steroidal anti-inflammatory drugs (NSAIDs) both over-the-counter (aspirin, ibuprofen, naproxen) as well as prescription (celecoxib, diclofenac, etc.), paracetamol, quinine, and macrolide antibiotics. The link between NSAIDs and hearing loss tends to be greater in women, especially those who take ibuprofen six or more times a week.^[55] Others may cause permanent hearing loss.^[56] The most important group is the aminoglycosides (main member gentamicin) and platinum based chemotherapeutics such as cisplatin and carboplatin.^[57][58]

In 2007, the U.S. Food and Drug Administration (FDA) warned about possible sudden hearing loss from PDE5 inhibitors, which are used for erectile dysfunction.^[59]

Audiologic monitoring for ototoxicity allows for the (1) early detection of changes to hearing status presumably attributed to a drug/treatment regime so that changes in the drug regimen may be considered, and (2) audiologic intervention when handicapping hearing impairment has occurred.^[60]

Co-administration of anti-oxidants and ototoxic medications may limit the extent of the ototoxic damage^[61][62]

In addition to medications, hearing loss can also result from specific chemicals in the environment: metals, such as lead; solvents, such as toluene (found in crude oil, gasoline^[63] and automobile exhaust,^[63] for example); and asphyxiants.^[64] Combined with noise, these ototoxic chemicals have an additive effect on a person’s hearing loss.^[64]

Hearing loss due to chemicals starts in the high frequency range and is irreversible.

It damages the cochlea with lesions and degrades central portions of the auditory system.^[64] For some ototoxic chemical exposures, particularly styrene,^[65] the risk of hearing loss can be higher than being exposed to noise alone. The effects is greatest when the combined exposure include impulse noise.^[66][67]

A 2018 informational bulletin by the US Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) introduces the issue, provides examples of ototoxic chemicals, lists the industries and occupations at risk and provides prevention information.^[69]

There can be damage either to the ear, whether the external or middle ear, to the cochlea, or to the brain centers that process the aural information conveyed by the ears.

Damage to the middle ear may include fracture and discontinuity of the ossicular chain.

Damage to the inner ear (cochlea) may be caused by temporal bone fracture. People who sustain head injury are especially vulnerable to hearing loss or tinnitus, either temporary or permanent.^[70][71]

Sound waves reach the outer ear and are conducted down the ear canal to the eardrum, causing it to vibrate. The vibrations are transferred by the 3 tiny ear bones of the middle ear to the fluid in the inner ear. The fluid moves hair cells (stereocilia), and their movement generates nerve impulses which are then taken to the brain by the cochlear nerve.^[72][73] The auditory nerve takes the impulses to the brainstem, which sends the impulses to the midbrain. Finally, the signal goes to the auditory cortex of the temporal lobe to be interpreted as sound.^[74]

Hearing loss is most commonly caused by long-term exposure to loud noises, from recreation or from work, that damage the hair cells, which do not grow back on their own.^[75][76]

Older people may lose their hearing from long exposure to noise, changes in the inner ear, changes in the middle ear, or from changes along the nerves from the ear to the brain.^[77]

Identification of a hearing loss is usually conducted by a general practitioner medical doctor, otolaryngologist, certified and licensed audiologist, school or industrial audiometrist, or other audiometric technician. Diagnosis of the cause of a hearing loss is carried out by a specialist physician (audiovestibular physician) or otorhinolaryngologist.

A case history (usually a written form, with questionnaire) can provide valuable information about the context of the hearing loss, and indicate what kind of diagnostic procedures to employ.

Case history will include such items as:

In case of infection or inflammation, blood or other body fluids may be submitted for laboratory analysis.

Hearing loss is generally measured by playing generated or recorded sounds, and determining whether the person can hear them.

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.

Other method for quantifying hearing loss is a hearing test using a mobile application or hearing aid application, which includes a hearing test.^[78][79] Hearing diagnosis using mobile application is similar to the audiometry procedure. As a result of hearing test, hearing thresholds at different frequencies (audiogram) are determined. Despite the errors in the measurements, application can help to diagnose hearing loss.^[78] Audiogram, obtained using mobile application, can be used to adjust hearing aid application.^[79]

Another method for quantifying hearing loss is a speech-in-noise test.

As the name implies, a speech-in-noise test gives an indication of how well one can understand speech in a noisy environment.

A person with a hearing loss will often be less able to understand speech, especially in noisy conditions.

This is especially true for people who have a sensorineural loss – which is by far the most common type of hearing loss.

As such, speech-in-noise tests can provide valuable information about a person's hearing ability, and can be used to detect the presence of a sensorineural hearing loss.

A recently developed digit-triple speech-in-noise test may be a more efficient screening test.^[80]

Otoacoustic emissions test is an objective hearing test that may be administered to toddlers and children too young to cooperate in a conventional hearing test.

The test is also useful in older children and adults and is an important measure in diagnosing auditory neuropathy described above.

Auditory brainstem response testing is an electrophysiological test used to test for hearing deficits caused by pathology within the ear, the cochlear nerve and also within the brainstem.

This test can be used to identify delay in the conduction of neural impulses due to tumours or inflammation but can also be an objective test of hearing thresholds.

Other electrophysiological tests, such as cortical evoked responses, can look at the hearing pathway up to the level of the auditory cortex.

MRI and CT scans can be useful to identify the pathology of many causes of hearing loss.

They are only needed in selected cases.

Hearing loss is categorized by type, severity, and configuration.

Furthermore, a hearing loss may exist in only one ear (unilateral) or in both ears (bilateral).

Hearing loss can be temporary or permanent, sudden or progressive.

The severity of a hearing loss is ranked according to ranges of nominal thresholds in which a sound must be so it can be detected by an individual.

It is measured in decibels of hearing loss, or dB HL. The measurement of hearing loss in an individual is conducted over several frequencies, mostly 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The hearing loss of the individual is the average of the hearing loss values over the different frequencies. Hearing loss can be ranked differently according to different organisations; and so, in different countries different systems are in use.

Hearing loss may be ranked as slight, mild, moderate, moderately severe, severe or profound as defined below:

The 'Audiometric Classifications of Hearing Impairment' according to the International Bureau Audiophonology (BIAP) in Belgium is as follows:^[81]

Hearing loss may affect one or both ears.

If both ears are affected, then one ear may be more affected than the other.

Thus it is possible, for example, to have normal hearing in one ear and none at all in the other, or to have mild hearing loss in one ear and moderate hearing loss in the other.

For certain legal purposes such as insurance claims, hearing loss is described in terms of percentages.

Given that hearing loss can vary by frequency and that audiograms are plotted with a logarithmic scale, the idea of a percentage of hearing loss is somewhat arbitrary, but where decibels of loss are converted via a legally recognized formula, it is possible to calculate a standardized "percentage of hearing loss", which is suitable for legal purposes only.

There are three main types of hearing loss, conductive hearing loss, sensorineural hearing loss. Combinations of conductive and sensorineural hearing losses are called a mixed hearing loss.^[12] An additional problem which is increasingly recognised is auditory processing disorder which is not a hearing loss as such but a difficulty perceiving sound.

Conductive hearing loss is present when the sound is not reaching the inner ear, the cochlea. This can be due to external ear canal malformation, dysfunction of the eardrum or malfunction of the bones of the middle ear. The eardrum may show defects from small to total resulting in hearing loss of different degree. Scar tissue after ear infections may also make the eardrum dysfunction as well as when it is retracted and adherent to the medial part of the middle ear.

Dysfunction of the three small bones of the middle ear – malleus, incus, and stapes – may cause conductive hearing loss. The mobility of the ossicles may be impaired for different reasons including a boney disorder of the ossicles called otosclerosis and disruption of the ossicular chain due to trauma, infection or ankylosis may also cause hearing loss.

Sensorineural hearing loss is one caused by dysfunction of the inner ear, the cochlea or the nerve that transmits the impulses from the cochlea to the hearing centre in the brain.

The most common reason for sensorineural hearing loss is damage to the hair cells in the cochlea. Depending on the definition it could be estimated that more than 50% of the population over the age of 70 has impaired hearing.^[82]

Damage to the brain can lead to a central deafness.

The peripheral ear and the auditory nerve may function well but the central connections are damaged by tumour, trauma or other disease and the patient is unable to process speech information.

Mixed hearing loss is a combination of conductive and sensorineural hearing loss.

Chronic ear infection (a fairly common diagnosis) can cause a defective ear drum or middle-ear ossicle damages, or both. In addition to the conductive loss, a sensory component may be present.

This is not an actual hearing loss but gives rise to significant difficulties in hearing.

One kind of auditory processing disorder is King-Kopetzky syndrome, which is characterized by an inability to process out background noise in noisy environments despite normal performance on traditional hearing tests. An auditory processing disorders is sometimes linked to language disorders in persons of all ages.

The shape of an audiogram shows the relative configuration of the hearing loss, such as a Carhart notch for otosclerosis, 'noise' notch for noise-induced damage, high frequency rolloff for presbycusis, or a flat audiogram for conductive hearing loss. In conjunction with speech audiometry, it may indicate central auditory processing disorder, or the presence of a schwannoma or other tumor. There are four general configurations of hearing loss:

1. Flat: thresholds essentially equal across test frequencies.

2. Sloping: lower (better) thresholds in low-frequency regions and higher (poorer) thresholds in high-frequency regions.

3. Rising: higher (poorer) thresholds in low-frequency regions and lower (better) thresholds in higher-frequency regions.

4. Trough-shaped ("cookie-bite" or "U" shaped): greatest hearing loss in the mid-frequency range, with lower (better) thresholds in low- and high-frequency regions.

People with unilateral hearing loss or single-sided deafness (SSD) have difficulty in:

In quiet conditions, speech discrimination is approximately the same for normal hearing and those with unilateral deafness; however, in noisy environments speech discrimination varies individually and ranges from mild to severe.

One reason for the hearing problems these patients often experience is due to the head shadow effect. Newborn children with no hearing on one side but one normal ear could still have problems.^[83] Speech development could be delayed and difficulties to concentrate in school are common. More children with unilateral hearing loss have to repeat classes than their peers. Taking part in social activities could be a problem. Early aiding is therefore of utmost importance.^[84][85]

It is estimated that half of cases of hearing loss are preventable.^[86] About 60% of hearing loss in children under the age of 15 can be avoided.^[2] A number of preventative strategies are effective including: immunization against rubella to prevent congenital rubella syndrome, immunization against H. influenza and S. pneumoniae to reduce cases of meningitis, and avoiding or protecting against excessive noise exposure.^[12] The World Health Organization also recommends immunization against measles, mumps, and meningitis, efforts to prevent premature birth, and avoidance of certain medication as prevention.^[87]

Noise exposure is the most significant risk factor for noise-induced hearing loss that can be prevented.

Different programs exist for specific populations such as school-age children, adolescents and workers.^[88] Education regarding noise exposure increases the use of hearing protectors.^[89] The use of antioxidants is being studied for the prevention of noise-induced hearing loss, particularly for scenarios in which noise exposure cannot be reduced, such as during military operations.^[90]

Noise is widely recognized as an occupational hazard. In the United States, the National Institute for Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Administration (OSHA) work together to provide standards and enforcement on workplace noise levels.^[91][92] The hierarchy of hazard controls demonstrates the different levels of controls to reduce or eliminate exposure to noise and prevent hearing loss, including engineering controls and personal protective equipment (PPE).^[93] Other programs and initiative have been created to prevent hearing loss in the workplace. For example, the Safe-in-Sound Award was created to recognize organizations that can demonstrate results of successful noise control and other interventions.^[94] Additionally, the Buy Quiet program was created to encourage employers to purchase quieter machinery and tools.^[95] By purchasing less noisy power tools like those found on the NIOSH Power Tools Database and limiting exposure to ototoxic chemicals, great strides can be made in preventing hearing loss.^[96]

Companies can also provide personal hearing protector devices tailored to both the worker and type of employment.

Some hearing protectors universally block out all noise, and some allow for certain noises to be heard.

Workers are more likely to wear hearing protector devices when they are properly fitted.^[97]

Often interventions to prevent noise-induced hearing loss have many components.

A 2017 Cochrane review found that stricter legislation might reduce noise levels.^[98] Providing workers with information on their noise exposure levels was not shown to decrease exposure to noise.

Ear protection, if used correctly, can reduce noise to safer levels, but often, providing them is not sufficient to prevent hearing loss.

Engineering noise out and other solutions such as proper maintenance of equipment can lead to noise reduction, but further field studies on resulting noise exposures following such interventions are needed.

Other possible solutions include improved enforcement of existing legislation and better implementation of well-designed prevention programmes, which have not yet been proven conclusively to be effective.

The conclusion of the Cochrane Review was that further research could modify what is now regarding the effectiveness of the evaluated interventions.^[98]

The United States Preventive Services Task Force recommends screening for all newborns.^[6]

The American Academy of Pediatrics advises that children should have their hearing tested several times throughout their schooling:^[38]

While the American College of Physicians indicated that there is not enough evidence to determine the utility of screening in adults over 50 years old who do not have any symptoms,^[99] the American Language, Speech Pathology and Hearing Association recommends that adults should be screened at least every decade through age 50 and at 3-year intervals thereafter, to minimize the detrimental effects of the untreated condition on quality of life.^[100] For the same reason, the US Office of Disease Prevention and Health Promotion included as one of Healthy People 2020 objectives: to increase the proportion of persons who have had a hearing examination.^[101]

Management depend on the specific cause if known as well as the extent, type and configuration of the hearing loss.

Sudden hearing loss due to and underlying nerve problem may be treated with corticosteroids.^[102]

Most hearing loss, that resulting from age and noise, is progressive and irreversible, and there are currently no approved or recommended treatments.

A few specific kinds of hearing loss are amenable to surgical treatment.

In other cases, treatment is addressed to underlying pathologies, but any hearing loss incurred may be permanent.

Some management options include hearing aids, cochlear implants, assistive technology, and closed captioning. This choice depends on level of hearing loss, type of hearing loss, and personal preference. Hearing aid applications are one of the options for hearing loss management.^[79]

Globally, hearing loss affects about 10% of the population to some degree.^[28] It caused moderate to severe disability in 124.2 million people as of 2004 (107.9 million of whom are in low and middle income countries).^[10] Of these 65 million acquired the condition during childhood.^[12] At birth ~3 per 1000 in developed countries and more than 6 per 1000 in developing countries have hearing problems.^[12]

Hearing loss increases with age.

In those between 20 and 35 rates of hearing loss are 3% while in those 44 to 55 it is 11% and in those 65 to 85 it is 43%.^[6]

A 2017 report by the World Health Organization estimated the costs of unaddressed hearing loss and the cost-effectiveness of interventions, for the health-care sector, for the education sector and as broad societal costs.^[103] Globally, the annual cost of unaddressed hearing loss was estimated to be in the range of $750–790 billion international dollars.

The International Organization for Standardization (ISO) developed the ISO 1999 standards for the estimation of hearing thresholds and noise-induced hearing impairment.^[104] They used data from two noise and hearing study databases, one presented by Burns and Robinson (Hearing and Noise in Industry, Her Majesty's Stationery Office, London, 1970) and by Passchier-Vermeer (1968).^[105] As race are some of the factors that can affect the expected distribution of pure-tone hearing thresholds several other national or regional datasets exist, from Sweden,^[106] Norway,^[107] South Korea,^[108] the United States^[109] and Spain.^[107]

In the United States hearing is one of the health outcomes measure by the National Health and Nutrition Examination Survey (NHANES), a survey research program conducted by the National Center for Health Statistics. It examines health and nutritional status of adults and children in the United States. Data from the United States in 2011-2012 found that rates of hearing loss has declined among adults aged 20 to 69 years, when compared with the results from an earlier time period (1999-2004). It also found that adult hearing loss is associated with increasing age, sex, race/ethnicity, educational level, and noise exposure.^[111] Nearly one in four adults had audiometric results suggesting noise-induced hearing loss. Almost one in four adults who reported excellent or good hearing had a similar pattern (5.5% on both sides and 18% on one side). Among people who reported exposure to loud noise at work, almost one third had such changes.^[112]

Abbé Charles-Michel de l'Épée opened the first school for the deaf in Paris at the deaf school. The American Thomas Gallaudet witnessed a demonstration of deaf teaching skills from Épée's successor Abbé Sicard and two of the school's deaf faculty members, Laurent Clerc and Jean Massieu; accompanied by Clerc, he returned to the United States, where in 1817 they founded American School for the Deaf in Hartford, Connecticut. American Sign Language (ASL) started to evolve from primarily French Sign Language (LSF), and other outside influences.^[113]

Post-lingual deafness is hearing loss that is sustained after the acquisition of language, which can occur due to disease, trauma, or as a side-effect of a medicine. Typically, hearing loss is gradual and often detected by family and friends of affected individuals long before the patients themselves will acknowledge the disability.^[114] Post-lingual deafness is far more common than pre-lingual deafness. Those who lose their hearing later in life, such as in late adolescence or adulthood, face their own challenges, living with the adaptations that allow them to live independently.

Prelingual deafness is profound hearing loss that is sustained before the acquisition of language, which can occur due to a congenital condition or through hearing loss before birth or in early infancy. Prelingual deafness impairs an individual's ability to acquire a spoken language in children, but deaf children can acquire spoken language through support from cochlear implants (sometimes combined with hearing aids).^[115][116] Non-signing (hearing) parents of deaf babies (90-95% of cases) usually go with oral approach without the support of sign language as the these families lack previous experience with sign language and cannot competently provide it to their children. Unfortunately, this may in some rare cases (late implantation or not sufficient benefit from cochlear implants) bring the risk of language deprivation for the deaf baby^[117] because the deaf baby wouldn't have a sign language if the child is unable to acquire spoken language successfully. The 5-10% of cases of deaf babies born into signing families have the potential of age-appropriate development of language due to early exposure to sign language by sign-competent parents, thus they have the potential to meet language milestones, but in sign language in lieu of spoken language.^[118]

There has been considerable controversy within the culturally deaf community over cochlear implants. For the most part, there is little objection to those who lost their hearing later in life, or culturally deaf adults choosing to be fitted with a cochlear implant.^[17]

Many in the deaf community strongly object to a deaf child being fitted with a cochlear implant (often on the advice of an audiologist); new parents may not have sufficient information on raising deaf children and placed in an oral-only program that emphasizes the ability to speak and listen over other forms of communication such as sign language or total communication. Many deaf people view cochlear implants and other hearing devices as confusing to one's identity. They feel a deaf person will never be a hearing person and therefore would be trying to fit into a way of living that is not their own. Other concerns include loss of deaf culture and identity and limitations on hearing restoration.^[17]

Jack Gannon, a professor at Gallaudet University, said this about Deaf culture: "Deaf culture is a set of learned behaviors and perceptions that shape the values and norms of deaf people based on their shared or common experiences." Some doctors believe that being deaf makes a person more social. Bill Vicar, from ASL University, shared his experiences as a deaf person, "[deaf people] tend to congregate around the kitchen table rather than the living room sofa... our good-byes take nearly forever, and our hellos often consist of serious hugs. When two of us meet for the first time we tend to exchange detailed biographies."^[119] Deaf culture is not about contemplating what deaf people cannot do and how to fix their problems, an approach known as the "pathological view of the deaf."^[120] Instead deaf people celebrate what they can do. There is a strong sense of unity between deaf people as they share their experiences of suffering through a similar struggle. This celebration creates a unity between even deaf strangers. Bill Vicars expresses the power of this bond when stating, "if given the chance to become hearing most [deaf people] would choose to remain deaf."^[121]

The United States-based National Association of the Deaf has a statement on its website regarding cochlear implants.^[122] The NAD asserts that the choice to implant is up to the individual (or the parents), yet strongly advocates a fully informed decision in all aspects of a cochlear implant. Much of the negative reaction to cochlear implants stems from the medical viewpoint that deafness is a condition that needs to be "cured," while the Deaf community instead regards deafness a defining cultural characteristic.

Many other assistive devices are more acceptable to the Deaf community, including but not limited to, hearing aids, closed captioning, email and the Internet, text telephones, and video relay services.

Sign languages convey meaning through manual communication and body language instead of acoustically conveyed sound patterns.

This involves the simultaneous combination of hand shapes, orientation and movement of the hands, arms or body, and facial expressions to express a speaker's thoughts.

"Sign languages are based on the idea that vision is the most useful tool a deaf person has to communicate and receive information".^[123]

Those who are deaf (by either state or federal standards) have access to a free and appropriate public education.

If a child does qualify as being deaf or hard of hearing and receives an individualized education plan, the IEP team must consider, "the child's language and communication needs. The IEP must include opportunities for direct communication with peers and professionals. It must also include the student’s academic level, and finally must include the students full range of needs"^[124][125]

In part, the Department of Education defines deafness as "... a hearing impairment that is so severe that the child is impaired in processing linguistic information through hearing, with or without amplification...."

Hearing impairment is defined as "... an impairment in hearing, whether permanent or fluctuating, that adversely affects a child's educational performance but that is not included under the definition of deafness...."^[126]

In a residential school where all the children use the same communication system (whether it is a school using ASL, Total Communication or Oralism), students will be able to interact normally with other students, without having to worry about being criticized.

An argument supporting inclusion, on the other hand, exposes the student to people who are not just like them, preparing them for adult life.

Through interacting, children with hearing disabilities can expose themselves to other cultures which in the future may be beneficial for them when it comes to finding jobs and living on their own in a society where their disability may put them in the minority.

These are some reasons why a person may or may not want to put their child in an inclusion classroom.^[125]

The communication limitations between people who are deaf and their hearing family members can often cause difficulties in family relationships, and affect the strength of relationships among individual family members.

It was found that most people who are deaf have hearing parents, which means that the channel that the child and parents communicate through can be very different, often affecting their relationship in a negative way.

If a parent communicates best verbally, and their child communicates best using sign language, this could result in ineffective communication between parents and children.

Ineffective communication can potentially lead to fights caused by misunderstanding, less willingness to talk about life events and issues, and an overall weaker relationship.

Even if individuals in the family made an effort to learn deaf communication techniques such as sign language, a deaf family member often will feel excluded from casual banter; such as the exchange of daily events and news at the dinner table.

It is often difficult for people who are deaf to follow these conversations due to the fast-paced and overlapping nature of these exchanges.

This can cause a deaf individual to become frustrated and take part in less family conversations.

This can potentially result in weaker relationships between the hearing individual and their immediate family members.

This communication barrier can have a particularly negative effect on relationships with extended family members as well.

Communication between a deaf individual and their extended family members can be very difficult due to the gap in verbal and non-verbal communication.

This can cause the individuals to feel frustrated and unwilling to put effort into communicating effectively.

The lack of effort put into communicating can result in anger, miscommunication, and unwillingness to build a strong relationship.^[127]

People who have hearing loss can often experience many difficulties as a result of communication barriers among them and other hearing individuals in the community.

Some major areas that can be impacted by this are involvement in extracurricular activities and social relationships.

For young people, extracurricular activities are vehicles for physical, emotional, social, and intellectual development.

However, it is often the case that communication barriers between people who are deaf and their hearing peers and coaches/club advisors limit them from getting involved.

These communication barriers make it difficult for someone with a hearing loss to understand directions, take advice, collaborate, and form bonding relationships with other team or club members.

As a result, extracurricular activities such as sports teams, clubs, and volunteering are often not as enjoyable and beneficial for individuals who have hearing loss, and they may engage in them less often.

A lack of community involvement through extracurricular activities may also limit the individual’s social network.

In general, it can be difficult for someone who is deaf to develop and maintain friendships with their hearing peers due to the communication gap that they experience.

They can often miss the jokes, informal banter, and "messing around" that is associated with the formation of many friendships among young people.

Conversations between people who are deaf and their hearing peers can often be limited and short due to their differences in communication methods and lack of knowledge on how to overcome these differences.

Deaf individuals can often experience rejection by hearing peers who are not willing to make an effort to find their way around communication difficulties.

Patience and motivation to overcome such communication barriers is required by both the deaf or hard of hearing and hearing individuals in order to establish and maintain good friendships.^[127]

Many people tend to forget about the difficulties that deaf children encounter, as they view the deaf child differently from a deaf adult.

Deaf children grow up being unable to fully communicate with their parents, siblings and other family members.

Examples include being unable to tell their family what they have learned, what they did, asking for help, or even simply being unable to interact in daily conversation.

Deaf children have to learn sign language and to read lips at a young age, however they cannot communicate with others using it unless the others are educated in sign language as well.

Children who are deaf or hard of hearing are faced with many complications while growing up, for example some children have to wear hearing aids and others require assistance from sign language (ASL) interpreters.

The interpreters help them to communicate with other individuals until they develop the skills they need to efficiently communicate on their own.

Although growing up for deaf children may entitle more difficulties than for other children, there are many support groups that allow deaf children to interact with other children.

This is where they develop friendships.

There are also classes for young children to learn sign language in an environment that has other children in their same situation and around their same age.

These groups and classes can be very beneficial in providing the child with the proper knowledge and not to mention the societal interactions that they need in order to live a healthy, young, playful and carefree life that any child deserves.

There are three typical adjustment patterns adopted by adults with hearing loss.

The first one is to remain withdrawn into your own self.

This provides a sense of safety and familiarity which can be a comforting way to lead your life.

The second is to act "as if" one does not even have hearing loss.

A positive attitude will help people to live a life with no barriers and thus, engage in optimal interaction.

The final and third pattern is for the person to accept their hearing loss as a part of them without undervaluing oneself.

This means understanding that one is forced to live life with this disability, however it is not the only thing that constitutes life’s meaning.

Furthermore, many feel as if their inability to hear others during conversation is their fault.

It's important that these individuals learn how to become more assertive individuals who do not lack fear when it comes to asking someone to repeat something or to speak a little louder.

Although there is much fatigue and frustration that is produced from one’s inability to hear, it is important to learn from personal experiences in order to improve on one’s communication skills.

In essence, these patterns will help adults with hearing loss deal with the communication barriers that are present.^[128]

In most instances, people who are deaf find themselves working with hearing colleagues, where they can often be cut off from the communication going on around them.

Interpreters can be provided for meetings and workshops, however are seldom provided for everyday work interactions.

Communication of important information needed for jobs typically comes in the form of written or verbal summaries, which do not convey subtle meanings such as tone of voice, side conversations during group discussions, and body language.

This can result in confusion and misunderstanding for the worker who is deaf, therefore making it harder to do their job effectively.

Additionally, deaf workers can be unintentionally left out of professional networks, informal gatherings, and casual conversations among their collogues.

Information about informal rules and organizational culture in the workplace is often communicated though these types of interactions, which puts the worker who is deaf at a professional and personal disadvantage.

This could sever their job performance due to lack of access to information and therefore, reduce their opportunity to form relationships with their co-workers. Additionally, these communication barriers can all affect a deaf person’s career development. Since being able to effectively communicate with one's co-workers and other people relevant to one's job is essential to managerial positions, people with hearing loss can often be denied such opportunities.^[127]

To avoid these situations in the workplace, individuals can take full-time or part-time sign language courses.

In this way, they can become better able to communicate with the deaf and hard of hearing.

Such courses teach the American Sign Language (ASL) language as most North Americans use this particular language to communicate.

It is a visual language made up of specific gestures (signs), hand shapes, and facial expressions that contain their own unique grammatical rules and sentence structures^[129] By completing sign language courses, it ensures that deaf individuals feel a part of the workplace and have the ability to communicate with their co-workers and employer in the manner as other hearing employees do.

Not only can communication barriers between deaf and hearing people affect family relationships, work, and school, but they can also have a very significant effect on a deaf individual’s physical and mental health care. As a result of poor communication between the health care professional and the deaf or hard of hearing patient, many patients report that they are not properly informed about their disease and prognosis.^[130] This lack of or poor communication could also lead to other issues such as misdiagnosis, poor assessments, mistreatment, and even possibly harm to patients.

Poor communication in this setting is often the result of health care providers having the misconception that all people who are deaf or hard of hearing have the same type of hearing loss, and require the same type of communication methods.

In reality, there are many different types and range of hearing loss, and in order to communicate effectively a health care provider needs to understand that each individual with hearing loss has unique needs.

This affects how individuals have been educated to communicate, as some communication methods work better depending on an individual’s severity of hearing loss.

For example, assuming every deaf or hard of hearing patient knows American Sign Language would be incorrect because there are different types of sign language, each varying in signs and meanings.

A patient could have been educated to use cued speech which is entirely different from ASL.^[130] Therefore, in order to communicate effectively, a health care provider needs to understand that each individual has unique needs when communicating.

Although there are specific laws and rules to govern communication between health care professionals and people who are deaf, they are not always followed due to the health care professional’s insufficient knowledge of communication techniques.

This lack of knowledge can lead them to make assumptions about communicating with someone who is deaf, which can in turn cause them to use an unsuitable form of communication.

Acts in countries such as the Americans with Disabilities Act (ADA) state that all health care providers are required to provide reasonable communication accommodations when caring for patients who are deaf.

These accommodations could include qualified sign language interpreters, CDIs, and technology such as Internet interpretation services.

A qualified sign language interpreter will enhance communication between a deaf individual and a health care professional by interpreting not only a health professional’s verbal communication, but also their non-verbal such as expressions, perceptions, and body language.

A Certified Deaf Interpreter (CDI) is a sign language interpreter who is also a member of the Deaf community.^[131] They accompany a sign language interpreter and are useful for communication with deaf individuals who also have language or cognitive deficits.

A CDI will transform what the health care professional communicates into basic, simple language.

This method takes much longer, however it can also be more effective than other techniques.

Internet interpretation services are convenient and less costly, but can potentially pose significant risks.

They involve the use of a sign language interpreter over a video device rather than directly in the room.

This can often be an inaccurate form of communication because the interpreter may not be licensed, is often unfamiliar with the patient and their signs, and can lack knowledge of medical terminology.^[132]

Aside from utilizing interpreters, healthcare professionals can improve their communication with deaf or hard of hearing patients by educating themselves on common misconceptions and proper practices depending on the patient’s needs.

For example, a common misconception is that exaggerating words and speaking loudly will help the patient understand more clearly.

However, many individuals with hearing loss depend on lip-reading to identify words.

Exaggerated pronunciation and a raised voice can distort the lips, making it even more difficult to understand.

Another common mistake health care professionals make are the use of single words rather than full sentences.

Although language should be kept simple and short, keeping context is important because certain homophonous words are difficult to distinguish by lip-reading.

Health care professionals can further improve their own communication with their patients by eliminating any background noise and positioning themselves in a way where their face is clearly visible to the patient, and suitably lit.

The healthcare professional should know how to use body language and facial expressions to properly communicate different feelings.^[130]

A 2005 study achieved successful regrowth of cochlea cells in guinea pigs.^[133] However, 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 that gene therapy targeting Atoh1 can cause hair cell growth and attract neuronal processes in embryonic mice. Some hope that a similar treatment will one day ameliorate hearing loss in humans.^[134]

Recent research, reported in 2012 achieved growth of cochlear nerve cells resulting in hearing improvements in gerbils,^[135] using stem cells.

Also reported in 2013 was regrowth of hair cells in deaf adult mice using a drug intervention resulting in hearing improvement.^[136] The Hearing Health Foundation in the US has embarked on a project called the Hearing Restoration Project.^[137] Also Action on Hearing Loss in the UK is also aiming to restore hearing.^[138]

Researchers reported in 2015 that genetically deaf mice which were treated with TMC1 gene therapy recovered some of their hearing.^[139][140] In 2017, additional studies were performed to treat Usher syndrome^[141] and here, a recombinant adeno-associated virus seemed to outperform the older vectors.^[142][143]

Besides research studies seeking to improve hearing, such as the ones listed above, research studies on the deaf have also been carried out in order to understand more about audition.

Pijil and Shwarz (2005) conducted their study on the deaf who lost their hearing later in life and, hence, used cochlear implants to hear.

They discovered further evidence for rate coding of pitch, a system that codes for information for frequencies by the rate that neurons fire in the auditory system, especially for lower frequencies as they are coded by the frequencies that neurons fire from the basilar membrane in a synchronous manner.

Their results showed that the subjects could identify different pitches that were proportional to the frequency stimulated by a single electrode.

The lower frequencies were detected when the basilar membrane was stimulated, providing even further evidence for rate coding.^[144]