People become deaf for a wide range of reasons, from genetics they’re born with to damage accumulated over a lifetime. Over 430 million people worldwide currently have disabling hearing loss, and that number is projected to reach 700 million by 2050. Understanding the causes starts with how the ear works and the specific points where things can go wrong.
How Hearing Works and Where It Breaks Down
Sound travels through three sections of the ear before reaching the brain. The outer ear funnels sound waves inward. The middle ear contains tiny bones that amplify those vibrations and pass them to the inner ear. The inner ear, specifically a snail-shaped structure called the cochlea, converts vibrations into electrical nerve signals that travel to the brain.
Hearing loss falls into categories based on where the problem occurs. Conductive hearing loss involves the outer or middle ear, where something physically blocks or disrupts the transmission of sound. Sensorineural hearing loss involves the inner ear or the hearing nerve itself, and it accounts for the majority of permanent deafness. Some people have both types simultaneously, called mixed hearing loss. Each type has different causes and different treatment possibilities.
Genetic Causes
Genetics is one of the most common reasons people are born deaf. Mutations in a single gene called GJB2 account for roughly 50% of nonsyndromic hearing loss, the type where deafness occurs without other medical conditions. This gene provides instructions for building a protein that forms channels between cells in the cochlea. When those channels don’t work properly, the cochlea can’t develop normally. One of the key failures is that a structure called the tunnel of Corti never opens, which is essential for the inner ear’s sound-processing machinery to function.
Hundreds of other genes also play a role. Some cause deafness present at birth, while others lead to progressive hearing loss that shows up later in childhood or adulthood. Genetic deafness can be inherited from parents who are deaf, from parents who carry a gene variant without being deaf themselves, or from spontaneous mutations. In many families, two hearing parents can have a deaf child if both carry a copy of the same recessive gene.
Noise Exposure
Loud sound physically damages the delicate sensory hair cells inside the cochlea. These hair cells have tiny projections called stereocilia that bend in response to sound vibrations. When sound is too intense, stereocilia can splay apart, fuse together, or break off entirely. Animal studies show that even a two-hour exposure at 103 decibels (roughly the volume of a loud concert or power tool) causes measurable stereocilia damage, and there’s no evidence of recovery in the weeks following exposure.
In humans, the damage accumulates. A single loud event can cause temporary hearing loss that resolves, but repeated exposure kills hair cells permanently. Mammals cannot regrow these cells. This is why noise-induced hearing loss is irreversible and why it’s one of the most preventable causes of deafness. Occupational noise from construction, manufacturing, military service, and music performance are among the most common culprits.
Aging
Age-related hearing loss, called presbycusis, affects more than 25% of people over 60. It’s progressive, irreversible, and typically affects both ears equally. High-pitched sounds become harder to hear first, with the loss gradually spreading to lower frequencies over time.
Several things happen inside the aging cochlea. Sensory hair cells at the base of the cochlea (the region that processes high-frequency sounds) deteriorate from a lifetime of accumulated injury: noise, medications, oxidative stress from normal metabolism, and other factors. The stria vascularis, a tissue that maintains the chemical environment the hair cells need to function, can atrophy. Auditory nerve fibers throughout the cochlea can also be lost. In most people, aging hearing loss involves some combination of all three processes. Because oxidative damage plays a significant role, researchers have explored whether antioxidants could slow the progression, though no proven preventive treatment exists yet.
Infections
Several infections can cause deafness, and some of the most consequential ones occur before birth. Cytomegalovirus (CMV) is a leading infectious cause of childhood hearing loss. A recent estimate attributes about 21% of all hearing loss present at birth to congenital CMV infection. What makes CMV particularly tricky is that less than half of CMV-related hearing loss is detectable at birth. Between 33% and 50% of cases are late-onset, appearing during the preschool or early school years. By age four, roughly 25% of all childhood hearing loss is likely tied to CMV.
Bacterial meningitis is another serious threat. The infection triggers a powerful inflammatory response that can invade all compartments of the inner ear. In animal models, scar tissue forms within four days of infection, and new bone starts growing inside the cochlea within two weeks. This bone growth can destroy the delicate structures needed for hearing and also makes cochlear implantation more difficult if it progresses too far. Auditory nerve cells are frequently lost as well. Not everyone who gets meningitis loses their hearing, but when it happens, it can be rapid and severe.
Medications and Chemical Exposure
Certain medications can damage hearing as a side effect. A class of antibiotics called aminoglycosides, used for serious bacterial infections, can enter sensory hair cells and trigger the production of harmful molecules that cause the cells to die. A chemotherapy drug called cisplatin works similarly, damaging the energy-producing structures inside hair cells and initiating cell death. Because human hair cells don’t regenerate, the hearing loss from these drugs is typically permanent.
Some substances cause only temporary hearing changes. High doses of aspirin and certain anti-malaria drugs can reduce hearing, but the effect usually reverses once the medication is stopped. Loop diuretics, used to treat fluid retention, fall into this reversible category as well.
Workplace chemicals pose their own risk. Industrial solvents like toluene, styrene, and xylene can damage outer hair cells in the cochlea. Metals including mercury and lead compounds, asphyxiants like carbon monoxide, and certain pesticides are also known to be ototoxic. The danger increases significantly when chemical exposure combines with noise. Being exposed to both loud sound and ototoxic chemicals causes more hearing damage than either one alone, and some chemicals can create hearing risk at levels below existing safety guidelines when noise is also present.
Auditory Neuropathy
Not all deafness comes from damage to the ear itself. Auditory neuropathy spectrum disorder is a condition where the inner ear detects sound relatively well, but the signal traveling from the ear to the brain is disrupted. People with this condition often pass basic hearing screenings because they can detect tones, yet they struggle enormously to understand speech, especially in noisy environments. Their difficulty with speech is disproportionate to what their hearing test results would predict.
The core problem is one of timing. The auditory nerve fails to fire in a synchronized pattern, so the brain receives a scrambled version of the sound signal. This is essentially the opposite of what happens with typical cochlear damage, where the ability to detect quiet sounds is impaired but the timing of nerve signals stays relatively intact. Auditory neuropathy can be present from birth or develop later, and its severity varies widely from person to person.
Physical and Structural Causes
Conductive hearing loss often has more straightforward mechanical explanations. Chronic ear infections can damage the eardrum or the tiny bones of the middle ear. Fluid buildup in the middle ear, common in young children, temporarily blocks sound transmission. Abnormal bone growth near the middle ear (a condition called otosclerosis) can freeze the small bones in place so they no longer vibrate. Structural differences present from birth, such as a missing or narrowed ear canal, can also prevent sound from reaching the inner ear.
Many conductive causes are treatable with surgery or medical management, which distinguishes them from most sensorineural causes. This is why identifying the type of hearing loss matters: it determines whether the damage is potentially reversible or permanent.