People become deaf when something disrupts the path sound takes from the outer ear to the brain. That disruption can happen before birth, develop gradually over decades, or occur suddenly from injury, infection, or loud noise. Roughly 1 in 5 people worldwide have some degree of hearing loss, and the causes fall into a few broad categories depending on where in the hearing system the problem occurs.
How Hearing Normally Works
Sound enters the ear canal, vibrates the eardrum, and passes through three tiny bones in the middle ear. Those bones amplify the vibration and push it into the fluid-filled cochlea in the inner ear. Inside the cochlea, thousands of microscopic hair cells convert those vibrations into electrical signals. The auditory nerve then carries those signals to the brain, which interprets them as sound. A breakdown at any point along this chain, from the ear canal to the brain itself, can cause deafness.
Conductive Hearing Loss
Conductive hearing loss happens when sound waves physically cannot reach the inner ear. The blockage sits in the outer or middle ear. Common causes include impacted earwax, fluid buildup from chronic ear infections, a perforated eardrum, or abnormalities in the three small bones that transmit vibration. Because the inner ear and auditory nerve still work, this type of hearing loss is often treatable or partially reversible through medication or surgery.
Sensorineural Hearing Loss
Sensorineural hearing loss is the most common type and accounts for the majority of all hearing loss. It results from damage to the hair cells inside the cochlea, to the auditory nerve, or to the brain’s sound-processing centers. Unlike conductive hearing loss, it is usually permanent.
The hair cells are the critical link. Each human cochlea contains about 15,000 of them, and they do not grow back once destroyed. Birds and amphibians can regenerate damaged hair cells within days, but mammals lost this ability. Scientists believe this trade-off came as the mammalian inner ear evolved to detect higher-frequency sounds, which required more rigid, specialized structures that can no longer divide and regrow. There was also little evolutionary pressure to preserve hearing into old age, since most hair cell loss happens after reproductive years.
When hair cells are damaged by noise, for example, the internal scaffolding that gives each cell its stiffness begins to break apart. The tiny filaments on top of the cell (called stereocilia) lose their structural proteins, fuse together, or snap at their roots. Once enough of these cells die, the frequencies they were responsible for detecting go silent permanently.
Genetic Causes
Genetics are the leading cause of deafness present at birth. The most common culprit is a mutation in a gene called GJB2, which provides instructions for building a protein that forms tiny channels between cells in the inner ear. These channels shuttle potassium ions and nutrients like glucose between neighboring cells. When the protein is missing or defective, potassium builds up outside the hair cells, starving them of nutrients and eventually triggering cell death. Hearing loss from GJB2 mutations actually begins before the hair cells visibly degenerate, meaning the chemical environment in the cochlea fails before the cells themselves break down.
Hundreds of other genes can also cause deafness, either on their own or as part of broader syndromes that affect multiple organs. Some genetic hearing loss is present at birth, while other forms don’t appear until childhood or even adulthood, making it possible for someone with no early symptoms to gradually lose hearing from a condition they were born with.
Infections Before and After Birth
Certain infections during pregnancy can cross the placenta and damage a developing baby’s auditory system. Cytomegalovirus (CMV) is the most significant. About 50% of infants born with symptomatic CMV infection develop hearing loss, and even among babies who show no symptoms at birth, roughly 7% go on to develop hearing problems that emerge later, progress over time, or fluctuate unpredictably. Rubella, toxoplasmosis, syphilis, herpes, and Zika virus can all cause similar damage to the fetal inner ear.
Viruses harm the ear in two ways: they can directly destroy hair cells, supporting cells, or the organ of Corti (the structure inside the cochlea where hair cells sit), or they can trigger an immune response that inadvertently damages those same structures. After birth, bacterial meningitis is one of the most common infectious causes of deafness in children, as the inflammation can spread to the cochlea and destroy it rapidly.
Noise Exposure
Loud sound is one of the most preventable causes of permanent hearing loss. An estimated 12.5% of teenagers and adolescents aged 6 to 19 have already suffered permanent hearing damage from recreational noise exposure, including concerts, headphones, and power tools. For newborns in intensive care, sustained noise levels above 45 decibels can pose a risk to developing ears.
The damage is cumulative. A single extremely loud blast can destroy hair cells instantly, but more often the loss builds over years of moderate overexposure. Each episode of loud sound causes microscopic structural damage to the stereocilia. Some of that damage can partially repair itself if the cells survive, but repeated insults eventually push cells past the point of recovery.
Medications That Damage Hearing
Certain medications are toxic to the inner ear. The two most well-known categories are aminoglycoside antibiotics (used for serious bacterial infections) and platinum-based chemotherapy drugs like cisplatin (used to treat cancers including neuroblastoma, brain tumors, and bone cancers in children). These drugs cause dose-dependent hearing loss, meaning the more you receive, the greater the damage. The hearing loss is typically permanent and affects high-frequency sounds first.
Age-Related Hearing Loss
Gradual hearing loss with aging, called presbycusis, is extremely common. It results primarily from the slow, lifelong loss of hair cells in the inner ear. Unlike noise-induced damage, which tends to create a sharp drop at specific frequencies, age-related loss usually affects higher pitches first and progresses to lower ones over time. Genetics, cumulative noise exposure, cardiovascular health, and medications all influence how quickly it develops.
When the Ear Works but the Brain Doesn’t Get the Signal
In a condition called auditory neuropathy, the outer hair cells in the cochlea function normally, but the signal never reaches the brain properly. The problem may lie in the inner hair cells (which are responsible for converting vibrations into nerve impulses), in the auditory nerve itself, or in the connections between the two. People with auditory neuropathy can sometimes detect that sound is present but cannot understand speech, especially in noisy environments. It can be caused by genetic mutations, oxygen deprivation at birth, or nerve damage from other conditions.
Degrees of Hearing Loss
Deafness exists on a spectrum. The World Health Organization classifies hearing loss by the quietest sound a person can detect in their better ear:
- Mild (20 to 34 dB): No trouble in quiet rooms, but difficulty following conversation in background noise.
- Moderate (35 to 49 dB): Difficulty hearing a normal speaking voice even in quiet settings.
- Moderately severe (50 to 64 dB): Needs loud speech to hear in quiet; great difficulty in noise.
- Severe (65 to 79 dB): Can only hear loud speech spoken directly into the ear.
- Profound (80 to 94 dB): Unable to hear or understand even a shouted voice.
Disabling hearing loss, defined by the WHO as greater than 35 dB in the better ear, affects hundreds of millions of people globally. Many people with profound hearing loss identify culturally as Deaf and communicate primarily through sign language, while others use hearing aids, cochlear implants, or a combination of tools depending on the type and severity of their loss.
How Hearing Loss Is Identified
The most familiar test is pure-tone audiometry, where you wear headphones and indicate when you hear beeps at different pitches and volumes. This maps out exactly which frequencies you can and cannot hear, and how loud a sound needs to be before you detect it. Comparing results from sounds played through headphones (air conduction) versus a device placed on the bone behind your ear (bone conduction) helps determine whether the loss is conductive or sensorineural. If both pathways show equal loss, the problem is in the inner ear or beyond. If only air conduction is reduced, the blockage is in the outer or middle ear.
For newborns and young children who cannot respond to beeps, auditory brainstem response testing measures electrical activity in the hearing nerve and brain in response to sound played through tiny earphones. Another test, otoacoustic emissions, checks whether the outer hair cells in the cochlea are functioning by detecting faint sounds the healthy ear naturally produces in response to stimulation. These tests are routinely used in newborn hearing screenings and are how conditions like auditory neuropathy are caught early.