Otitis externa, commonly called swimmer’s ear, is an infection or inflammation of the outer ear canal. It happens when the ear canal’s natural defenses break down, allowing bacteria or fungi to take hold. The most common trigger is trapped moisture, but anything that disrupts the thin skin lining the ear canal can set the stage for infection.
How the Ear Canal Protects Itself
Understanding what causes otitis externa starts with understanding what normally prevents it. The ear canal has a surprisingly effective defense system built around earwax. Cerumen (the technical name for earwax) is a mixture of fats, immune proteins, and enzymes secreted by glands in the outer third of the ear canal. It maintains an acidic environment with a pH between 5.2 and 7.0, which discourages bacterial and fungal growth. Earwax also has direct antibacterial and antifungal properties, and it physically coats the canal’s skin to keep moisture out.
The canal also has a self-cleaning mechanism. Skin cells migrate outward from the eardrum, carrying old wax and debris toward the ear opening. When this system works properly, the canal stays dry, slightly acidic, and inhospitable to infection. Otitis externa develops when something disrupts one or more of these protective layers.
Moisture: The Most Common Trigger
Water that stays trapped in the ear canal is the single most frequent cause. The moisture softens the skin lining, strips away the protective wax layer, and raises the local pH, creating ideal conditions for bacteria to multiply. This is why the condition is so closely associated with swimming, earning it the nickname “swimmer’s ear.”
But you don’t need to be a swimmer to get it. Heavy sweating, prolonged humid weather, and water left in the ear after showering can all produce the same effect. Anyone who regularly exposes their ears to moisture is at higher risk, especially during summer months or in tropical climates where humidity is persistently high.
Mechanical Damage to the Ear Canal
The skin inside the ear canal is remarkably thin and delicate. Even minor scratches create entry points for bacteria. The most common source of this damage is self-cleaning with cotton swabs, which can scrape the canal lining and push wax deeper, compacting it against the eardrum. This removes the protective cerumen layer while simultaneously injuring the tissue underneath.
Other objects cause the same problem. Hearing aids, earbuds, and earplugs worn for extended periods trap moisture against the skin and can cause friction-related irritation. Ear irrigation, sometimes performed to remove impacted wax, carries a risk of otitis externa as well, particularly if the procedure damages the canal skin. Even fingernails or hairpins used to scratch an itch inside the ear can introduce enough trauma to start an infection.
Which Bacteria and Fungi Are Responsible
Most cases of otitis externa are bacterial. Staphylococcus aureus is the most commonly isolated organism, followed closely by Pseudomonas aeruginosa, a bacterium that thrives in warm, moist environments. These two species account for the majority of acute infections.
Fungal infections, called otomycosis, tend to appear in chronic or recurring cases. Aspergillus species are the dominant fungal cause, responsible for 54 to 80 percent of fungal ear infections. Candida species are also frequently isolated. Fungal otitis externa is more common in tropical regions and in people who have used antibiotic ear drops for extended periods, since killing off bacteria can allow fungi to flourish unopposed.
Skin Conditions That Raise Your Risk
Certain dermatological conditions make the ear canal more vulnerable. Eczema, seborrheic dermatitis, and psoriasis can all affect the skin of the ear canal, causing dryness, flaking, and cracking that compromises the skin barrier. Psoriasis of the ear, while uncommon, is a recognized risk factor for developing secondary otitis externa. These conditions also tend to cause itching, which leads to scratching, which compounds the problem by adding mechanical trauma on top of already compromised skin.
Allergic reactions to hair products, earrings, or certain ear drop ingredients can produce contact dermatitis in the ear canal, creating similar vulnerability. If you notice that ear infections seem to follow the use of a particular product, the product itself may be the underlying cause.
Necrotizing Otitis Externa: A Serious Complication
In rare cases, otitis externa can progress into a dangerous condition called necrotizing (or malignant) otitis externa, where the infection spreads beyond the ear canal into the surrounding bone and soft tissue. This is not a typical ear infection. It involves osteomyelitis of the skull base and can become life-threatening.
Pseudomonas aeruginosa is the primary culprit, involved in 50 to 90 percent of cases. Diabetes is the single strongest risk factor. In large studies, roughly 55 to 75 percent of patients with necrotizing otitis externa had diabetes, and people with the disease have about ten times the odds of developing this complication compared to those without it. The condition predominantly affects older adults, though it can occur at any age.
Other high-risk groups include people with HIV, those undergoing cancer treatment, and patients who have received radiation therapy to the head and neck. That said, necrotizing otitis externa can also appear in otherwise healthy individuals, particularly younger patients, who tend to respond better to treatment.
How to Reduce Your Risk
Since moisture and mechanical trauma are the two main drivers, prevention focuses on keeping the ear canal dry and leaving it alone. The CDC recommends several practical steps:
- Dry your ears thoroughly after water exposure. Tilt your head to each side so the ear faces downward, and pull the earlobe in different directions to help water drain. A towel works for the outer ear. If water remains, a hair dryer on the lowest heat and fan setting, held several inches away, can evaporate residual moisture.
- Use a bathing cap or earplugs when swimming. Custom-fitted swim molds offer the best seal.
- Skip the cotton swabs. The ear canal cleans itself. Inserting anything into it removes protective wax and risks scratching the skin.
- Ask about ear-drying drops. Acidifying drops used after swimming can help restore the canal’s natural pH and evaporate trapped water. These should not be used if you have ear tubes, a perforated eardrum, or an active infection.
For people with underlying skin conditions like eczema or psoriasis, managing the skin disease itself is an important part of preventing recurrent ear infections. Keeping flare-ups under control reduces the cracking and itching that make the ear canal vulnerable in the first place.