Blepharitis is caused by a combination of bacterial overgrowth on the eyelid margins and dysfunction of the tiny oil glands embedded in the eyelids. It comes in two main forms, each with a distinct trigger: anterior blepharitis, driven primarily by bacteria at the base of the eyelashes, and posterior blepharitis, caused by clogged or malfunctioning oil glands. Many people have both at the same time, and the condition tends to be chronic because the underlying causes are difficult to fully eliminate.
Bacterial Overgrowth and Anterior Blepharitis
The most common form of anterior blepharitis starts with Staphylococcus bacteria, which naturally live on the skin but can proliferate along the eyelid margin. When these bacteria multiply, they produce enzymes called lipases that break down the natural oils on your eyelid into irritating free fatty acids. Bacterial debris and white blood cells then clump together into hard, brittle scales at the base of each lash. As lashes grow, these scales form ring-like crusts called collarettes, one of the hallmark signs of the condition.
The bacteria also release toxins that dilate blood vessels along the lash line, creating the persistent redness and swelling that characterizes blepharitis. In some people, the immune system develops a hypersensitivity reaction to the bacteria or their toxins, which amplifies the inflammation beyond what the bacterial load alone would cause. This is why two people with similar amounts of bacteria on their eyelids can have very different levels of symptoms.
How Bacterial Biofilms Keep It Chronic
One reason blepharitis is so persistent is that bacteria on the eyelid margin don’t just sit on the surface individually. They organize into biofilms: structured colonies encased in a protective layer that shields them from both your immune system and topical treatments. Within these biofilms, bacteria communicate chemically. When their population reaches a critical mass, they collectively activate genes that produce toxins, inflammatory enzymes, and other virulence factors designed to break down surrounding tissue and liberate nutrients for the growing colony.
This strategy is deliberate. The bacteria delay triggering a strong immune response until they’re numerous enough to survive it, protected inside their biofilm “armor.” This is why blepharitis can seem to improve with treatment only to flare again. Unless the biofilm itself is physically disrupted through consistent lid hygiene, the colony rebuilds and the cycle repeats.
Meibomian Gland Dysfunction and Posterior Blepharitis
Your upper and lower eyelids contain roughly 30 to 40 meibomian glands each, small structures that secrete an oily substance onto the tear film to prevent it from evaporating too quickly. In posterior blepharitis, these glands malfunction. The openings of the glands become blocked by a buildup of hardened skin cells, a process called hyperkeratinization. Over time, the blocked glands dilate, and if the obstruction continues long enough, the glands can atrophy and lose their ability to function entirely.
The oil these glands produce also changes in composition, becoming thicker and more viscous, which makes it even harder to flow out normally. When the glands do release oil, it’s often of poor quality. The altered lipids, lipid debris, and inflammatory byproducts spill onto the eye’s surface, damaging the tear film rather than stabilizing it. This is why posterior blepharitis and dry eye so frequently overlap. The tear film loses its protective oil layer, evaporates faster, and leaves the eye feeling gritty, irritated, and dry.
Demodex Mites
Microscopic mites called Demodex live in hair follicles across the face and are especially drawn to eyelash follicles. Most people carry some Demodex without problems, but when populations grow too large, they contribute to eyelid inflammation. An eye doctor can detect excess mites by examining a few pulled eyelashes under a microscope. Demodex-related blepharitis has its own characteristic appearance, often producing a cylindrical dandruff-like crust at the lash base rather than the brittle scales seen with bacterial blepharitis.
Cosmetic Procedures and Products
Certain beauty treatments around the eyes are a well-documented trigger. About 15% of people who get eyelash extensions develop conjunctivitis or blepharitis afterward and need to see an eye doctor. Eyebrow and eyelash tinting poses a separate risk because the dyes often contain a chemical called para-phenylenediamine (PPD), which can cause severe allergic inflammation of the eyelids and the eye’s surface.
Lash and brow lamination treatments are another culprit. These procedures use chemicals that break down the structural bonds in hair to reshape it, but those same chemicals aggressively irritate the skin and mucous membranes. They trigger the release of inflammatory compounds in the eyelid tissue, and within 12 to 48 hours, many people develop allergic inflammation of the eyelids and conjunctiva. Repeated lamination procedures can also cause eyelash loss due to hair brittleness and dehydration. Permanent makeup, microblading, and periocular tattooing carry risks of infection, pigment migration, and eyelash loss, all of which can either cause or worsen blepharitis.
Other Contributing Factors
Several skin conditions make blepharitis more likely. Seborrheic dermatitis, the same condition that causes dandruff on the scalp, frequently affects the eyelids and produces oily, flaky scales along the lash line. Rosacea is strongly linked to meibomian gland dysfunction, and people with rosacea are significantly more likely to develop the posterior form. Allergic conditions, including seasonal allergies and contact dermatitis from eye drops or cosmetics, can also inflame the eyelid margins and set the stage for chronic blepharitis.
Age plays a role too. Meibomian gland function naturally declines over time, which is why posterior blepharitis becomes more common in middle-aged and older adults. Hormonal changes, particularly declining androgen levels, can reduce oil gland output and change the composition of the oils they produce. Contact lens wear, prolonged screen time (which reduces blink rate and therefore reduces natural oil expression from the glands), and dry indoor environments all add to the risk.
How Blepharitis Is Identified
Diagnosis is primarily visual. An eye doctor examines the eyelid margins using a magnifying instrument, looking for redness, crusting patterns, clogged gland openings, and changes to the lash line. In some cases, a swab of the oil or crust on the eyelid is sent for bacterial culture. If Demodex mites are suspected, a few eyelashes may be pulled and examined under a microscope.
Most cases are confirmed by how they respond to initial treatment. If standard lid hygiene and warm compresses don’t improve things, particularly if only one eye is affected, further testing may be done to rule out other conditions, including eyelid skin cancer or immune-system disorders that can mimic blepharitis.