Eye inflammation has dozens of possible causes, ranging from seasonal allergies and infections to autoimmune diseases that affect the whole body. The specific cause depends on which part of the eye is inflamed. The surface (conjunctiva), the white outer wall (sclera), the colored middle layer (uvea), or the eyelids can all become inflamed independently, and each points to a different set of triggers.
Where Inflammation Occurs in the Eye
Not all eye inflammation is the same condition. Conjunctivitis affects the clear membrane covering the white of your eye and inner eyelids. Uveitis targets the middle layer of the eye, which includes the iris and the tissue behind it. Scleritis involves the tough white outer coating, and episcleritis affects the thinner tissue sitting on top of it. Blepharitis is inflammation of the eyelids themselves. Keratitis targets the cornea, the clear dome at the front of the eye.
These distinctions matter because the causes, severity, and outlook differ significantly. Conjunctivitis from allergies is uncomfortable but rarely dangerous. Scleritis from an autoimmune disease can threaten your vision if untreated. Uveitis affects roughly 26 out of every 10,000 people in the United States, and it’s one of the leading preventable causes of blindness worldwide.
Infections: Bacteria, Viruses, Fungi, and Parasites
Infections are among the most common and straightforward causes. Bacterial conjunctivitis produces the thick, yellowish discharge many people associate with “pink eye.” Viral conjunctivitis, often caused by the same family of viruses behind the common cold, tends to produce a thinner, watery discharge and frequently spreads from one eye to the other.
Herpes simplex virus (the cold sore virus) and varicella-zoster virus (the shingles virus) can both cause deeper inflammation inside the eye, including uveitis. When shingles affects the nerve branch that runs to the forehead and eye, it can trigger painful inflammation of the iris and surrounding structures. Syphilis and tuberculosis, though less common, are also recognized infectious causes of uveitis.
Fungal eye infections are rarer but tend to be more serious. The fungi most often involved, Fusarium and Aspergillus, live in soil and on plants. These infections typically happen after an eye injury involving plant material, like a twig scratch, or in people who wear contact lenses with poor hygiene. Candida, a yeast that normally lives on the body, can also infect the eye, particularly in people with weakened immune systems. A single-celled parasite called Acanthamoeba, found in tap water, pools, and hot tubs, causes a particularly painful corneal infection almost exclusively in contact lens wearers.
Allergies and Environmental Irritants
Allergic conjunctivitis is one of the most frequent reasons for red, itchy, watery eyes. It works through a specific chain reaction: when an allergen like pollen, pet dander, or dust mites contacts the eye’s surface, immune cells release histamine and other inflammatory chemicals. This is what causes the intense itching, swelling, and redness. Some people experience it only during pollen seasons, while others deal with it year-round from indoor triggers like mold or dust.
Air pollution is a less obvious but well-documented trigger. Particulate matter, nitrogen dioxide, sulfur dioxide, and ground-level ozone, primarily from vehicle exhaust and industrial activity, irritate the eye’s surface and promote inflammation. Research from Paris found that higher concentrations of nitrogen-based pollutants correlated with more severe conjunctivitis cases. A study in Taiwan linked increased visits to eye clinics for conjunctivitis directly to spikes in fine particulate matter and ozone levels.
Indoor air quality matters too. Tobacco smoke, cooking fumes, volatile organic compounds from cleaning products, and formaldehyde from building materials all contribute. Nicotine and acrolein in cigarette smoke trigger an inflammatory response in corneal cells. Long-term exposure to fine particulate matter from indoor sources can alter tear composition, damage the eye’s surface, and lead to chronic dry eye and inflammation.
Autoimmune and Systemic Diseases
Sometimes inflamed eyes are the visible tip of a body-wide inflammatory condition. Several autoimmune diseases have strong links to specific types of eye inflammation, and in some cases, the eye symptoms appear before the underlying disease is diagnosed.
Ankylosing spondylitis, a form of inflammatory arthritis that primarily affects the spine, is one of the most common systemic causes of anterior uveitis. Up to 50% of people with this type of uveitis carry a genetic marker called HLA-B27, and about 75% of those individuals have an associated systemic condition like ankylosing spondylitis, reactive arthritis, or psoriatic arthritis.
Rheumatoid arthritis is frequently linked to scleritis, the deep, painful inflammation of the eye’s white outer wall. It can also cause episcleritis, keratitis, and uveitis. When scleritis occurs alongside rheumatoid arthritis or a condition called granulomatosis with polyangiitis (a type of blood vessel inflammation), it can lead to serious complications including tissue destruction and vision loss.
Sarcoidosis deserves special attention. This condition, which causes clusters of inflammatory cells in various organs, produces uveitis in 20% to 50% of affected patients. The eye inflammation often appears before any other symptoms of sarcoidosis, making an eye exam sometimes the first step toward diagnosing the disease. Lupus tends to affect the blood vessels in the retina, while Sjögren’s syndrome, which attacks moisture-producing glands, primarily causes severe dry eye that leads to chronic surface inflammation. Inflammatory bowel diseases like Crohn’s and ulcerative colitis can also trigger uveitis and scleritis.
Contact Lenses
Contact lenses are medical devices that sit directly on the cornea, and improper use is a major cause of eye inflammation and infection. Wearing lenses longer than recommended, sleeping in lenses not designed for overnight use, swimming or showering in contacts, or failing to clean and replace them on schedule all increase the risk. These habits allow bacteria, fungi, and parasites to accumulate on the lens surface and invade corneal tissue that’s already slightly oxygen-deprived from the lens sitting on top of it.
Microbial keratitis, an infection of the cornea, is the most serious contact lens complication. It can cause permanent scarring and vision loss if not treated quickly. Even without infection, contact lenses can cause a non-infectious inflammatory response from protein buildup on the lens, preservatives in lens solutions, or mechanical irritation from a damaged or poorly fitting lens.
Trauma and Surgery
Any physical injury to the eye triggers inflammation as part of the body’s healing response. A scratch on the cornea, blunt force, a foreign object lodged under the eyelid, or a chemical splash can all set off rapid swelling, redness, and pain. Chemical injuries are particularly urgent because alkali substances (like certain cleaning products) continue to penetrate and damage tissue even after the initial exposure.
Eye surgery, even routine procedures like cataract removal, causes some degree of postoperative inflammation. During surgery, manipulation of the iris and other internal structures disrupts the barrier between blood vessels and the fluid inside the eye. This allows immune cells and proteins to flood into the eye’s interior. Complications during surgery, such as a torn lens capsule, increase this risk significantly because they involve more tissue disruption, longer operating time, and sometimes the need to remove fragments of the natural lens from deeper in the eye.
When Inflammation Has No Clear Cause
In a significant number of cases, particularly with uveitis and scleritis, no specific cause is found even after thorough testing. These are classified as idiopathic, meaning they arise without an identifiable infection, autoimmune disease, or injury. This doesn’t mean nothing is wrong. It simply means current testing can’t pinpoint the trigger. These cases are still treated to control inflammation and prevent damage, and doctors may continue to monitor for an underlying condition that reveals itself over time.
Signs That Need Urgent Attention
Most eye inflammation from allergies or mild conjunctivitis resolves on its own or with simple treatment. But certain symptoms signal something more serious: sudden changes in vision, severe eye pain, sensitivity to light combined with headache or nausea, seeing halos around lights, swelling in or around the eye, or being unable to keep the eye open. A chemical splash or penetrating injury to the eye always warrants immediate care, regardless of how severe it initially appears.