Red eyes happen when tiny blood vessels on the surface of the eye become dilated or inflamed, making the white of the eye look pink or bloodshot. The causes range from completely harmless (a long day at the computer) to potentially serious (a sudden spike in eye pressure). Most cases in adults trace back to a handful of common culprits: allergies, dry eye, conjunctivitis, contact lens irritation, environmental triggers, or a broken blood vessel.
Why Eyes Turn Red
The white part of your eye is covered by a thin, transparent membrane packed with microscopic blood vessels. When something irritates or inflames the eye, those vessels widen to deliver immune cells and other protective molecules to the area. Histamine, cytokines, and other inflammatory signals drive this response. The result is visible redness, sometimes with swelling, tearing, or discomfort. This process is the same whether the trigger is a grain of pollen or a serious infection.
Allergies
Allergic conjunctivitis is one of the most frequent reasons adults develop red eyes, especially seasonally. Pollen, pet dander, dust mites, and mold trigger histamine release on the eye’s surface, producing mild to moderate redness, clear and watery discharge, and often intense itching. Both eyes are usually affected at the same time. The redness tends to flare when you’re exposed to the allergen and ease once you’re away from it or use antihistamine eye drops.
Dry Eye Disease
Dry eye is a chronic inflammatory condition driven by an unstable or overly concentrated tear film. When tears evaporate too quickly or aren’t produced in sufficient volume, the surface cells of the eye become stressed and release inflammatory signals. Those signals recruit immune cells, damage the protective barrier of the cornea, and reduce the mucus-producing cells the eye depends on for lubrication. This creates a self-reinforcing cycle: inflammation worsens the tear film, which worsens the inflammation.
The redness from dry eye is typically mild but persistent, often worse at the end of the day or after prolonged screen time. You may also notice a gritty or burning sensation, blurred vision that clears temporarily when you blink, or eyes that water excessively as a reflex response to dryness. Spending hours in air-conditioned or heated rooms, staring at screens without breaks, and aging all increase your risk.
Pink Eye (Conjunctivitis)
Conjunctivitis simply means inflammation of that surface membrane, and the three main types look and feel different enough to tell apart in many cases.
Viral conjunctivitis usually starts in one eye and spreads to the other within a day or two. The redness is moderate, and the hallmark is a gritty, sandy sensation like something is stuck in the eye. Light sensitivity can be significant. There’s no specific treatment; like a cold, it runs its course over two to three weeks.
Bacterial conjunctivitis produces a yellow or green discharge that can be dramatic, sometimes crusting the eyelashes shut overnight. The redness may look worse than it feels, as pain is typically minimal. Antibiotic drops or ointment usually improve symptoms within three to four days, though finishing the full course prevents recurrence.
Allergic conjunctivitis stands apart by its clear, watery discharge and itching. It almost always affects both eyes simultaneously and lacks the crusty discharge or sandy pain of the infectious types.
Digital Eye Strain and Screen Use
Staring at a computer, tablet, or phone for extended periods reduces your blink rate by as much as half. Fewer blinks means the tear film isn’t being refreshed, which leads to surface drying, irritation, and redness. This is often lumped together with dry eye, but it can happen even in people who don’t have chronic dry eye disease. Taking breaks every 20 minutes, consciously blinking more, and adjusting screen brightness all help reduce it.
Contact Lens Irritation
Wearing contact lenses too long, sleeping in lenses not designed for overnight use, or poor cleaning habits can cause redness through several routes. Lenses reduce the amount of oxygen reaching the cornea, which triggers blood vessel dilation as the eye tries to compensate. Protein and debris buildup on the lens surface irritates the tissue underneath the upper eyelid. In the worst cases, bacteria get trapped beneath the lens and infect the cornea. Corneal infections from contacts can scar the eye and permanently affect vision, so persistent redness, pain, or light sensitivity while wearing lenses is worth taking seriously.
Environmental and Chemical Irritants
Tobacco smoke, smog, chlorinated pool water, wind, and very dry air can all inflame the eye’s surface and produce temporary redness. These exposures typically irritate both eyes at once, and the redness fades once you’re no longer exposed. Wearing wraparound sunglasses in windy or smoky conditions and using swim goggles in chlorinated pools are straightforward ways to prevent it.
Broken Blood Vessel
A subconjunctival hemorrhage looks alarming: a bright red patch that covers part or all of the white of the eye. It happens when a tiny blood vessel on the surface breaks and leaks blood beneath the membrane. Common triggers include coughing, sneezing, vomiting, straining on the toilet, heavy lifting, or rubbing the eye too hard. People with high blood pressure, diabetes, or who take blood thinners are more susceptible.
Despite the dramatic appearance, these are painless and don’t affect vision. The blood reabsorbs on its own, typically within two to three weeks, shifting from bright red to yellow-green as it clears, much like a bruise on the skin.
Blepharitis
Blepharitis is chronic inflammation of the eyelid margins, usually caused by clogged oil glands or an overgrowth of normal skin bacteria near the lash line. The eyelids become red, swollen, and crusty, and the irritation spills over to redden the eye itself. It tends to come and go, often worsening in the morning. Regular warm compresses and gentle lid cleaning help manage flare-ups.
More Serious Causes
Acute Angle-Closure Glaucoma
This is a medical emergency. Fluid drainage inside the eye becomes suddenly blocked, causing pressure to spike to dangerous levels. The eye turns red with a distinctive pattern of vessel dilation concentrated around the edge of the cornea. The cornea may look hazy or cloudy, the pupil may not react normally to light, and the pain is severe, often accompanied by nausea, vomiting, headache, and seeing halos around lights. Without rapid treatment, permanent vision loss can occur within hours.
Uveitis
Uveitis is inflammation of the middle layer of the eye wall. It causes redness, deep aching pain, light sensitivity, and blurred vision. Unlike glaucoma, it doesn’t typically produce a hazy cornea or extremely elevated eye pressure, but it still requires prompt treatment to prevent complications like scarring inside the eye.
When Red Eyes Signal Something Urgent
Most red eyes resolve on their own or with simple measures. But certain combinations of symptoms point to conditions that can threaten your vision. Seek immediate care if your red eye comes with any of the following: sudden changes in vision, severe pain, sensitivity to light with nausea or vomiting, halos around lights, a sensation that something is stuck in the eye that won’t resolve with rinsing, swelling in or around the eye, fever, or inability to keep the eye open. A red eye caused by a chemical splash or direct injury also warrants emergency evaluation regardless of how it feels.