There are many things that can cause your eye to turn red.
The eye looks red when the blood vessels that are in the conjunctiva (the mucous membrane that covers the white of your eye and the backside of your eyelids) becomes dilated.
Those blood vessels often dilate when the eye gets irritated. This irritation can originate from a problem occurring inside the eye or factors from outside the eye.
The most common external factors that can cause the eye to become red are exposure to infectious organisms (mostly viruses and bacteria), environmental irritants (smoke, chemicals, sunlight), or allergens.
Infectious organisms can cause infectious conjunctivitis, or what is more commonly referred to as “pink eye.” This condition often presents with the eye being red and a mucous discharge being produced, often to such a degree that the eyelids are crusted over upon awaking in the morning. Infectious conjunctivitis can be extremely contagious and it is often advised that you severely limit your exposure to others while the problem is active. Infectious conjunctivitis caused by bacteria can be treated with antibiotic eye drops but viral conjunctivitis currently has no treatment and must run its course like the common cold.
Environmental irritants can make the eye look red for a short period of time during and immediately after exposure. The irritation is usually self-limited but may resolve more quickly with the use of over-the-counter lubricating drops or artificial tears. It is very important to understand exactly which irritant you were exposed to because there are some chemicals (acids and bases) that can cause extreme damage to the eye. So if you’re exposed to a caustic chemical you need to immediately rinse your eye out with water and seek emergency medical attention.
Allergens can cause allergic conjunctivitis, which can look very similar to pink eye but usually has significantly less mucous discharge and is usually accompanied by fairly severe itching. Allergic conjunctivitis is not contagious and can usually be treated with anti-allergy eye drops.
Infectious and allergic conjunctivitis can cause mild discomfort and itching but they rarely cause significant pain or loss of vision. A red eye with significant pain, especially when accompanied by severe light sensitivity and vision loss, often indicates more significant problems such as iritis, angle closure glaucoma or a corneal ulcer, all of which require immediate medical attention. If your eye is red and there is significant pain do not assume you have pink eye--see your eye doctor immediately!
