There are many possible causes of a red eye or eyes. Some are cause for concern, some are medical emergencies. Others are of no consequence or concern at all. The degree of redness or appearance of blood usually does not correlate to how serious the situation is. It is generally more important whether you also have eye pain or impaired vision.

Bloodshot eyes appear red because the vessels in the surface of the white portion of the eye (sclera) become enlarged and irritated. This may result from extremely dry air, sun exposure, dust, foreign body, an allergic reaction, infection, trauma, or other conditions.

One common cause of a red eye is straining or coughing. This can lead to a bright red, uniformly dense bloody area on the sclera. This is called a subconjunctival hemorrhage. Although this bloody area may appear alarming, it is a fairly common occurrence and of little significance. If you notice a bloody blotch in one eye that doesn't hurt, but just looks bad, don't worry. It generally clears up on its own within a week or two.

Eye infections or inflammation can occur in different locations. They cause redness as well as possible itching, discharge, pain, or vision problems:

• Blepharitis -- inflammation of the eyelash follicles along the eyelid. It is caused by skin bacteria. Itching is common and your eyelids may appear greasy or crusty.
• Conjunctivitis -- inflammation or infection of the membrane that lines the eyelids and coats the surface of the eye (the conjunctiva). This condition is often referred to as "pink eye". It may be caused by a virus, bacteria, allergy, or irritation. If caused by an organism, this is highly contagious.
• Corneal ulcers -- often caused by a bacterial or viral infection. (The cornea is the outer covering of the eye.)
• Uveitis -- inflammation of the uvea, which includes the iris, ciliary body, and choroid. This is often related to an autoimmune disorder, infection, or exposure to toxins. Often, only the iris is inflamed, which is called iritis.

Other potential causes include:

• Cold or allergies.
• Foreign objects in the eye that cut or damage the eye. (See eye emergencies.)
• Acute glaucoma -- a sudden increase in eye pressure that is extremely painful and causes serious visual disturbances. This is a medical emergency. Most times, glaucoma is chronic and gradual.
• Corneal scratches caused by sand, dust, or overuse of contacts.
• Bleeding problems (for example, from excess use of blood thinning drugs).

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Rodriguez JO. Prevention and treatment of common eye injuries in sports. Am Fam Physician. 2003; 67(7): 1481-1488.