Enlarged, dilated blood vessels lead to redness on the surface of the eye, according to Medline Plus, the National Institutes of Health website. The redness becomes even more pronounced because the vessels enlarge on the surface of the white portion of the eye, or sclera. Bloodshot eyes sometimes indicate underlying medical problems and at other times result in no serious consequences. Bloodshot eyes should be a concern if accompanied by eye pain or impaired vision.
Eye Strain
Fatigue or eye strain causes eye redness, which can be relieved by resting the eyes. Some people who are fatigued and suffer eyestrain may constantly rub their eyes, which makes the problem worse. Sometimes eye strain or coughing leads to a bright red bloody area on the sclera. Although it looks serious, it is a common occurrence and usually resolves in a week or two, Medline Plus notes.
Irritants
Pet dander, pollen and dust are common causes of bloodshot eyes for people with allergies, according to EyeCareSource.com. Many people suffer from eye redness because of environmental irritants such as dry air, blowing wind or sun exposure. Dry eyes usually lead to redness. Common colds and flu often cause bloodshot eyes, especially when accompanied by sneezing, coughing or lack of sleep. Simple activities such as swimming in a chlorinated swimming pool can irritate the eyes and cause redness.
Eye Damage
Foreign objects that cut or damage the eye can cause bloodshot eyes and require immediate medical treatment, Medline Plus says. The cornea, the outer covering of the eye, can also be scratched by sand, dust or overuse of contacts, causing eye redness. Corneal ulcers result from bacterial or viral infection result in bloodshot eyes as well.
Acute Glaucoma
Eye redness accompanied by extreme pain and visual disturbances may be a sign of acute glaucoma, a sudden increase in eye pressure. Medical treatment is necessary for this condition which can threaten eyesight, Medline Plus warns.
Inflammation
Conjunctivitis is an inflammation or infection of a membrane covering the surface of the eye. It is sometimes contagious and may be caused by a virus, bacteria, allergy or irritation. Redness and itching occur when the eyelash follicles along the eyelid become inflamed with a condition called blepharitis. The eyelids may become crusty. Bloodshot eyes occur when infection, exposure to toxins or immune disorders inflame the layer of the eyeball surrounding the lens called the uvea, which includes the iris. The inflammation is called uveitis.
