Red or bloodshot eyes are the result of blood vessel swelling that can be brought on by a wide variety of factors and conditions. For example, things as innocuous as dry air can cause redness in the whites of the eyes.
Most of the time, reddened, bloodshot eyes are not a sign of any serious illness. According to the National Institutes of Health, relatively harmless normal factors such as dust and allergies can cause the blood vessels in the eye to become dilated, or swollen, leading to increased redness in the whites of the eyes. Red eyes can be the result of something more serious, including an injury to the eye or conditions such as conjunctivitis (pink eye), corneal ulcers or acute glaucoma. In some cases, as with pink eye, the redness is a sign of a contagious illness, and those suffering from this ailment should take care to avoid touching their eyes lest they spread the infection to the other eye or to other people.
Though most of the health problems that cause redness in the whites of the eyes are relatively harmless, this symptom can also occasionally be the sign of something serious. Paying attention to other symptoms will help those with reddened eyes understand the severity of their condition. People who are experiencing symptoms like extreme pain, vision problems, nausea, vomiting or dizziness along with red eyes should contact their doctors. Otherwise, the red eye issues should clear up in a matter of days to weeks.