SINGAPORE – Have you ever looked in a mirror and seen a startling red blotch in the white of one or both eyes? What causes red eyes and when should you be worried?

Doctors tell The Straits Times that they most commonly treat red eyes caused by allergic or viral conjunctivitis. The resulting “pink eye” can last for weeks or months.

Dry eyes and wearing contact lenses for too long can also irritate the eyes, leading to redness.

Other causes of red eyes are eyelid infections or clogged oil glands at the eyelid margin.

Red eyes may also signal underlying infections or more serious health conditions. Here is what you need to know.

Doctors say that the eyes turn red when blood vessels in and around the surface of the eye become inflamed.

Blood flow to the conjunctiva or thin layer covering the white of the eye increases as part of the body’s immune response to viral or bacterial infection, or to allergens. As blood vessels dilate and become more prominent, the eye appears pink or red. This inflammation is known as conjunctivitis.

Blood vessels may also become inflamed in response to trauma, foreign bodies such as dust or dust mites, eyelid infection, or irritation from wearing contact lenses too long.

Sometimes a blood vessel below the conjunctiva may burst. This subconjunctival haemorrhage leads to a red patch in the white of the eye.

Consultant ophthalmologist Low Jin Rong of Asian Healthcare Specialists explains that the blood vessels in the eye are fragile and can burst spontaneously on sneezing or coughing, or lifting heavy loads.

People with high blood pressure, or who are on blood thinners, may be more prone to such burst blood vessels.

Senior consultant ophthalmologist Jean Chai from Eagle Eye Centre says that a subconjunctival haemorrhage usually resolves on its own in a couple of weeks.

Patients ma...