The sclera, or white part of the eye, is covered by a clear membrane called the conjunctiva. Sometimes the conjunctiva thickens, grows and encroaches onto the cornea, or clear tissue at the front of the eye - this is called a pterygium. Pterygia most commonly appear on the nasal (inner) sclera but can also appear on the outside. They are sometimes confused with cataracts, but cataracts are a clouding of the lens inside the eye whereas pterygia are on the outside of the eye. Pterygia are not a cancer or tumor, but are mainly caused by exposure to UV radiation so are more commonly seen in people who spend long hours outdoors and/or live in hot, sunny environments. They tend to exacerbate symptoms of dry eye. Wearing sunglasses when outdoors is perhaps the best way of avoiding pterygia; if they threaten to extend too far onto the cornea they are usually removed surgically under local anesthetic.