As you age, proteins normally found in the lens of the eye can be
slowly destroyed by certain chemicals called free radicals. These chemicals are
formed during normal cell function (metabolism). Increased levels of free
radicals can also be caused by environmental factors, such as ultraviolet
radiation (as in sunlight) and tobacco smoke.
As lens proteins are destroyed by free radicals, the lining (capsule)
surrounding the lens may allow excess water to enter the lens and cause it to
become cloudy. This cloudiness of the lens is referred to as a cataract.
Age-related cataracts usually progress slowly, but occasionally they progress
rapidly.