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 the environment, such as from ultraviolet radiation (as in sunlight) and tobacco smoke.

As lens proteins are destroyed by free radicals, the lining (capsule) surrounding the lens may let excess water enter the lens and make it cloudy. This cloudiness of the lens is referred to as a cataract. Age-related cataracts usually progress slowly. But sometimes they progress rapidly.