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.