Contrary to what some people believe, cataract (clouding of the normally clear lens of the eye) is not caused by a film blanketing the eye, nor is it related to overuse of the eyes. It does not spread from one eye to the other -- although the condition may develop in both eyes.
Aging and accompanying changes in the chemical composition of the lens are the most common causes. Many cataracts develop as an exaggeration of normal aging sclerosis, in which the lens becomes less resilient, less transparent, and often thicker. Fibers in the lens become compressed, and the lens becomes more rigid. Clarity fades as proteins coagulate, or clump together, creating tiny specks or wheel-like spokes in the periphery of the lens. In later stages, the milkiness becomes denser and occurs in the center, making it truly difficult to see. The change in the lens is similar to what happens when you cook an egg white; it goes from clear to opaque. Early on, before cataracts blur vision, they can cause nearsightedness, double vision, or distorted vision.
Although age is the factor most likely to cause cataract, other factors such as family history, eye injuries, use of some medications (particularly corticosteroids), and certain health problems (such as diabetes) can contribute to cataract as well. Several studies have linked cataract with alcohol consumption and smoking. Even if you have smoked for many years, quitting now will help lower the chances of cataracts forming in the future. Long-term exposure to high levels of ultraviolet-B (UVB) rays from the sun is another hazard, and studies have found a greater prevalence of cataract in people who live in areas with considerable sunlight. Wearing sunglasses can help protect eyesight and minimize cataract formation.