In others, it occurs when the tear film, the natural fluid covering of the eye, is altered in some way. Dry eye also can occur when the eyelid cannot be completely closed. As you get older, particularly after 40 to 50 years of age, the eyes also are generally drier because tear production is lower. Irritants and allergens in the air, although not believed to be causative, also can worsen dry eye. Your eyes have two main protective mechanisms-blinking and tears.
Tears are one of the body's natural defense mechanisms.
Each time you blink, a protective coating of tears is spread like a film over the front of the eye. This tear film carries essential nourishment to the eye and also acts as a shield to protect it against wind, heat, smog, dust, or other foreign objects. The tear film is a built-in lubricating and cleansing system. It has three distinct fluid layers. The outer layer contains fatty substances, which keep the fluid that is in close contact with the eye from evaporating too quickly.