Dry Eyes - Tears

"Dry eye syndrome is a chronic lack of sufficient lubrication and moisture in the eye. Its consequences range from irritation to ocular inflammation of the anterior part of the eye.
Tears bathe the eye, washing out dust and debris and keeping the eye moist. They also contain enzymes that neutralize the microorganisms that colonize the eye. Tears are essential for good eye health.
In dry eye syndrome, the eye doesn't produce enough tears, or the tears have a chemical composition that causes them to evaporate too quickly.
Dry eye syndrome has several causes. It occurs as a part of the natural aging process, as a side effect of many medications, such as antihistamines, antidepressants, certain blood pressure medicines, Parkinson's medications, and birth control pills; or because you live in a dry, dusty or windy climate. If your home or office has air conditioning or a dry heating system, that too can dry out your eyes. Another cause is insufficient blinking, such as when you're staring at a computer screen

Dry eyes are also a symptom of systemic diseases such as lupus, rheumatoid arthritis, rosacea or Sjogren's syndrome (a triad of dry eyes, dry mouth, and rheumatoid arthritis or lupus).
Long-term contact lens wear is another cause; Dry eye syndrome makes contact lenses feel uncomfortable, and the rubbing of the lenses against the conjunctiva seems to be a cause of dry eyes.
Incomplete closure of the eyelids, eyelid disease and a deficiency of the tear-producing glands are other causes. Tears are composed of three layers: the outer, oily, lipid layer; the middle, watery, lacrimal layer; and the inner, mucous or mucin layer. Each layer is produced by a different part of the eye, so a problem with any of those sources can result in dry eyes.

Dry eye syndrome is more common in women, possibly due to hormone fluctuations. Recent research suggests that smoking, too, can increase your risk of dry eye syndrome. With increased popularity of eyelid surgery for improved appearance, dry eye complaints now occasionally are associated with incomplete closure of eyelids following a procedure."