Dry eyes are caused by a lack of adequate tears. Your tears are a complex mixture of water, fatty oils, proteins and electrolytes. This mixture helps make the surface of your eyes smooth and clear, and it helps protect your eyes from infection.

For some people, the cause of dry eyes is an imbalance in the composition of their tears. Other people don't produce enough tears to keep their eyes comfortably lubricated. Eyelid problems, medications and other causes, such as environmental factors, also can lead to dry eyes.

Poor tear quality
The tear film has three basic layers: oil, water and mucus. Problems with any of these layers can cause dry eyes symptoms.

• Oil. The outer layer of the tear film, produced by small glands on the edge of your eyelids (meibomian glands), contains fatty oils called lipids. These smooth the tear surface and slow evaporation of the middle watery layer. If your oil glands don't produce enough oil, the watery layer evaporates too quickly, causing dry eyes. Dry eyes are common in people whose meibomian glands are clogged. Meibomian dysfunction is more common in people with inflammation along the edge of their eyelids (blepharitis), rosacea and other skin disorders.
• Water. The middle layer is mostly water with a little bit of salt. This layer, produced by the tear glands (lacrimal glands), cleanses your eyes and washes away foreign particles or irritants. If your eye produces inadequate amounts of water, the oil and mucus layers can touch and cause a stringy discharge.
• Mucus. The inner layer of mucus helps spread tears evenly over the surface of your eyes. If you don't have enough mucus to cover your eyes, dry spots can form on the front surface of the eye (cornea).

Decreased tear production
Dry eyes can occur when you're unable to produce enough tears. The medical term for this condition is keratoconjunctivitis sicca (ker-uh-to-kun-junk-ti-VIE-tis sik-uh).

You may not produce enough tears if you:

• Are older than 50. Tear production tends to diminish as you get older. Dry eyes are common in people older than 50.
• Are a postmenopausal woman. A lack of tears is more common among women, especially after menopause. This may be due in part to hormonal changes.
• Have a medical condition that reduces your tear production. Dry eyes are also associated with some medical conditions such as diabetes, rheumatoid arthritis, lupus, scleroderma, Sjogren's syndrome, thyroid disorders and vitamin A deficiency.
• Have had laser eye surgery. Refractive eye surgeries such as laser-assisted in-situ keratomileusis (LASIK) also may cause decreased tear production and dry eyes. Symptoms of dry eyes related to these procedures are usually temporary.
• Have tear gland damage. Damage to the tear glands from inflammation or radiation can hamper tear production.

Eyelid problems
Blinking spreads a continuous thin film of tears across the surface of your eyes. If you have an eyelid problem that makes it difficult to blink, tears may not be spread across your eye adequately or your tears may evaporate too quickly, causing dry eyes. Eyelid problems can include an out-turning of the lids (ectropion) or an in-turning of the lids (entropion).

Medications that cause dry eyes
Medications that can cause dry eyes include:

• Certain types of drugs used to treat high blood pressure, such as central-acting agents and diuretics
• Antihistamines and decongestants
• Birth control pills
• Certain antidepressants
• Pain relievers, such as ibuprofen (Advil, Motrin, others) and naproxen (Aleve)
• Isotretinoin-type drugs for treatment of acne

Other dry eyes causes
Other causes of dry eyes include:

• Wind
• High altitude
• Dry air
• Tasks that require concentration, such as working at a computer, driving or reading