There are numerous conditions that can cause eye pain. Some causes may be apparent, such as eyestrain or dust, an eyelash or another foreign body in the eye, or being splashed with an irritant. Other potential causes of eye pain may be divided into certain categories:

• Allergies. An allergy is an excessive response by the body’s immune system to some substance. Many allergies produce eye pain because the conjunctiva (mucous membrane that lines the eyelid) contains mast cells. Allergens trigger mast cells to release a substance called histamine, which enlarges blood vessels and causes itching and watery eyes.
  
  
• Injuries and burns. Any trauma to an eye can cause pain. Chemical irritants, juices, shampoo or chlorine can all cause redness and pain. More serious chemicals can damage the cornea. Overexposure to light may cause painful eye burns. This may occur when people do not wear safety goggles for such activities as welding or using tanning booths. Even exposure to strong indoor light, such as stage lights, can dry the eyes. A blow to the eye can cause an orbital bone fracture, black eye and/or bruising of the eye socket.
  
  
• Corneal abrasions. The cornea is extremely susceptible to painful abrasions. Its concentrated set of highly sensitive nerves can create extreme pain that may spread to other parts of the head, such as around the eye socket and upward to the forehead. If a corneal abrasion is superficial but becomes infected, it may lead to a corneal ulcer, which is a far more serious condition. Corneal abrasions may be caused by a foreign object in the eye or by contact lenses that are older, are poorly cleaned or do not fit well.
  
  
• Infections. Infections may attack many parts of the eye and can cause inflammation. Some originate in the eye. Others may be the result of other conditions. Some common painful eye infections include:
  
  
• Conjunctivitis. Bacterial or viral inflammation of the conjunctiva, the membrane that lines the eyelid and sclera. This inflammation reddens the sclera and may swell the eyelid. It is extremely contagious and is usually spread from one eye to the other by touching.
  
  
• Stye (hordeolum). Gland inflammation that produces a bump on the edge of the eyelid.
  
  
• Orbital cellulitis. Infection and/or inflammation of the tissues surrounding the eye (lids, eyebrow and cheek). Sometimes it has spread from the sinuses, though usually from the overlying skin.

Several viruses related to herpes may cause eye pain and keratitis (inflammation of the cornea). These include the varicella zoster virus, which causes chickenpox and shingles, and the herpes simplex virus 1, which causes cold sores on the mouth and lips. Risk factors for fungal keratitis such as Fusarium include trauma, an impaired immune system and, in rare cases, use of contact lenses, according to the American Academy of Ophthalmology.

• Dry eye. This condition develops when the body is no longer able to produce enough tears to adequately moisten, protect and cleanse the eyes, or the tears made are not good ones, or the lids and orbit do not spread the tears well over the surface of the eye.

Common causes of dry eyes are the normal aging process and autoimmune conditions such as Sjogren’s syndrome. Certain medications may cause dry eyes, including oral contraceptives (birth control pills), antidepressants, antihistamines, decongestants, anti–anxiety agents, and diuretics or other blood pressure pills.

• Blepharitis. Inflammation of eyelash follicles and skin causes crusty, swollen eyelids and burning and itching eyes.
  

• Reiter’s syndrome. This type of arthritis causes uveitis, inflammation of the uvea.
  
  
• Temporal arteritis. This condition involves inflammation of arteries in the body. It can present with inflammation and pain in the temporal artery in the scalp, where a biopsy can aid in diagnosis. Temporal arteritis often causes eye pain and may cause visual impairments such as blurred or double vision. It can cause a devastating optic neuropathy (arteritic anterior ischemic optic neuropathy) in the elderly.
  
  
• Glaucoma. Most forms of this eye disease, which causes damage to the optic nerve, involve a gradual, painless loss of vision. However, there is also a less common type called angle-closure or narrow-angle glaucoma. The acute form of angle-closure glaucoma usually involves sudden, excruciating eye pain and can cause blindness within a day or two if not treated by laser or surgery. The chronic form of angle-closure glaucoma has less apparent symptoms but should also be treated promptly.
  
  
• Growths or lumps. Some conditions that appear as bumps in the eye area may cause eye pain. A chalazion is a lump caused by a blocked oil gland that irritates the eye. Tumors in the soft tissues behind the eye may put pressure on nerves and cause eye pain. Eye cancer is rare, and only rarely and usually at an advanced stage does eye cancer cause pain.
  
  
• Optic neuritis. An inflammation of the optic nerve is often due to multiple sclerosis, a progressive neurological disease. It can also result from other conditions, including Lyme disease, viruses, syphilis or tuberculosis.
  
  
• Juvenile rheumatoid arthritis. This inflammatory joint condition often inflames the eyes. The child will often not experience eye pain, but vision loss  oftenoccurs nonetheless.
  
  
• Sarcoidosis. This is a disease in which inflammatory lumps (granulomas) develop in organs. Sarcoidosis can produce eye pain and lead to uveitis, cataracts or glaucoma.
  
  
• Behcet’s disease. An autoimmune disorder that often involves inflammation inside the eye.
  
  
• Fuch’s dystrophy. A slowly progressive eye disease in which the cornea swells painfully in late stages.
  
  
• Stevens-Johnson syndrome. This skin disorder, also called erythema multiforme major, can also cause painful inflammation in the eyelids.
  
  
• Ciliary muscle spasm.
  
  
• Improper prescription for corrective lenses or improper use of cosmetic lenses.
  
  
• Cranial neuralgias. In addition to causing facial pain, inflammation of the cranial nerves in the head can, in rare cases, cause eye pain as well.