Conjunctivitis has many causes, all of which affect the clear lining that covers the white of the eye. Infection from bacteria or viruses causes bacterial and viral conjunctivitis, respectively.

Allergic conjunctivitis is a reaction to some type of allergen. When the eye comes into contact with a substance to which a person is allergic, histamine is released from mast cells within the eye, causing blood vessels in the membrane covering the eye to swell. A large percentage of those who suffer from allergic rhinitis also develop associated allergic conjunctivitis.

Common allergens that trigger allergic conjunctivitis include:

• Pollen. Small, powdery grains of flowering plants that can easily become airborne. These are most often produced by trees, grass and weeds.
  

  
  

• Dust. Mixture that contains tiny particles of soil, plant material, human and animal skin, hair, fabric fibers and insect parts.
  
  
• Dander. Small scales or flakes of dead skin cells that are continually shed by all animals.
  
  
• Molds and mildews. Tiny fungus spores that can become airborne. These often thrive outdoors in soil, vegetation and rotting wood. They can be found indoors as well, especially in damp areas, basements and bathrooms.

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• Eye drops/contact lens solutions. Preservatives in these formulas sometimes can make individuals more sensitive to an allergic reaction.
  
  
• Other irritants. Irritants known to trigger allergic conjunctivitis include tobacco smoke, chalk dust and perfume. Avoid these whenever possible.