Allergies affect over 20% of the general population and are on the rise in many developed countries. The specific reason for the increasing prevalence of allergies has not been determined; some believe it is due to the effects of pollution in urban areas while others consider that modern lifestyles don't allow for enough childhood exposure to allergens to become acclimated to them.
Ocular allergies are often accompanied by allergic rhinitis (nasal symptoms). This is partly due to the fact that the eyes and nose are so closely connected by the tear ducts; substances that enter the eye can then flow downward into the nose. Symptoms include:
When exposed to these allergens, the response in the eye is nearly immediate: First, itching sets in and can become persistent and extremely bothersome if exposure to the allergen is continued. Shortly after itching, redness of the eyes follows and can last somewhat longer, as the blood vessels of the eye are slower to recover. The swelling of the eyes and eyelids comes to its peak later than the itching, as swelling is also caused by the blood vessels letting fluids out into the tissue around them. Swelling is the sign of ocular allergies slowest to disappear, due to the time it takes for the displaced fluid to be removed. The itching can be considered the most uncomfortable symptom, whereas the swelling and redness are most bothersome in terms of appearance.