Hay fever is caused by an airborne allergen coming into contact with the mucous membranes of the nose and sinuses, triggering an allergic reaction that involves the release of histamine. This in turn causes the mucous membranes to become inflamed and secrete large quantities of mucous.
Seasonal hay fever involves pollens and grasses, and consequently is more likely to occur during spring and summer, when increased levels of pollen are present in the atmosphere.
Perennial rhinitis involves other inhaled allergens such as pet hair, dust mite or mould, and symptoms tend to be constant throughout the year.