Hay fever affects up to 30% of all people worldwide, including up to 10% of U.S. children under 17 years of age and 7.8% of U.S. adults. The medical cost of allergic rhinitis is approximately $3.4 billion, mostly due to the cost of prescription medications. These figures are probably an underestimate because many of those affected may attribute their discomfort to a chronic cold. Although childhood hay fever tends to be more common, this condition can occur at any age and usually occurs after years of repeated inhalation of allergic substances. The incidence of allergic disease has dramatically increased in the U.S. and other developed countries over recent decades.

"Hay fever" is a misnomer. Hay is not a usual cause of this problem, and it does not cause fever. Early descriptions of sneezing, nasal congestion, and eye irritation while harvesting field hay promoted this popular term. Allergic rhinitis is the correct term used to describe this allergic reaction, and many different substances cause the allergic symptoms noted in hay fever. Rhinitis means "inflammation of the nose" and is a derivative of rhino, meaning nose. Allergic rhinitis that occurs during a specific season is called "seasonal allergic rhinitis." When it occurs throughout the year, it is called "perennial allergic rhinitis." Rhinosinusitis is the medical term that refers to inflammation of the nasal lining as well as the lining tissues of the sinuses. This term is sometimes used because the two conditions frequently occur together.

Runny nose
Itchy eyes, mouth or skin
Sneezing
Stuffy nose due to blockage or congestion
Fatigue (often reported due to poor quality sleep as a result of nasal obstruction

When you have hay fever, your immune system identifies a harmless airborne substance as harmful. Your immune system then produces antibodies to this harmless substance. The next time you come in contact with the substance, these antibodies signal your immune system to release chemicals such as histamine into your bloodstream, which cause a reaction that leads to the signs and symptoms of hay fever.