Seasonal allergic rhinitis, often referred to as hay fever, affects about 10 percent of the U.S. population. Substances called allergens, of which the most common is pollen, cause this seasonal allergy. A wide variety of grasses, trees and weeds release pollen in the spring and fall, causing the runny nose and itchy, watery eyes associated with hay fever. Depending on where they live, different people will be allergic to different pollens at different times of the year.
There are thousands of types of molds, but only a few are recognized as allergenic. The spores, or reproductive parts of the mold, are carried in the air and can cause allergic rhinitis in some people. While not seasonal, molds are affected by wind, rain and temperature. They typically flourish in moist, shady areas outdoors and in damp basements, bathrooms or closets in the home. Molds in certain foods, such as cheese, can also trigger symptoms.
A dust mite is a microscopic organism found in all homes and workplaces. House dust mites live in bedding, upholstered furniture and carpets. Certain proteins in the mites and their waste products provoke an allergic reaction in some people. Similarly, proteins in the saliva and waste products of cockroaches are also known to be allergenic. All of these allergens are carried by the air and, when breathed in, can trigger a reaction.