The most important allergens that cause allergic rhinitis are airborne substances. These airborne allergens can be divided into those that are present only seasonally versus those that are present year-round (also called “perennial”). Seasonal allergens are generally present in the outdoor environment and include tree, grass, and weed pollens as well as outdoor molds. In the United States, tree pollen can usually be found in the early spring (February through April), grass pollen in the summer months (May, June, and July), and weed pollen in the late summer and early fall (August, September, and October). All of these plants produce small and light pollen granules that can be carried through the air for miles as compared with fruit- and flower-bearing plants, which rely upon insects for pollination.
Outdoor molds, the most common of which are Cladosporium and Alternaria, can often be found in the outdoors in high levels throughout the year, but peak levels usually occur in the late summer and fall, particularly in the Midwest. Perennial allergens are usu-ally found indoors and include house dust mites, animal fur or skin, indoor molds (like Aspergillus and Penicil-lium), and cockroaches. Food allergy is not a common cause of nasal allergies and is more likely an issue in young children who also have eczema. However, certain foods and beverages, particularly alcohol, may cause vasodilation of the blood vessels in the nose, which leads to nasal congestion.
Vasodilation
The widening of blood vessels.