Also known as hay fever or allergic rhinitis, seasonal allergies occur during certain seasons of the year--spring, summer or fall--when normally harmless airborne substances such as pollen and molds are released into the air. The immune system of a hypersensitive person mistakes the inhaled pollen or mold spores for foreign invaders and releases chemicals that trigger the symptoms of allergies. Some persons may be sensitive to more than one trigger and experience symptoms all year long.
Tree Pollen
Pollen allergy, one of the most chronic diseases in the United States, usually starts in the early spring when trees release pollen grains to reproduce. Trees such as oak, elm, ash, maple, hickory, alder, birch, juniper and olive have light, dry pollen that gets carried by the wind and triggers allergies. Although each tree pollinates at specific times each season, the warmer regions of the southern United States have earlier periods of pollination than the north. For example, the juniper mountain cedar is the most common cause of pollen allergy in the western United States from December to March.
Grass Pollen
From late spring to early summer, pollen from grasses such as Bermuda, timothy, sweet vernal, orchard and Johnson grass are the main causes of seasonal allergies. Like tree pollen, different grasses release their pollen at specific periods of the season. However, trees and grasses may pollinate at overlapping times and increase the total pollen count at any particular period. High pollen counts usually cause more severe symptoms and affect even less-sensitive individuals.
Ragweed Pollen
Pollen from ragweed is the major cause of seasonal allergies from late summer to late fall. Seventeen types of ragweed grow throughout the United States, and each plant is able to produce more than 1 billion pollen grains per year. Other weed species related to ragweed, such as sage, burweed marsh elder, rabbit brush, mugworts, groundsel bush and eupatorium, also cause seasonal allergies, according to the Asthma and Allergy Foundation of America.
Mold Spores
Mold spores are another common cause of seasonal allergies, triggering symptoms when the humidity is high. Outdoor molds are abundant in damp places such as soil, dead plants and rotting leaves, and indoor molds are often found in damp basements and bathrooms. Because molds can grow everywhere, persons allergic to mold may experience symptoms all year long. The most common molds that cause allergies are Alternaria, Cladosporium, Aspergillus, Penicillium, reports the AAFA.
