Year-round or perennial allergies results from exposure to substances like dust from inside the home are present throughout the year.
Perennial allergies may occur at any time of year, unrelated to the season or may last year-round. Perennial allergies are often a reaction to house dust. House dust may contain a number of things like mold and fungal spores, animal dander, or dust mites. Substances in and on cockroaches are often the cause of allergic symptoms. These substances are present in houses year-round but may cause more severe symptoms during the cold months when more time is spent indoors.
Seasonal allergies happen due to an exposure to airborne substances such as pollens that appear only during certain times of the year.
Seasonal allergies or hay fever are common. They occur only during certain times of the year, particularly in the spring, summer, or fall depending on what a person is allergic to.
The term hay fever is somewhat misleading because symptoms do not occur only in the summer when hay is traditionally gathered and do not include fever. Hay fever is usually a reaction to pollens and grasses. The pollens that cause hay fever vary by season:
- Spring: Usually trees such as oak, elm, maple, alder, birch, juniper, and olive
- Summer: Grasses such as Bermuda, timothy, sweet vernal, orchard, and Johnson grass. Weeds such as Russian thistle and English plantain
- Fall: Ragweed
Also, different parts of the country have very different pollen seasons. In the western United States, mountain cedar (a juniper) is one of the main sources of tree pollen from December to March. In the Southwest, grasses pollinate for much longer, and in the fall, pollen from weeds, such as sagebrush and Russian thistle, can cause hay fever. People may react to one or more pollen, so their pollen allergy season may be from early spring to late fall. Seasonal allergy is also caused by mold spores, which can be airborne for long periods of time during the spring, summer, and fall.