Seasonal allergies, often called hay fever or allergic rhinitis, are triggered by airborne substances, primarily pollen. This condition involves the immune system overreacting to harmless outdoor particles by releasing chemicals like histamine. This reaction causes familiar symptoms such as sneezing, nasal congestion, and itchy, watery eyes. The duration of symptoms is variable and depends entirely on which specific outdoor allergens a person reacts to and when those allergens are released into the air.
Defining the Allergy Season
While a single pollination period for one type of plant, such as ragweed, typically lasts between six and ten weeks, the overall “allergy season” is much longer. The total duration a person suffers from symptoms is extended because the pollen seasons for different plants occur sequentially and often overlap. In many parts of the United States, airborne pollen can be present from late February through October. This extended timeframe means individuals allergic to multiple types of pollen may experience symptoms for several consecutive months.
Duration Based on Specific Allergens
The specific timeline for seasonal allergies is determined by the three main categories of wind-pollinated plants. Tree pollen is the first to arrive, starting the season in late winter or early spring, generally running from February or March through May. Common contributors include oak, maple, birch, and cedar trees.
As tree pollen subsides, grass pollen takes over, peaking in late spring and throughout the summer months, typically from May through July. Grasses like Bermuda, Timothy, and Kentucky bluegrass are frequent causes of allergic reactions. The final wave is dominated by weed pollen, most notably ragweed, which begins releasing particles in late summer. Ragweed season generally runs from mid-August, peaks in September, and continues until the first hard frost, often lasting into late October or November.
Geographic and Climate Factors Affecting Length
The geography and climate of a region dramatically alter the beginning, end, and intensity of the allergy season. In warmer, southern climates, the growing season starts earlier and ends much later, extending allergen exposure to nine months or more; tree pollen may appear as early as December or January. Conversely, the first hard frost acts as a natural end for the fall weed pollen season across many temperate zones. Mild winters and early spring warm-ups accelerate the start of the season, effectively lengthening it by weeks. Furthermore, climate change has extended the pollen season in North America by an average of 20 days since 1990.
Distinguishing Seasonal from Year-Round Allergies
When symptoms persist for nine months or more, they are defined as perennial allergies, which are year-round problems. These are typically caused by indoor allergens present regardless of the outdoor growing cycle, such as dust mites, pet dander, indoor mold spores, and cockroach debris. Seasonal allergies are linked to outdoor pollen fluctuations, while perennial allergies are caused by persistent indoor exposure. If symptoms continue through the winter months when outdoor pollen is inactive, the cause is likely one of these indoor triggers. It is also possible to have both types, where year-round symptoms worsen during seasonal pollen peaks.