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What are Seasonal Allergies?

By Leo Galland ; Updated August 14, 2017


An allergy is a misguided immune response to an external trigger called an allergen. Seasonal allergies are allergic reactions you experience to allergens that you’re exposed to during particular times of the year.

English general practitioner John Bostock made the first description of seasonal allergic symptoms, which was called “hay fever,” in 1819. It was considered to be a rare, new condition. Although its incidence increased in Britain during the 19th century, it was still quite unusual by the start of the 20th century.

The term “allergy” was coined by Viennese pediatrician Claude von Pirquet in 1906 to describe sneezing among schoolchildren exposed to pollen. He conceived the term to mean “altered reactivity,” a state that affected only a few people, but not most.

The Allergy Epidemic

Although some allergic phenomena were described in ancient medical texts, current seasonal hay fever appears to be a post-industrial disease, which has increased progressively over the past 200 years. Nasal allergy (allergic rhinitis), the most common manifestation of seasonal allergy, has become more prevalent in the present day, with a dramatic increase throughout the U.S. over the past few decades, rising from 10 percent of the population in 1970 to 30 percent by 2008.

Triggers for Seasonal Allergies

The most common triggers for seasonal allergies are pollen grains and mold spores. Your level of exposure to these varies not only with the season, but also with your location and the weather.

The continental United States can be divided into 10 geographical regions with distinct pollen patterns: Northeast, Southeast, Subtropical, Great Lakes, North Central, South Central, Rocky Mountains, Desert, Southwest Coastal and Northwest. As a general rule, trees in all regions tend to pollinate in late winter and early spring, grasses pollinate in mid to late spring and most weeds pollinate in summer. The farther south and the warmer the average annual temperature, the earlier that plants release pollen and the later the pollen seasons finish. Local pollen production will end with the first frost.

In any region, pollen exposure is greatest on clear, windy days, because wind spreads pollen grains. Ragweed pollen, for example, is the major late-summer allergen throughout much of the United States. Its grains have been found 400 miles out to sea and two miles high in the atmosphere due to wind currents. Wind may also spread pollen from one region to the next.

Mold spores, in contrast, may be found at any time of year. Their levels increase with temperature, so they usually peak in mid or late summer and are very low in the dead of winter. They are the main seasonal allergens in autumn. Mold spores have been divided into “dry” and “wet.” The dry molds behave like pollen. Their count is greatest in the afternoon and on windy days. The spore count for wet molds is greatest in the pre-dawn hours and in damp weather. Mold spore counts can reach their highest levels just before thunderstorms. These peaks have been associated with an increase in asthma attacks.

You can find out how much pollen and mold spores are in your area on any given day through the website of the American Academy of Allergy and Immunology’s National Allergy Bureau.

Many people with seasonal allergies are also affected by substances like house dust and dust mites, to which they are exposed year-round. Dust mites are microscopic insects that feed on organic matter in house dust, primarily human skin cells. They thrive in humid environments and soft places like mattresses, pillows, stuffed animals and upholstery. Their excretions contain some of the most potent indoor allergens. Because people spend comparatively more time indoors in the winter than in the summer, allergic symptoms that peak in the winter may be caused by an allergy to dust or mites.

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