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Your Top Fall Allergen Questions, Answered
As the leaves start changing and the air becomes cooler, allergens are released right under our noses. Autumn brings not only seasonal allergens but also a slew of questions.
We consulted the Asthma and Allergy Foundation of America (AAFA) to tackle common queries about fall allergy season, including regional effects and duration in regions across the United States.
What’s behind the seasonal sneezes?
Fall allergies are primarily caused by airborne plant pollen, and when it comes to pollen allergies, weed pollen is the overwhelming culprit. These are a few plants that cause the most allergy symptoms in the fall:
- Ragweed¹
- Burning bush
- Cocklebur
- Lamb’s-quarters
- Pigweed
- Sagebrush
- Mugwort
- Tumbleweed
- Russian thistle¹
Ragweed is one of the most prevalent, growing wild in 49 states.¹ In fact, in one season, a single ragweed plant can produce up to 1 billion pollen grains that can easily travel long distances in the air.²
In addition to weed pollen, mold spores can also run rampant through the early fall. Mold thrives on rotting wood and fallen leaves, so mold counts tend to be higher in the fall season. Outdoor mold spores travel either through the air in places with dry, windy fall weather or with fog or dew in places with high humidity.³
When the body’s immune system is met with an allergen like any of the ones mentioned above, it treats it as a harmful invader. As a result, your body produces histamines that cause allergy symptoms.⁴
When does fall allergy season hit in different regions, and how long does it last?
Weed pollen: Weed pollen season generally appears in the late summer and persists into the fall. For most of the United States, ragweed typically blooms and releases pollen starting in August through as late as November, with the highest pollen levels occurring in early to mid-September. However, ragweed can start pollinating as early as July in some southern states.⁵
Mold: In the northern United States, outdoor mold growth begins after the first spring thaw and peaks later in the summer and into fall. In western and southern areas of the U.S., outdoor mold grows year-round.¹ Unlike pollen, molds do not die with the first killing frost. Most outdoor molds become inactive during the winter however the “seeds,” called spores, travel through the air.⁶
Are fall allergies more severe in certain regions?
Yes, the 20 most challenging places to live in the U.S. with pollen allergies are spread across the Southwest, Midwest and Southeast regions, according to this year’s Allergy Capitals report. Ragweed, considered one of the most allergenic weeds, grows in every state except Alaska and is most prevalent in the Midwest and on the East Coast.⁶
While the severity and timing of fall allergy season varies across the country, the following factors can also influence allergen impact:
- Pollen levels tend to peak in the morning.
- Pollen counts rise on warm and windy days.
- High humidity and heat makes mold grow faster.
- Grass, tree and weed pollens flourish during warm days and cool nights.¹
Sources
1. “Pollen Allergy.” Asthma and Allergy Foundation of America. 2022.
2. “Climate Change Indicators: Ragweed Pollen Season.” EPA. 2023.
3. “Mold Allergy.” Asthma and Allergy Foundation of America. 2022.
4. “Fall allergen.” Harvard Health Publishing. 2023.
6. “2024 Allergy Capitals.” Asthma and Allergy Foundation of America. 2024.(PDF, 3.0 MB)
