Climate MattersMarch 8, 2023

Allergy Season: Earlier, Longer, and Worse


  • Plants are leafing and blooming earlier, and the overall growing season is lasting longer across much of the U.S. 

  • Analysis of temperature data for 203 U.S. cities shows the freeze-free season lengthened by more than two weeks (15 days) on average since 1970.

  • For millions of Americans that suffer from seasonal allergies to pollen and mold, climate change is bringing an earlier, longer, and overall worse allergy season.

  • Climate Central’s new report Seasonal Allergies: Pollen and Mold details more of the weather and climate trends that are worsening allergy season and the associated health risks.

CM: Longer Growing Season 2023 (EN)
Click the downloadable graphic: Longer Growing Season

Warming climate, longer pollen season, worse allergies

The first leaves and blooms of spring are arriving days to weeks early in parts of the U.S., according to the USA National Phenology Network (USA-NPN). Some areas in the East and South are seeing the earliest spring on record. 

This is bad news for people with seasonal allergies—about one-quarter of adults (26%) and 19% of children in the U.S., according to the Centers for Disease Control and Prevention.

Earlier spring and longer periods of freeze-free days mean that plants have more time to flower and release allergy-inducing pollen. A recent study found that North American pollen seasons became longer (by 20 days on average) and more intense (21% increase in concentrations) from 1990 to 2018.

Seasonal allergies can already last from early spring through late fall. But warming temperatures and shifting seasonal patterns—both linked to climate change and greenhouse gas emissions—are expanding allergy season and its impacts on respiratory health

Climate Central’s new report, Seasonal Allergies: Pollen and Mold, details weather and climate trends that affect allergy season locally. 

CM: Longer National Growing Season 2023 (EN)
Click the downloadable graphic: Longer National Growing Season

Longer growing season across the U.S.

To analyze how the growing season has changed in the U.S., Climate Central assessed temperature data for 203 cities since 1970. 

  • The freeze-free season lengthened across the country by more than two weeks (15 days) on average.

  • 85% (172) of the cities saw their freeze-free seasons lengthen.

  • In 31 cities, the season between the last and first freeze grew by at least a month. 

  • The growing season in Reno, Nev., increased by 99 days—among the biggest increases in the country.

  • Since 1970, the freeze-free season lengthened the most in the West (27 days). 

  • The freeze-free season lengthened by more than two weeks in the Southeast (16 days), Northeast (15 days), and South (14 days). 

  • The Central region saw the freeze-free season lengthen by 13 days.

CM: Airborne Seasonal Allergens 2023 (EN)
Click the downloadable graphic: Airborne Seasonal Allergens

More than pollen: mold spores cause seasonal allergies, too. 

Plant pollen typically peaks in spring, summer, or fall, depending on the species and location. This video from researchers at the University of Michigan shows how pollen season blooms across the U.S. 

In addition to pollen, some molds (fungi that grow on soil and dead plants) can be allergenic. Different kinds of molds may release tiny spores throughout the year, but tend to peak in late summer and fall. 

For people who have both pollen and mold allergies, this can mean allergies that last for much of the year. Although outdoor mold isn’t as well-studied as pollen, climate change is likely affecting how both allergens impact people with allergies and asthma

Climate change is affecting allergy season in other ways.

Warming temperatures and more freeze-free days are key ways that climate change is affecting allergy season. But other connections between climate change and seasonal allergies are becoming clearer as research advances.

Climate Central’s new report, Seasonal Allergies: Pollen and Mold, details weather and climate trends that affect allergy season locally—including how increased carbon dioxide in the atmosphere boosts pollen production, and why thunderstorms can increase the risk of asthma attacks. 


Find local pollen and mold counts.

There are pollen and mold spore monitoring stations across the U.S. Local allergen counts and forecasts can be found through resources such as the National Allergy Bureau. State or tribal agencies for environmental protection or public health may also have relevant air quality reports.

See where your city ranks.

The severity of the allergy season varies across the country. The Asthma and Allergy Foundation of America (AAFA) releases an annual report on the Allergy Capitals in the U.S., ranking cities based on pollen scores, over-the-counter medicine use, and the availability of board-certified allergists. Check out how your city ranked on AAFA’s Allergy Capitals list in 2022—and the 2023 report is scheduled for release on March 15.


Lewis Ziska, PhD
Associate Professor, Environmental Health Sciences
Mailer School of Public Health, Columbia University Irving Medical Center
Relevant expertise: connections between climate change, carbon dioxide, plant biology, and public health
Media contact: Stephanie Berger, 

Jesse Bell, PhD (he/him/his)
Claire M. Hubbard Professor of Water, Climate and Health
College of Public Health, University of Nebraska Medical Center
Relevant expertise: extreme weather, climate change, and health

Brooke Lappe, MPH
PhD candidate
Rollins School of Public Health, Emory University
Relevant expertise: climate change, air quality, pollen and health


Find a local allergist or immunologist listed through the American Academy of Allergy, Asthma and Immunology.

Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts. 

Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.  

Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices. 

Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area. 


The growing season is the difference between the last day below 32°F from January through July and the first day below 32°F from July through December. Years with growing seasons of less than two weeks were dropped from the analysis (e.g., beginning June 30, ending July 3). This condition only impacted a handful of years in Bend, Ore. and Butte, Mont. Forty-four stations that did not have a regular growing season and were on average frost-free for most of the year were excluded completely.