Climate MattersMarch 24, 2021

Pollen & Allergy Season

Pollen & Allergy Season


  • The dreaded allergy season is upon us and climate change will only make it worse for allergy and asthma sufferers. 

  • Rising temperatures from human-induced climate change are the dominant contributor to recent changes in the timing and length of the pollen season in North America. Over the last half-century, the growing season (and the pollen season) lengthened in 82% of the cities Climate Central analyzed.

  • The amount of carbon dioxide in the atmosphere also directly impacts pollen concentrations because it can stimulate plant growth. As carbon dioxide levels rise due to the burning of fossil fuels, pollen concentrations will increase and the allergy season will worsen.

  • Seasonal allergies can have serious consequences for those with respiratory problems like asthma. A longer allergy season and higher pollen concentrations will put a burden on the most vulnerable populations, including lower-income and minority communities.

CO2 and Pollen - Pollen & Allergy Season
CO2 and Pollen
Longer Growing Season - Pollen & Allergy Season
Longer Growing Season

As springtime arrives, so does the dreaded allergy season. According to the Centers for Disease Control and Prevention (CDC), more than 25 million Americans have seasonal pollen allergies. Millions of people are suffering every year, and climate change is only making it worse. 

According to a new study, rising temperatures from human-induced climate change are the dominant contributor to recent changes in the timing and length of the pollen season in North America. Spring-like temperatures are arriving earlier in the year, and fall-like temperatures linger later, therefore, the growing season (the time between the last spring freeze and first fall freeze) and pollen season are getting longer.

Climate Central examined the length of the growing season for more than 200 locations. Over the past half-century, the season lengthened in 82% (166 of 203) of the places analyzed. It also extended by at least four weeks in 38 of those places, with Bend, Ore. and Reno, Nev. tying for first place with 99 additional growing season days.

But the impacts of climate change on seasonal allergies don't just change the start time or length of pollen season, they also intensify it. The amount of carbon dioxide in the atmosphere directly impacts pollen concentrations because it can stimulate plant growth. One study reported that grass pollen levels doubled when carbon dioxide levels were increased from current levels (about 400ppm) to 800ppm. Such high levels of carbon dioxide, and pollen, are possible at the end of the century if current emissions trends continue and new mitigation efforts are not implemented. However, new analyses suggest that the US can meet the goals of the Paris Climate Accord and reach net-zero emissions by mid-century, leading to fewer additional allergy and asthma attacks.

Many people may experience allergies as a minor inconvenience, but seasonal allergies can have serious consequences and decreased quality of life for those with respiratory problems like asthma. About 60% of the 25 million Americans with asthma have allergic asthma一where pollen can trigger an asthmatic attack. With a longer pollen season and high pollen concentrations, asthma and allergy reactions can become even more severe and expensive to treat一and unfortunately this burden is placed on the most vulnerable populations. In the U.S., the total cost of allergies is more than $18 billion a year and the medical cost of asthma is just above $3,200 per year per person. Effective medications and therapy to manage symptoms can be a burden on lower-income families, especially since asthma is more prevalent in families living below the poverty line. Currently, the highest asthma diagnoses, hospitalizations, and deaths are disproportionately found in minority communities (Black, Hispanic, and American Native/Alaskan Native) due to a combination of determinants. One example is discriminatory housing policies restricting minorities to older, less ventilated homes and neighborhoods with elevated levels of air pollutants.


What are pollen concentrations like in your area? Which plant species are producing the pollen?
Nearly 25 million people in the U.S are estimated to have asthma, and research suggests that climate change might already be contributing to longer pollen seasons and increased pollen concentrations. You can search for local pollen counts and sign up for pollen level alerts through the American Academy of Allergy, Asthma, and Immunology. You can also use the Allergy and Botany Research Library to discover which plant species contribute to pollen reactions in your area. 

Are you located in an allergy capital? 
The severity of the allergy season varies across the country. The Asthma and Allergy Foundation of America conducted a 2021 report on the top 100 Allergy Capitals in the U.S., ranking cities on spring and fall pollen scores, over-the-counter medicine use, and the availability of board-certified allergists. Check out where your city placed on the worst allergy capital list. 

How are certain racial and enthic groups in my area disproportionately affected by a longer allergy season?
Check out the AAFA’s recent report on Asthma Disparities in the U.S. You can also look at specific infographics and statistics on Black, Hispanic, and Indigenous American populations.


The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on the extended pollen and allergy season in your area. The American Association of State Climatologists is a professional scientific organization composed of all 50 state climatologists. In addition, the American Academy of Allergy, Asthma, & Immunology has a directory of allergists and immunologists across the country. Other health professionals and experts can be found on The Medical Society Consortium on Climate and Health.


  • Dr. Lewis Ziska, Ph.D.
    Environmental Health Sciences, Columbia University Irving Medical Center
    Contact: Timothy Paul,

  • Dr. Jennifer Albertine, Ph.D.
    Visiting Lecturer in Environmental Studies, Mount Holyoke College 

  • To get in contact with other asthma and allergy experts:
    Asthma and Allergy Foundation of America (AAFA)


The length of the growing season is the number of days between the last spring freeze and first fall freeze. Locations were only included in the analysis if they had a “freeze season” of at least 90 days of a temperature minimum less than or equal to 32°F. As a result, 204 stations are included in the analysis. Wheeling, W. Va was removed from any summaries statistics due to large data gaps. 

Dr. Jennifer M. Albertine and colleagues conducted experiments where they grew Timothy grass under different carbon dioxide concentrations. Their analysis suggests that pollen production increases linearly with carbon dioxide levels. We used their published relationship to estimate the amount of pollen in 2000 (397ppm CO2), 2020 (419ppm CO2), and in 2040, 2060, and 2080. For the future periods, we used CO2 concentrations from the high-emissions scenario (RCP8.5), the reduced emissions scenario (RCP4.5), and a scenario with aggressive emissions reductions (RCP2.6). 

Reference: Albertine JM, et al. (2014) Projected Carbon Dioxide to Increase Grass Pollen and Allergen Exposure Despite Higher Ozone Levels. PLoS ONE 9(11): e111712. doi:10.1371/journal.pone.0111712