Climate Change Intensifies Hay Fever As Pollen Seasons Grow Longer, Stronger

Climate change is amplifying the global burden of hay fever, according to a new review published in The Laryngoscope.

Health professionals have observed a steady rise in allergy symptoms, and the new findings support a growing consensus: rising temperatures and higher carbon dioxide levels are extending pollen seasons and intensifying allergic reactions.

In industrialized nations, hay fever—also known as allergic rhinitis (AR)—is increasing by 2% to 3% annually, leading to significant health care costs and lost productivity.

In North America, the spring pollen season now starts up to 20 days earlier than in previous decades.

The new study is a scoping review, examining scientific literature from 2000 to 2023 to determine how climate change affects hay fever severity and season length.

Researchers identified 30 studies that met strict criteria, focusing on climatological mechanisms linked to worsening allergy symptoms.

“We were very specific in our inclusion and exclusion criteria,” said Alisha Pershad, a third-year medical student at George Washington University and the study’s lead author.

“By minimizing variability in our included studies, we were able to improve the strength of our conclusions.”

More than half of the reviewed studies reported longer pollen seasons or increased pollen concentrations directly linked to climate change.

One US-based projection estimated that pollen emissions could rise by 16% to 40% by 2100, with pollen seasons lengthening by an additional 19 days beyond the current 20-day increase.

Five studies found that these changes primarily occur at the beginning of the season.

In Europe, ragweed pollen—known as Ambrosia—is also expected to rise due to rising temperatures.

Specific studies further solidify the connection.

A 2021 study from Australia found that from 2016 to 2020, daily maximum temperatures, atmospheric CO₂ levels, and the grass pollen index were all higher compared to the period between 1994 and 1999.

A 2017 European model projected that between 2041 and 2060, the number of people allergic to ragweed could grow from 33 million to 77 million, as the plant spreads into new areas.

In China, a 2025 study found increasing pediatric outpatient visits for AR, corresponding with higher peak pollen levels.

Children are especially susceptible due to greater outdoor exposure, higher respiration rates, and more frequent mouth breathing.

The studies also examined underlying mechanisms linking climate change to allergy severity.

Two studies—one field-based and one lab-based—found that higher humidity and carbon dioxide levels boost allergenic pollen dispersal, while increased rainfall helps reduce pollen levels by cleansing the air.

Another study showed that the mold allergen Aspergillus thrives under current CO₂ concentrations compared to pre-industrial levels.

Not all populations are affected equally.

Environmental and socioeconomic disparities mean that Black and Hispanic communities, low-income groups, and older adults are more vulnerable.

Urban residents also face greater risks, despite fewer trees and plants, due to the urban heat island effect caused by concrete and asphalt.

“Communities historically impacted by environmental inequities such as red-lining tend to live in regions that experience warmer daytime temperatures,” said Pershad.

Mold exposure is another concern, particularly in low-income areas.

“(Mold) is especially a concern for lower income communities that may not have the resources to address the water damage to their home as quickly as necessary to avoid mold growth,” she added.

“Global warming is exacerbating weather extremes such as hurricanes and flooding, which increase the risk of mold growth, a common environmental allergen.”

Health professionals are increasingly aware of the trend.

A 2022 study from Italy found that 56% of pulmonologists reported an earlier and longer pollen season, 45% saw an increase in AR patients, and 61% observed a rise in pediatric cases.

Nearly all—97%—expressed a desire to learn more about climate change’s role in hay fever.

“Physicians are uniquely positioned to witness the impact of allergic rhinitis on patient outcomes and can adapt their practice as climate change intensifies,” said Pershad.

“As trusted voices in the community, they should leverage their frontline experience to advocate for meaningful change in addressing the climate crisis.”