Morning Chronicle - Climate Change Extends U.S. Allergy Season by Weeks, Worsening Health Risks

A warming climate is extending the duration and intensity of allergy seasons across the United States, creating prolonged exposure to pollen for millions of people with seasonal allergies and asthma. According to a new analysis by Climate Central, rising temperatures have lengthened the freeze-free growing season in 83% of analyzed U.S. cities since 1970, leading to earlier spring blooms and later fall pollen release.

The organization’s assessment of temperature trends in 197 U.S. cities found that the freeze-free season—the period between the last spring freeze and the first autumn freeze—lengthened by an average of 19 days across 164 cities where data was complete. In 60 of these cities, the growing season expanded by at least three weeks.

The expansion varied significantly by location. Reno, Nevada, saw the largest increase with 95 additional freeze-free days, followed by Las Cruces, New Mexico, with 65 more days; Medford, Oregon, with 61 days; Boise, Idaho, with 51 days; and Tupelo, Mississippi, with 50 days. All nine major U.S. climate regions experienced an average lengthening of at least 11 days since 1970. The Northwest region saw the most substantial regional increase, with the freeze-free season extending by more than two weeks on average, specifically 26 additional days. The Southeast and Northeast each saw increases of 15 days, while the Upper Midwest experienced an average gain of 14 days.

These climatic shifts are directly linked to earlier arrivals of spring leaves and blooms in large parts of the southern and central United States, according to the USA National Phenology Network. Carbon pollution, primarily from burning coal, oil, and natural gas, traps heat in the atmosphere, contributing to these warming trends. Warmer temperatures allow plants more time to grow and release pollen earlier in the spring and later into the fall.

Higher levels of carbon dioxide (CO2) in the air further exacerbate the issue by boosting pollen production in plants, particularly grasses and ragweed. A recent study cited by Climate Central projects that with continued high rates of CO2 pollution, the U.S. could face up to a 200% increase in pollen production by the end of this century.

The health consequences of these changes are significant, particularly for children. Approximately one-quarter of adults and one-in-five children in the U.S. suffer from seasonal allergies. Around 19% of children experience symptoms such as sneezing, coughing, itchy or watery eyes, and runny noses. Pollen is also a major trigger for asthma, which affects 6.5% of U.S. children. According to the Asthma and Allergy Foundation of America (AAFA), allergic asthma is the most common type of asthma and is most prevalent in early childhood.

Seasonal allergies and asthma can negatively impact children’s quality of life, affecting school performance, mood, and sleep, particularly in adolescents aged 10 to 19. The AAFA notes that Black and Puerto Rican children are between two and three times more likely to have asthma than white children, placing them at higher risk for allergic asthma.

The U.S. Environmental Protection Agency (EPA) has analyzed the potential future impacts of continued warming on these health outcomes. Their analysis found that 2°C (3.6°F) of warming could result in a 17% annual increase in asthma-related emergency room visits among children due to pollen exposure, relative to baseline conditions from 1986 to 2005. The projected increases were largest in Indiana, Kentucky, Ohio, Vermont, and West Virginia.

The EPA analysis also highlighted disparities in exposure, finding that oak pollen exposure disproportionately impacts Hispanic, Asian, and Black children, as well as low-income, limited English-speaking, and uninsured children.

Climate Central’s report, "Seasonal Allergies: Pollen and Mold," reviews the weather and climate trends affecting pollen seasons and related health risks. Their fact sheet, "Climate Change & Children’s Health: Seasonal Allergies," details how these trends affect children’s well-being. A recent study found that human-caused warming was a primary driver of North American pollen seasons lengthening by an average of 20 days from 1990 to 2018.

Experts advise parents and caregivers to take specific steps to protect children’s health amid these changing conditions. Recommendations include responding to symptoms rather than relying on traditional seasonal calendars, as growing seasons expand and shift. Caregivers are encouraged to check local air quality reports and allergen forecasts before outdoor activities and limit time outside when pollen concentrations are high.

Indoor safety measures are also critical. Closing windows and doors can minimize aeroallergens inside homes, while using high-efficiency particulate air (HEPA) filters in living spaces and bedrooms can help clean the air. Furthermore, experts emphasize the need for rapid, sustained cuts to carbon pollution from burning fossil fuels, as reducing CO2 levels will bring benefits to allergy sufferers of all ages by curbing pollen production.

Pollen and mold spore monitoring stations operate across the U.S., with local allergen counts and forecasts available through resources such as the National Allergy Bureau. State or tribal agencies for environmental protection or public health may also provide relevant air quality reports. The severity of allergy seasons varies geographically; the AAFA releases an annual report on "Allergy Capitals" in the U.S., ranking cities based on pollen scores, over-the-counter medicine use, and the availability of board-certified allergists.

The data underpinning Climate Central’s analysis was derived from daily minimum temperature records from 1970 to 2023 obtained from the Applied Climate Information System. The length of the annual freeze-free season was determined based on the annual count of consecutive days with minimum temperatures above 32°F (0°C). Years with freeze-free seasons of less than two weeks were excluded from the analysis. Of 247 total stations assessed, 44 had an average growing season length of less than three months or a year-round lack of sustained freeze and were therefore excluded. Reported long-term trends are based on linear regression.

The report is authored by Lisa Patel, Clinical Associate Professor of Pediatrics at Stanford Medicine Children’s Health and Executive Director of the Medical Society Consortium on Climate and Health; Susan Anenberg, Professor and Chair of the Environmental and Occupational Health Department at George Washington University; and Lauren Gentile, Geographer in the EPA’s Climate Change Division. The content is licensed under CC BY 4.0 by Climate Central.

M.Brown--MC-UK