The fresh scents and floral delights of spring can be torture for many people for whom their arrival signals sneezing fits, tickly throats, and itchy eyes. And they’re all just on the horizon.
The dreaded allergy season has been extending over the past decades because of climate change. More carbon in the air is both lengthening the allergy season and intensifying the allergens. That’s bad news for about a quarter of American adults and a fifth of the country’s children who suffer from seasonal allergies—and it’s particularly extreme in some areas of the country.
In a new report from nonprofit Climate Central, researchers studied temperature data for 203 U.S. cities, from 1990 to 2018, finding that more carbon dioxide in the atmosphere is extending seasonal allergy periods. Winters are warming, and spring is arriving more quickly; according to the EPA, the final frost of the year in spring is occurring earlier than at any point since 1895.
The report found that 85% of U.S. cities had a longer spring and summer season—meaning plants are leafing and blooming earlier, extending the pollen season by 20 days on average across the U.S.
For instance, it’s extending the seasonal span of ragweed, a weed that’s part of the daisy family and the biggest culprit of fall allergies; one plant produces a billion pollen grains, which can spread hundreds of miles from the source. In some parts of the Midwest, ragweed season now starts in early August and extends through October—nearly three weeks longer than it used to be.
Reno, Nevada, has the most prolonged pollen season: 99 days longer in 2018 than in 1990. Lewis Ziska, one of the researchers and an associate professor in environmental health sciences at Columbia University, explains that higher-altitude places, like Reno, are now having more warmer days than before, so pollen is staying longer.
Not only is the season increasing, but so is the intensity of the allergens. More carbon dioxide is allowing plants to thrive through photosynthesis, so they produce more energy to grow larger and increase in number—and consequently produce more pollen. In the same time period, pollen concentrations have increased by 21%.
And pollen isn’t the only trigger. Many people are also sensitive to various molds that proliferate from July through the fall. Growing on rotting logs and fallen leaves, fungal spores are released in the air and cause allergies similar to pollen. In warmer and wetter conditions—including places with increased tropical storms—more spores are produced and they’re able to travel faster.
A city like Dallas, for example, which ranked second on a 2023 report of the worst U.S. cities for allergies, has all the triggering seasonal allergens. It has pollen-producing trees in the spring, and grass and mold in the summer, all of which spread widely due to the hot and humid weather. In the fall, there’s ragweed, which can last for months. And in the winter: “cedar fever,” the nickname for pollen from mountain cedar trees, which are indigenous to Texas and have the largest pollen counts in the state.
These increased allergens mean sufferers feel all the familiar symptoms, including sneezing and itchiness. But the health effects could be much more pronounced for those with asthma or respiratory problems, which the pollen and mold can aggravate. Ziska himself has extreme asthma. “When I’m out, particularly in the spring,” he says, “I have to always have my inhaler with me to make sure that I don’t go into a full-blown asthma attack.”
The trend could continue to worsen in the future if carbon levels remain high. Another study that modeled future pollen production predicted it could double by the end of this century, and that the season could start up to 40 days earlier.
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