Allergies hitting early? Climate change may be to blame, report says.

Norberto Perez was interviewed for a story about a poll we commissioned with MassInc on Massachusetts' views on climate change. He says he thinks about climate change a lot because it made his allergies debilitating.Jonathan Wiggs/Globe Staff

INTO THE RED: Climate and the fight of our lives

Scientists have long warned that climate change is making allergy season worse. A new study shows just how much more brutal it has become.

As the world warms as a result of burning fossil fuels, winters across much of the United States are getting shorter and springs longer, giving plants more time to grow, flower, and release pollen. In other words, allergy season is lengthening thanks to climate change, and the Wednesday analysis by climate research nonprofit Climate Central found exactly by how much: 15 days on average since 1970.

“I think a lot of times people think climate change is this very nebulous entity,” said Climate Central meteorologist Lauren Casey. “But it’s happening now, and it’s impacting ... our health on a daily basis.”

The northeast has seen growing season — the period between the last freeze of spring and the first freeze of fall — lengthen by the same amount of time as the rest of the nation, about 15 days on average, according to the new study, based on data from 203 US cities.

In Boston, allergy seasons have gotten 13 days longer since 1970. Some cities saw even more severe changes.

In 31 cities, the season between the last and first freeze grew by at least a month, and Reno, Nev.’s season increased by a stunning 99 days.

“When we have warm weather earlier in the year, buds open earlier,” said Richard B. Primack, a biology professor at Boston University who focuses on climate change and did not work on the new report. “And allergy season can also last longer into fall if it stays warm.”

More than 24 million people in the United States suffer from pollen-induced respiratory allergies, according to Centers for Disease Control and Prevention data.

Though we “normalize” allergen-triggered diseases like hay fever — technically known as allergic rhinitis — we shouldn’t minimize their effect, said Caroline Sokol, a physician in Massachusetts General Hospital’s Allergy and Clinical Immunology Unit.

“Allergic rhinitis takes a huge toll in terms of missed time at work, because people feel horrible, people feel sick,” she said. “So many people are walking through a fog right now because their immune system is basically fighting off an allergen just like it would a virus, making us feel exhausted and miserable.”

Seasonal allergies can also have dangerous health impacts, including by triggering or worsening asthma. Other environmental factors, like poor air quality, can exacerbate risk.

“All these problems with allergies are going to be compounded by air pollution,” Primack said.

As carbon pollution warms the planet and lengthens spring seasons, it’s also increasing allergen production, the new report says, making allergy season not just longer but also more severe.

“Plants use carbon dioxide and photosynthesis. They take that carbon dioxide and transform it into energy,” Casey said. “So you see more plant growth which means more pollen.”

Research also suggests climate change-fueled increases in rainfall and extreme weather are increasing the amount of nitrogen polluting rivers and other waterways. That is likely causing some allergens, like ragweed — a flowering plant common to New England which is often found along streets and vacant lots — to produce more pollen, Primack said.

“There’s really much more nitrogen in the soil now than there was, say, 50 years ago,” he said. “And ragweed does better in high nitrogen environments, as well as high CO2 environments.”

An earlier study published in the journal Nature Communications last year looked specifically at 15 types of pollen from different plants found in the United States and found, in computer simulations, that pollen counts are increasing. And if the world keeps emitting carbon, things could get worse. The authors found that by the end of the century, pollen production could double.

Pollen isn’t the only trigger of seasonal allergies. Mold can also exacerbate allergy season, and climate change is making that worse too, the report says.

Mold thrives in warm, wet conditions, which are becoming more common in much of the United States as climate change drives up temperatures and increases the frequency of extreme rain events.

The report also highlights the link between allergens and thunderstorms, which research shows are becoming more severe amid the climate crisis. During torrential downpours, pollen and mold spores can spread through the air more efficiently. Strong upward winds during storms can lift up pollen grains, which then get dispersed across a wider distance when downdraft winds begin. To make matters worse, when pollen grains get wet, they rupture, and they break into smaller pieces.

“Once those tinier bits dry out, they’re more easily dispersed by the wind and they’re more easily inhaled,” Casey said.

Respiratory issues triggered by pollen can be exacerbated by air pollution, which tends to be more severe in highly industrialized urban centers, and often has the worst effects for poorer areas with higher percentages of people of color, Primack said.

Allergies can also pose financial challenges, said Patrick Kinney, a professor at Boston University’s School of Public Health. Medications can be expensive, as can devices like air filters that can help lessen symptoms.

“Allergy season is a big deal from a quality of life and health perspective, as well as from a medical expenditures perspective,” he said.