Why pollen season is starting earlier and how it could affect allergies

As the U.S. welcomes spring, it also means the arrival of allergy season, which affects millions of people.

High pollen levels can lead to a variety of symptoms including sneezing, runny nose, itchy and watery eyes, hives and coughing -- and Americans may have to deal with them for much longer than in past years.

Typical allergy seasons are getting longer and more intense in North America because of climate change, research shows.

One 2020 study found that between 1990 and 2018, North American pollen seasons became 20 days longer on average with pollen concentrations increasing up to 21% during the same period.

Plants have more time to bloom and sprout leaves, releasing allergy-inducing pollen when there are longer periods of freeze-free days. With no mitigation, these trends will persist, further impacting respiratory health in the coming decades.

Parts of the Midwest, including the Ohio River Valley, are currently experiencing the earliest spring leaf out on record, two to three weeks earlier than average, according to the USA National Phenology Network.

Allergies occur when the immune system views food, medicine, things in the environment or something else as harmful and overreacts. The immune system then releases chemical compounds such as histamine, which causes those hallmark symptoms of allergies.

Reactions can range from mildly annoying symptoms to life-threatening reactions including anaphylactic shock, which occurs when blood pressure drops suddenly and the organs can't get enough oxygen.

In 2021, approximately 81 million people in the U.S. were diagnosed with seasonal allergies, otherwise known as hay fever, according to the Asthma and Allergy Foundation of America (AAFA).

However, the warming planet is likely making allergy seasons longer, as more carbon dioxide (CO2) is released into the air.

"We've known for a long time, several decades, that when you turn up the temperatures and increase the CO2 levels around plants, they tend to grow bigger and produce more pollen," Dr. William Anderegg, an associate professor in the school of biological sciences at the University of Utah and the director of the Wilkes Center for Climate Science and Policy, told ABC News.

Anderegg, a lead author on the 2020 study, explained that warmer temperatures drive more pollen from plants and higher pollen levels will mean more intense allergies and asthma.

Plants are incredibly sensitive to temperature and so warming really trigger plants to think, 'Well, it's time, it's the growing season. Time to put out pollen.' And that really kicks off our allergy seasons," he said.

This has been seen in other events, including the bloom of the iconic cherry blossoms around the Tidal Basin in Washington, D.C.

Peak bloom occurs when 70% of the cherry blossoms open. This year, the National Park Service announced the trees reached a near-record early peak bloom on Sunday, March 17.

The peak bloom date for the trees at the Tidal Basin is occurring earlier than it did in the past, shifting about a week earlier since 1921.

Higher pollen counts don't just mean earlier seasons or more severe allergies.

Dr. Rita Kachru, chief of clinical immunology and allergy and associate professor in the department of medicine at the David Geffen School of Medicine at UCLA, said earlier and more intense pollen seasons due to global warming mean people are dealing with a more diverse and intense variety of allergens, which can trigger a more persistent and severe inflammatory response.

She added that increased pollen and mold due to climate change can exacerbate the effects air pollutants have on certain individuals. Exposure to these pollutants can also make new groups sensitive to seasonal allergies.

Anderegg said the results from his study and others like it show that climate change is already affecting people's health. His warning for those who suffer from seasonal allergies? Be prepared.

"Earlier and longer pollen seasons are something we need to prepare for," he said. "This is something that climate change is going to keep driving in the coming years. So preparing for that earlier start is a good way to try to minimize some of the health impacts."