Why Climate Change Is Making Pollen Allergies Worse Than Ever

in Asthma, Grass & Ragweed, Outdoor Allergies, Tree Pollen
Published: June 6, 2023
Why Climate Change Is Making Pollen Allergies Worse Than Ever
Photo: Getty

Across the U.S. and North America, climate change is making pollen allergy season longer and more intense. If you’re feeling the misery of symptoms, you’ve got plenty of company. 

Research shows allergy season starts an average of three weeks earlier than it used to. There’s also a far higher concentration of pollen in the air.

This combination creates a potpourri of suffering for the 26 percent of U.S. adults and 19 percent of children with seasonal allergic rhinitis (also known as hay fever). 

Worsening Pollen Allergy Symptoms

When he started practicing 40 years ago, Dr. Leonard Bielory says the typical patient was sensitized to one or two types of pollen. The New Jersey allergist-immunologist, who has studied the impact of climate on allergies, says such patients had mild or moderate allergy symptoms. 

Today, many of his patients are sensitized to numerous types of pollen and have more severe symptoms. They’re also coming in earlier in the spring, desperate for relief from symptoms such as clogged noses, relentless sneezing, itchy throat, coughing and fatigue. He is also seeing a rash of patients tormented by an underappreciated seasonal allergy symptom – itchy, red, puffy, watery eyes. 

“I had a college baseball player who said he wasn’t going to be able to play because he could not be out on the field. I had another child who couldn’t go out for recess,” Bielory says. “We always thought of allergies being of the nose and only mildly of the eyes. But if you look at the data, the ocular allergy symptoms are as much a problem as the nasal symptoms.”

The greater pollen load also appears to be triggering the immune system to react in patients who were never bothered by symptoms in the past. Repeated exposure to pollen, over longer periods, may lead people to become sensitized who weren’t before, explains Dr. Kathleen May, president of the American College of Allergy, Asthma and Immunology.

The pollen-laden air can also have a “priming effect” on the immune system. This means it “takes much less allergen at the start of a season to trigger symptoms,” May says. With a hair trigger immune system ready to respond to the slightest provocation, “this creates an extended season for symptoms, unfortunately.” 

Here’s more to help you understand how climate change is impacting pollen allergies. 

When Pollen Takes Flight

To reproduce, male plants release pollen, a fine, yellowish powder. Showier plants with big flowers typically create heavier pollen that is spread by insects and birds. But with allergies, the troublemaking plants are those that cloud the air with light pollen grains that are carried on the wind in search of a female counterpart. 

In North America, trees spreading allergenic pollens in spring include birch, oak, cedar and alder. In summer, grass pollens take over, while weeds like ragweed create havoc in the fall.

For people who are sensitized, it’s the wind-borne pollen that sets off most symptoms, says William Anderegg, an associate professor of biology and director of the Wilkes Center for Climate Science and Policy at University of Utah. 

Trees release massive amounts of pollen in the spring. Fall’s ragweed is also a formidable pollinator, with one plant able to spew up to a billion pollen grains. When these seasonal grains get in your nose and eyes, cue the sneezing, sniffling, stuffiness, itching, coughing. For some with allergic asthma, medication adjustments may be needed to control wheezing. 

Climate Change: Driving Pollen Production 

The biggest factor in the increase in pollen levels is warmer temperatures, which cause plants to bloom earlier. A second factor is higher concentrations of carbon dioxide (CO2) in the atmosphere. Those concentrations spur growth and enable plants to produce more pollen. 

Study author William Anderegg, PhD

But first, some background. Climate change refers to long-term shifts in temperatures and weather patterns caused by human activity, mainly burning fossil fuels like coal, oil and gasoline. Burning fossil fuels generates emissions such as carbon dioxide, nitrous oxides and methane. These are sometimes called “greenhouse” gases because they act like greenhouse glass. They surround the Earth, trapping heat, and warming the planet. 

The warmer temperatures are lengthening our pollen seasons. In a recent study, Anderegg and his colleagues used pollen data from 60 sites across the U.S. and North America and found that pollen season started on average 20 days earlier in 2018 than it did just 30 years ago. The data also showed that pollen concentrations were 21 percent higher than in 1990. 

“Pollen seasons are starting earlier than the used to a couple of decades ago. And there is about 20 percent more pollen in the air than there used to be,” Anderegg says. “Our study and others have shown that climate change is a primary driver of that.” 

Research in the lab has shown that plants thrive in hotter, higher CO2 environments, Anderegg says. “You tend actually to see bigger plants on average that make slightly more flowers, and those flowers make slightly more pollen.”

Japan’s Cherry Blossoms: Dramatic Change

Those effects are showing up in the real world. A study in Atlanta, for example, found that the average daily pollen concentration of the city’s oak trees increased by 5 percent per year over the past 27 years. 

The changes aren’t limited to North America. This is a worldwide phenomenon. In Kyoto, Japan, researchers recorded the earliest cherry blossom blooms ever in 2020 and 2021. With centuries of records on when the famous pink and white blossoms reached their peak bloom, the scientists could see an obvious trend. 

“This change in flowering is one of the most widely observed patterns with climate change,” Anderegg says. “The timing of when cherry trees blossom is incredibly strongly linked to spring temperatures. You can see for 500 years that the timing kind of bounces around. Then starting in the 1960s when human-caused climate change takes over, you see this trend of earlier and earlier blossoms in Japan.”

Climate Change and Summer, Fall Allergies

So far, the biggest changes in pollen production have been seen with the tree pollen blooms of the spring, Anderegg says. 

American common ragweed (Ambrosia artemisiifolia) causing allergy. Photo: Getty
Photo: Getty

But some research shows that grass species have also shifted their blooms to a bit earlier in the year. That means there is likely overlapping of spring and summer pollen seasons. So if you have pollen allergies to grass as well as trees, you may be getting hit by both at the same time. 

As for the fall, bountiful ragweed also is thriving in the balmier temperatures and higher CO2 concentrations. In the U.S., ragweed is the third most common allergen, affecting about 16 percent of the population.

In cities with higher CO2 concentrations, ragweed grows faster, flowers earlier and produces more pollen. Between 1995 and 2016, data from the U.S. Environmental Protection Agency found that the length of the ragweed pollen season grew by as much as 6 to 24 days across the U.S. and Canada, with the effect the more pronounced the farther north you go. 

Extreme Weather and Mold 

Climate change isn’t only about warmer temperatures. Planetary weather systems are interconnected. That means that rising sea levels and changing weather patterns can unleash all sorts of extreme weather events, such as droughts, wildfires, floods and ferocious storms. These can also affect allergies. 

Studies show that ragweed pollen disperses farther in places experiencing drought conditions. Conversely, floods can create wet, damp conditions where mold thrives. 

In people who are sensitized to mold, allergy symptoms can include itching in the nose, eyes and throat, sneezing, nasal congestion, or cough and wheezing in those with allergic asthma. Mold allergy symptoms can be severe. 

Thunderstorm Asthma, Other Triggers 

Thunderstorms are another weather event that can bring about allergy symptoms. Rain tends to wash pollen out of the air, temporarily lowering pollen counts and reducing pollen allergy symptoms. 

But cold downdrafts from thunderstorms can sweep pollen and mold particles high up into the air. There, the humidity and electricity shatter the particles into smaller pieces. These fragments can more easily be inhaled deep into the lungs, triggering asthma attacks, Bielory says. People with “thunderstorm asthma” usually also have a history of both asthma and seasonal allergy.

Over the past 30 years, there have been about 20 reports of “epidemics” of thunderstorm asthma. These are stormy days when large numbers of people suddenly rush to emergency departments experiencing coughing, wheezing and shortness of breath. 

The majority of these events occur in the spring and early summer, when pollen counts tend to be at their highest. As in the famous Australian thunderstorm asthma event of 2016, some of these have led to fatalities.

Another problematic combination for asthmatics is hot sunny days mixed with pollutants in the air, which can cause high levels of ozone, a lung irritant. Add to that high pollen counts, and “you have a witches’ brew,” Bielory says. 

How Much Worse Could It Get?

No one knows for sure, but researchers from University of Michigan used computer modeling to make some alarming predictions. 

July 2021 was declared the hottest month recorded in 142 years. If warming continues unchecked, spring pollen season could start as many as 40 days earlier by the end of century and extend 19 days longer, according to their research. Increasing CO2 levels could boost pollen production by as much as 200 percent annually. 

Still, Anderegg says “there are a lot of reasons for hope and optimism.” CO2 and global temperatures will keep rising only if we fail to cut fossil fuel usage. 

“We seem to be already partially tackling climate change. We have likely moved away from the highest scenarios of climate change already,” he says. “One of the things that my study and many others have emphasized is we probably need to be going a lot faster, and the faster we go to tackle climate change the more benefits we will see for our health.”

Related Reading:
Finding Pollen Allergy Relief: Which Pill, Shot or Spray?
Wild Weather and Our Allergies and Asthma
Allergic Living’s Outdoor Allergy Hub: Resources by Season
All About Ragweed Allergy: Signs, Symptoms and Avoidance