Climate Change Is Boosting Plant Pollens and Human Seasonal Allergies

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Source: Earth • Food • Life

Higher carbon dioxide levels and warmer temperatures are causing plants to increase their pollen production.

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Lucy Goodchild van Hilten is a writing fellow at Earth • Food • Life, a project of the Independent Media Institute. She has served as assistant editor of Microbiology Today and senior marketing communications manager for Life Sciences at Elsevier. She holds an MSc degree in the History of Science, Medicine, and Technology from Imperial College London.


When the season turns to spring, flowers begin to bloom, trees turn green and the sun shines longer. But if you’re like almost one-third of people in the U.S., you might be noticing watery eyes, a tickly throat, and a runny nose. With spring comes pollen, which makes breathing outdoor air a bit difficult.

But it’s getting worse: With climate change shifting weather patterns and causing an early, more extended pollen high, we could all be sneezing more than usual.

“If you live with seasonal allergies and feel like the pollen seasons feel longer and longer every year, you may be right,” wrote Paul Gabrielsen of the University of Utah, in 2021. “[P]ollen seasons start 20 days earlier, are 10 days longer, and feature 21 percent more pollen than in 1990—meaning more days of itchy, sneezy, drippy misery.”

Allergy specialist Dr. Kari Nadeau, chair of the Department of Environmental Health at the Harvard School of Public Health, blames global warming. “There are these extreme, chaotic conditions that climate change is associated with,” Nadeau told Boston 25 News in March 2023. “And that warming is affecting our pollen seasons.

“Trees are getting the wrong message and they’re releasing pollen earlier in the season,” said Nadeau. “So my patients, for example, otherwise would have started allergy season in March, now they’re having allergy season start January-February.”

Pollen: Pervasive Problem

Hay fever isn’t new. It was first described in 1819, when physician John Bostock presented a novel case to the Medical and Chirurgical Society, calling it a “case of a periodical affection of the eyes and chest.” It was the first recorded description of what he later called “catarrhus aestivus” or summer catarrh, which would become what we know today as hay fever.

It’s centuries old—and increasingly common: An estimated 30 percent of the U.S. population suffered from hay fever, or allergic rhinitis, in 2000, up from just 10 percent in 1970. The National Health and Nutrition Examination Survey (1988−1994), which collected information from nearly 40,000 people in the U.S., showed that 26.9 percent and 26.2 percent of the population were allergic to perennial ryegrass and ragweed respectively—at least twice as high as the previous survey (1976−1980).

It’s the pollen that’s to blame for their symptoms. When plants reproduce, they have to get their sex cells together. Pollen carries the male sex cells so it has to be transferred to the female plant. Many plants use insects, like bees, to transfer their pollen to other plants, and others rely on wind. The plants that are wind pollinated produce tiny, light pollen that can be carried on a breeze—fantastic for their reproduction, disastrous for our respiration.

Immune Response

When we inhale pollen grains, they can kickstart an immune response in which our body is trying to attack them. Our immune system can overreact to the harmless pollen: The sneezing, the watery eyes, and the histamines that make your nose itchy are designed to kill or eject the pollen. If you’re prone to allergic rhinitis, the more pollen you’re exposed to, the worse your symptoms.

Not every hay fever sufferer is allergic to every pollen. It tends to be seasonal: In the spring, tree pollens like those from the birch, oak, and mountain cedar cause the most problems, followed by grass and weeds like mugwort and nettle in the summer, with weeds like ragweed (the leading cause of hay fever nationwide) and fungus spores following in the autumn.

But one major factor is having an impact throughout the year: Climate change is causing an increase in pollen release, and it’s even making some pollens more potent hay fever inducers.

In 2015, the World Allergy Organization, a group of 97 medical societies from around the world, released a statement warning that climate change will have an impact on when, how long, and how bad the pollen season will be.

“The strong link between warmer weather and pollen seasons provides a crystal-clear example of how climate change is already affecting people’s health across the U.S.,” said William Anderegg, a biologist at the University of Utah, about research conducted by him and his team that was published in the Proceedings of the National Academy of Sciences in 2020.

“A number of smaller-scale studies—usually in greenhouse settings on small plants—had indicated strong links between temperature and pollen,” notes Anderegg. “This study reveals that connection at continental scales and explicitly links pollen trends to human-caused climate change.”

Warmer Weather Means More Pollen

The first culprit is the rise in temperature caused by climate change. “We have got good evidence of this—global warming due to climate change induces more airborne pollen,” said Amena Warner, head of clinical services at Allergy UK. “Pollen seasons are extending because of the warmer weather, which is having an impact on how much airborne pollen there is circulating.”

A 2015 study showed that in the decade between 2001 and 2010 in the U.S., pollen season started on average three days earlier than it did in the 1990s. What’s more, the amount of airborne pollen increased by more than 40 percent. “These changes are likely due to recent climate change and particularly the enhanced warming and precipitation at higher latitudes in the contiguous United States,” concluded the researchers.

This is also probably increasing the number of people suffering from hay fever. Scientists predict that by 2050, the number of people with allergic rhinitis in Japan will increase by 40 percent because of local temperature increases: Tokyo’s mean yearly temperature has risen by 3°C since 1890 and is set to hit 3.5°C by 2100.

Pollen Problem Fueled by Carbon Dioxide

While warmer temperatures have led to earlier and longer pollen seasons and even more potent pollen, rising carbon dioxide levels are also helping plants produce more pollen. Plants feed on carbon dioxide, so when there’s an abundance of it, they can go wild producing pollen. Couple that with warmer temperatures, and you’ve got the ideal growth and reproduction conditions for plants, which means more allergens for us.

Take the invasive and highly allergenic plant ragweed, for example. In 2000, Lewis Ziska, an expert on how plants are impacted by climate change, led a research team at the U.S. Department of Agriculture that grew ragweed in the lab under three conditions: pre-industrial levels of carbon dioxide, current levels, and the higher levels expected in the 21st century. They found that exposure to current levels increased the amount of pollen the ragweed produced by 131 percent compared to pre-industrial carbon dioxide, and under predicted future carbon dioxide levels, it skyrocketed to 320 percent. The researchers concluded that “continuing increase in atmospheric CO2 could directly influence public health by stimulating the growth and pollen production of allergy-inducing species such as ragweed.”

Ziska says the intensity of an allergic reaction depends on how much pollen is released, the duration of the exposure, and how allergenic the pollen is. In ragweed, these three factors work strongly together. “What’s unique about ragweed is that it produces so much pollen—roughly a billion grains per plant,” he wrote in the journal Environmental Health Perspectives in 2016. “And the Amb a 1 protein [in the pollen coat] is also highly reactive with the immune system.”

No Escape to the City

One might be tempted to think hay fever would be less of a problem in the city, away from all the trees and weeds, but the opposite appears to be true. Similar results have been observed outside the lab: In downtown Baltimore, where it’s 3˚C warmer and has 30 percent more carbon dioxide than the countryside, ragweed “thrived, growing bigger and puffing out larger plumes of pollen than its country counterpart,” reported Rachel Becker in The Verge.

Ragweed may thrive in our cities, but there’s a bigger—and taller—problem: The trees planted to provide shade and beauty are making our allergies worse.

“Many people believe that the more trees you have in a city’s green infrastructure, the more they act as a biofilter,” said Amena Warner of Allergy UK. “But are they the right kind of trees? In urban areas, particularly in London, there’s a lean towards planting birch trees, which are highly allergenic. When they’re in cities, people can’t escape the pollen easily, and it’s virtually indestructible unless it’s wet.”

That means the pollen that collects on your clothes, the bottom of your shoes, and in your hair during your afternoon stroll could plague you until it rains or is washed away. That, says Warner, extends the time you’re in contact with pollen, even out of pollen season. The UK has the third highest rates of allergic rhinitis and asthma prevalence in the world, so Allergy UK is seriously concerned about this.

“It’s important that the right tree is planted in the right place,” said Warner. “We want to raise awareness of why planting allergenic birch trees in urban areas can increase hay fever and other respiratory conditions.”

So if we know the pollen from birch trees (and lots of others) is causing allergic reactions, why are they still dominating our city streets? “Mainly because they seem to be fashionable,” said Warner. “They have this lovely silvery bark, and they’re long and graceful with a beautiful sweeping canopy that gently sways in the wind. And they don’t drop fruit—in a city, you want trees with a low cleanup cost.”

Keeping Hay Fever at Bay

There are alternatives: Not all tree pollen is allergenic. In 2010, a report by the National Wildlife Federation called on states, communities, and homeowners to “undertake smart community planning and landscaping, with attention to allergenic plants and urban heat island effects, to limit the amount of pollen and other allergens that become airborne.”

One way to reduce the impact of hay fever in cities would be to use the Ogren Plant Allergy Scale (OPALS), which rates trees in terms of how allergenic they are, to choose less allergenic trees. So when you’re choosing your tree, whether it’s in your garden or on the street, opt for something that won’t make people sneeze.

As the climate continues to change and we see an increase in hay fever, we’ll also notice a bigger impact on public health, not least because an estimated 30 percent of people with allergic rhinitis develop asthma later on. While urban planning may be out of our hands, there are some things we can do to reduce the pollen problem.

David Mizejewski, a naturalist at the National Wildlife Federation and a long-time allergy sufferer, gave readers of the Federation’s 2010 report some advice:

  • Get an allergy test—that way you can decide when’s best to go outside

  • Ask your doctor about allergens and what medication to take

  • Check daily pollen counts and go out when they’re low

  • Wash your clothes and yourself to remove trapped pollen, and use nasal sprays

  • Choose non-allergenic plants for your garden

  • Plant female trees and shrubs (it’s the males that produce pollen)

It’s important to remember that people with allergic rhinitis can develop asthma, which can be serious. So if your symptoms start to affect your breathing, it’s best to consult a doctor.

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