Can planting more trees make allergies worse?

October 21, 2026

AI Summary

5 min read

In 2013, epidemiologist Gina Lovasi and her team at Drexel University published a study that seemed to defy common sense. They tracked children from birth to age seven, comparing how much tree canopy covered their neighborhoods. The hope was that more trees early in life would protect kids from developing asthma. Instead, the data showed the opposite: among children living in areas with the least tree cover, 12 percent developed allergic sensitization to tree pollen. Among those with the most tree cover, that number more than doubled, to 26 percent. As Lovasi put it, "Importantly, we don't always get the result that we expect."

The perfect storm of pollen

The finding is counterintuitive because trees are widely understood to improve air quality. They filter pollutants, store carbon, and cool cities. But for the tens of millions of Americans with asthma and pollen allergies, the relationship is more complicated. Horticulturalist Tom Ogren, based in San Luis Obispo, California, has studied urban tree canopies for decades. He describes a "perfect storm" of factors driving pollen production upward. Climate change is a major driver: warmer temperatures cause trees to bloom earlier in the spring and keep blooming longer into the fall. At the same time, rising carbon dioxide levels in the atmosphere act as a fertilizer for male trees, prompting them to produce more pollen. "The

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What you'll learn

1 (00:40) **The Double-Edged Sword of Urban Trees** - Host Nate Rott and producer Juan Diego Ramirez introduce the tension: trees are essential for cities, but Ramirez, who has asthma, says they can also send him to the hospital.
2 (01:45) **Why Pollen Is Getting Worse: The "Super Pollen" Problem** - Horticulturalist Tom Oglen explains how climate change and air pollution are making pollen more potent and abundant.
3 (05:10) **A Personal Journey: Moving and Developing Asthma** - Ramirez shares his own story of moving to the Pacific Northwest and discovering he had asthma, which sparked his investigation.
4 (06:06) **The Study: Tree Canopy and Allergic Sensitization in Kids** - Ramirez introduces a 2013 study from *Environmental Health Perspectives* that yielded a counterintuitive result.
5 (07:15) **The Counterintuitive Finding: More Trees, More Allergies** - The study's core result is explained: living under more tree cover early in life is linked to a higher likelihood of developing allergies to those trees.
6 (08:01) **Practical Tip #1: Know Your Pollen Seasons** - Allergist Dr. Jay Portnoy explains that pollen seasons are predictable, allowing people to prepare.
7 (09:52) **Practical Tip #2: Keep the Outside Outside** - Dr. Portnoy offers actionable advice for minimizing pollen exposure during heavy seasons.

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Show Notes

Trees provide shade and clean air in congested cities, so planting more of them seems like a straightforward solution. But trees also produce a natural pollutant— pollen. In this episode, producer Juan Diego Ramirez reports on the link between growing urban canopies and allergy exposure, and what you can do to protect yourself.

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