Seattle's Pollen Season Starts Earlier and Lasts Longer

Climate change and rising CO2 levels are extending allergy season in the Pacific Northwest.

Apr. 20, 2026 at 3:09pm

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A minimalist, neon-outlined illustration of a flowering plant or pollen grains against a dark background, conveying the energy and urgency of Seattle's extended allergy season.As climate change extends Seattle's allergy season, a glowing neon illustration captures the urgency for residents to prepare for longer and more intense pollen exposure.Spokane Today

Seattle's allergy season is starting earlier and lasting longer, with tree pollen showing up weeks ahead of schedule and a second wave of grass and weed pollen hanging around for months. Experts attribute this to a longer frost-free growing season and increased pollen production due to climate change and higher CO2 levels.

Why it matters

The extended and intensified allergy season is a growing public health concern, affecting an estimated one in four adults and one in five children in the region. Doctors advise starting preventive treatment earlier and using mitigation strategies like HEPA filters to cope with the worsening conditions.

The details

National data shows the freeze-free growing season has lengthened by about 21 days on average in 173 of 198 U.S. cities since 1970, with the Northwest leading the pack at roughly a 31-day increase. In Washington state, Seattle's freeze-free season has grown by 12 days and Spokane's by 33 days. This gives plants more time to leaf out and release allergenic pollen, starting the season about 20 days earlier and lasting nearly a month longer than it did three decades ago.

  • Pollen season in Washington state now kicks off about 20 days earlier and lasts nearly a month longer than it did three decades ago.
  • Seattle's freeze-free season has lengthened by roughly 12 days since 1970.
  • Spokane's freeze-free season has lengthened by about 33 days since 1970.

The players

Washington State Department of Health

The state health agency is piloting an 11-monitor pollen network to provide more precise, localized monitoring to help doctors, schools, and residents prepare for high pollen days.

Climate Central

A nonprofit research organization that has analyzed climate records showing the lengthening of the freeze-free growing season across most U.S. cities.

Northwest Asthma & Allergy Center

A local allergist practice that posts daily pollen counts for Seattle online and advises patients to start preventive medication in advance of peak weeks.

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What’s next

The Washington State Department of Health is piloting an 11-monitor pollen network to provide more localized data to help residents and doctors prepare for high pollen days.

The takeaway

Seattle's extended and intensified allergy season is a growing public health issue driven by climate change, requiring early prevention, mitigation strategies, and long-term solutions to reduce emissions and improve monitoring.