Why Seattle has had the worst air quality in the world this week

SEATTLE - Seattle had the worst air quality in the world Thursday, with a heavy mix of fog and smoke blotting out the horizon, the surrounding mountains and the Space Needle.

It was the second day in a row that the city had the worst air quality on earth, beating out famously polluted cities like Beijing and Delhi. Seattle’s air quality index, or AQI, reached over 240 on Wednesday and Thursday - a level defined as “very unhealthy” for all groups. It was hard to see the top of a building a block away, and people wore masks to protect themselves from particulates in the air and the acrid smell of smoke.

According to the air quality monitoring site IQAir, Seattle’s concentration of PM 2.5 - tiny particles less than 2.5 microns in width - on Thursday afternoon was 38 times higher than the annual guideline recommended by the World Health Organization.

The cause was forest fires raging in the Cascade Mountains, combined with weeks of unusually dry and hot weather. On Sunday, Seattle broke a record for the hottest day this late in the fall, at 88 degrees Fahrenheit. Washington state has experienced precious little precipitation since June; 56 percent of the state is currently in drought, according to the National Oceanic and Atmospheric Administration.

Seattle having the worst air quality in the world is a “shocking statistic,” said Maddie Kristell, a meteorologist at the National Weather Service in Seattle. Part of the problem, she explained, is a persistent ridge of high pressure that has kept storms from settling over Seattle.

“That ridge was really strong and it was just not allowing a difference in weather patterns to come through,” she said. That, combined with above-normal temperatures, allowed the fires to burn on for longer than they might otherwise.

The city’s terrible air comes at a time when researchers are trying to understand whether wildfire smoke is worsening - and how it is affecting human health on the West Coast and beyond. According to a study published last month, the number of people in the United States experiencing an extreme smoke day has increased significantly over the last decade.

“Intuitively if you live in the West, you know that things have changed, it’s become smokier,” said Marshall Burke, a professor of earth science at Stanford University and one of the co-authors of the study. “But our goal was to try to quantify how much.”

Between 2006 and 2010, the researchers found, fewer than 500,000 people every year were exposed to a single day of extreme levels of PM2.5. But between 2016 and 2020, that number climbed to over 8 million. A hotter, drier climate - combined with a failure to plan and execute prescribed burns that can prevent giant wildfires - has caused an increase in the number of Americans exposed to choking, acrid air during the summer and fall.

Researchers know that PM2.5 is extremely bad for human health - it can exacerbate respiratory and cardiovascular problems, and frequent high exposures have been shown to affect cognition and children’s test scores. But, Burke said, nobody really knows how brief spikes of extremely high air pollution affect health and cognition.

Staying inside isn’t necessarily a fix: the air quality monitoring site PurpleAir showed that even many indoor air sensors in Seattle on Thursday were reporting AQIs of between 100 and 150, which can be dangerous for many vulnerable groups.

When faced with terrible air quality, Burke recommends staying inside, keeping doors and windows closed and using air purifiers or other filtration systems wherever possible. Those without air purifiers can make what are called “Corsi-Rosenthal boxes,” or box fans with air filters taped to them for a quick DIY solution for cleaning indoor air.

Ultimately, however, researchers are just beginning to scratch the surface of the fire-filled climate future. Poor air quality “affects our lives in a lot of ways,” Burke said. “We’re only beginning to understand the extent to which it matters.”

The air quality alert in effect for Seattle expires Friday morning. Rain is expected Friday afternoon into Saturday, a system the National Weather Service said should “help further improve air quality.”

- - -

The Washington Post’s Jason Samenow contributed reporting