Wildfire smoke clouds Pacific Northwest

• Air quality advisories are in effect until Friday morning as unhealthy conditions remain in place, per the National Weather Service in Seattle.

Of note: The air quality was so bad that residents in the Washington city were advised to wear a mask outdoors, keep their windows shut and avoid outdoor exercise, per air quality monitoring site IQAir.

By the numbers: The top 10 places in the U.S. with the worst air quality Thursday were all in Washington or Oregon, per the federal website airnow.gov.

• The worst was Oakridge, Oregon with an air quality index as high as 487, which is hazardous, according to federal site AirNow.
• Seattle had an air quality index of 207 and Portland's was 204, both considered very unhealthy.

The big picture: Much of the U.S> West is in a climate change-driven drought, exacerbating wildfire risk.

• There's an unusually high number of wildfires burning in the Pacific Northwest as much of the northwestern U.S. faces higher than normal temperatures and minimum relative humidity.

Though bubble, via Axios' Andrew Freedman: It's unusual to be facing wildfire-related air quality issues in Seattle, as well as Portland, Oregon, this late in the year.

• But a strong area of high pressure, dry weather and a strong burst of east winds last weekend help fan the flames of new and preexisting blazes.

What we're watching: Rain is headed toward the Pacific Northwest, with weather conditions changing markedly on Friday.

Go deeper: WHO revises air quality guidelines to reduce deaths from pollution