Air quality in the Puget Sound region dipped on Sunday to reach levels of "unhealthy for everyone," according to the Puget Sound Clean Air Agency. 

The agency on Sunday issued a warning for King, Kitsap, Pierce, and Snohomish County residents to remain indoors as winds pushed wildfire smoke from British Columbia and the Cascades toward Western Washington. In all four counties, officials rated the air quality to be bad for even healthy adults to walk outside. Air pollution is expected to remain at elevated levels until Wednesday. 

Seattle began ranking as the city with the worst air pollution worldwide on AirVisual, an air quality monitor—topping Vancouver, B.C. as of Monday afternoon. It remained in the top spot on Tuesday, followed by Portland. (The list only shows cities' current air quality, not overall pollution. Beijing, for example, tends to have its worst air polluting days in the winter season, said Craig Kenworthy, Puget Sound Clean Air Agency executive director.) 

"I can safely say that it’s the worst widespread pollution we’ve seen in the last 20 years," Kenworthy told PubliCola. And in the long term, "climate change is going to increase the likelihood of this happening." 

With drier and hotter conditions, Kenworthy said, the odds that the region will experience unhealthy air pollution go up, though it doesn't necessarily mean another smoky summer will happen next year or consecutively.

Particulates—microscopic particles about one-thirtieth the size of a human hair—left over from burning materials float along in the atmosphere and are so small, they can travel deeply into human lungs and pass into the bloodstream.  

Breathing the air can be especially harmful to "sensitive groups"—which includes kids, seniors, pregnant women, and people with respiratory or cardiac problems. But Kenworthy said some science suggests even healthy adults could have an increased risk of cardiovascular complications. In the short term, it also increases a person's chance of developing bronchitis or inflammation in the lungs.  

Seattle mayor Jenny Durkan sent out a press release Monday with Puget Sound Clean Air Energy's recommendations—including limiting physical activity outdoors, closing windows at home, avoiding driving, and suggesting masks.

Officials expect hotter temperatures Tuesday, which could exacerbate health problems for vulnerable populations.

Kenworthy said those with senior neighbors without air conditioning should try to check in on them and offer them air conditioning when possible. 

Updated 10:40am on August 21, 2018, to reflect an updated AirVisual list of cities with the worst air pollution.