The National Weather Service says a high pressure system over the Northwest is keeping the lid on air pollutants.
A stagnant air advisory is in effect for much of Washington, except for the mountains and part of the coast, until Friday afternoon.
Forecasters say the high pressure is trapping cooler air near the ground. With poor mixing of the lower atmosphere because of the inversion, wood smoke and car exhausts can build up.
And, due to continuing stagnant weather conditions and elevated air pollution levels, a Stage 1 burn ban remains in effect for Pierce and Snohomish counties.
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