The National Weather Service has extended the air stagnation advisory for the Tri-Cities through 10 a.m. Friday.
Conditions improved some with a storm that crossed Washington early Wednesday, largely to the north of the Tri-Cities.
However, strong high pressure continues to allow minimal mixing of air in the Tri-City area, according to the weather service. That increases pollution.
A cold front expected Friday should allow winds to stir up the stagnant air mass.