The stagnant air, combined with burning and other factors, deteriorated air quality in the Columbia Valley, where it reached unhealthy levels at times Wednesday and Thursday, Dec. 30 and 31.
At that level, health officials recommend people limit time outdoors.
The Environmental Protection Agency declared an outdoor burn ban starting 8 a.m. Friday on the Lummi and Nooksack reservations. It applies to all burning except ceremonial or traditional fires.
No similar ban had been issued for the rest of Whatcom County as of Thursday afternoon, though the Northwest Clean Air Agency has asked people to refrain from outdoor or indoor burning. Air quality in the rest of the county was good Thursday.
The air stagnation advisory issued by the National Weather Service continues through 10 a.m. Monday.