Upvalley Fire Scenes

Smoke rises from the western hills from Silverado Trail near Yountville Cross Road on Thursday. Air quality remains poor and a potential risk to general population.

J.L. Sousa, Register

The continuing strength of major wildfires in Napa and Sonoma counties will leave the air unhealthful to breathe through the weekend, air quality officials announced Thursday.

North Bay residents should continue staying indoors with windows closed whenever possible, due to high pollutant levels from the smoke billowing from the Atlas, Tubbs, Partrick and other fires, according to Tom Flannigan, spokesman for the Bay Area Air Quality Management District.

The air quality index, a measure of airborne soot, ash and other pollutants, remained north of 200 in the North Bay on Thursday, the district reported. A figure of 200 or more indicates pollution levels with serious health effects on most people, while an index of 100 or more points to health risks for seniors and those with existing illnesses.

Bay Area fires are only now starting to be contained, after spreading to tens of thousands of acres. While incoming sea breezes from the Pacific Ocean provide some afternoon relief, “we expect that every night the winds will settle down and pollution from the upper atmosphere will sink to the surface,” Flannigan said.

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A smoke advisory and Spare the Air alert will stay in effect through Sunday for the entire district, which governs air quality over nine Bay Area counties.

Napa County residents are advised to stay indoors, keep home and car climate-control systems recirculating inside air, and avoid physical exertion when they must be outside.

Those using respirator masks outdoors should use only models classified N95, meaning they filter out 95 percent of particulates, according to Flannigan. Masks also should seal tightly around the nose and mouth, without any gaps on the edges.