(BCN) — On Wednesday morning the Bay Area Air Quality Management District extended the air quality advisory for Contra Costa County until Thursday. BAAQMD says that winds during the daytime could disperse the smoke, but once the winds calm down overnight the air pollution levels may rise. At this time the pollutant levels are not expected to surpass the national 24-hour health standard. A Spare the Air Alert is not currently in effect and BAAQMD anticipates that the air quality will be in a moderate range overall. Smoke is expected to impact Pittsburgh, Antioch, Oakley and Brentwood.

Why the Marsh Fire in East Contra Costa County is still burning after six weeks is chiefly because of its fuel: peat, which isn’t easy to put out once it ignites. The peat that’s burning is spongy material formed by the partial decomposition of organic matter, usually plant material found in wetlands.

“Peat fires are very difficult, almost impossible, to extinguish,” said Steve Hill, spokesperson for the Contra Costa County Fire Protection District.

Much of the fire has burned in areas accessible only by foot, which also works against firefighters. The intermittent smoke it produces depends on the weather. The fire started in a homeless encampment near Solano and Suisun avenues in Bay Point on May 28, burning about 200 acres before firefighters got it under control. High winds flared the fire back up Saturday, causing it to burn into Pittsburg, threatening PG&E lines and getting close to homes.

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Firefighters from the county fire district and Cal Fire stopped its forward progress with a heavy aerial attack Saturday. The fire has burned about 500 acres, about 75 of which burned Saturday.

The fire moldered Monday until Tuesday, with firefighters monitoring the area in case it flares up again. There was enough smoke still in the area Monday to prompt Contra Costa Health Services to advise Pittsburg residents to stay indoors with windows and doors closed and to limit outdoor activity due to poor air quality in the area.

Con Fire officials say the fire is nearly out of fuel and, though still smoldering, shouldn’t be a danger to people or structures.

This was first reported by Bay City News.