DULUTH – Smoke from the state's largest wildfire drove air quality to very unhealthy levels in northeastern Minnesota on Thursday, with local officials telling residents to keep windows shut and stay indoors whenever possible.

An air-quality alert is in place for northeastern counties through 8 p.m. Friday, and east-central counties around Lake Mille Lacs should also expect hazardous air through 9 a.m. Friday. Some parts of the region will see thicker smoke as winds shift.

"Fine particle levels are expected to reach ... a level that is considered unhealthy for everyone," the Minnesota Pollution Control Agency said.

Rain forecast for Friday should temporarily improve air quality.

The Greenwood fire is burning on roughly 40 square miles — about 26,000 acres — in the Superior National Forest after growing by 4,200 acres Wednesday. The fire destroyed 12 residences and 57 outbuildings and damaged three more properties in the McDougal Lake area this week, according to the Lake County Sheriff's Office.

"Firefighters remain in that area to protect the remaining structures," the agency said Thursday. "This is still an active fire area and closure and it is not safe for property owners to return to their property yet."

Andover resident Rick Gottwaldt is one of three owners of a cabin on middle McDougal Lake, a property that has been in the family for at least 30 years, he said. Family members watched footage taken by a fire crew member and posted on social media that showed their cabin still standing, but he hasn't been able to confirm it. He says it appears they lost a sauna and an outhouse.

If the cabin still stands, he said, "it takes a lot off the mind. But I feel sorry for the other people who lost. You wouldn't believe all the balsams that are dead. I cleaned as much as I could. But it was just fuel for this. Bound to happen."

There are now 476 personnel fighting the Greenwood fire — an increase of about 50 from earlier in the week. Additional units have been requested, and a peak of 600 people will be battling the blaze in coming days.

Crews on Thursday improved on gains made Wednesday "with the support of air resources, engines, bulldozers and other equipment, to reduce the burnable natural fuels near homes or near the edge of the fire," according to an update from the interagency team fighting the fire.

Controlled fires have been intentionally set at the edge of the wildfire to reduce fuel, and are responsible for some of the smoke spreading across the region, officials said.

U.S. Sens. Amy Klobuchar and Tina Smith and Gov. Tim Walz joined a briefing Thursday afternoon to learn more about the extent of the fire and what resources — other than rain — are needed.

The Greenwood firefighting team, which includes crews from Florida to Alaska, is fatigued from an extra-long fire season during a year of exceptional drought throughout much of the country.

"We've been at it since March," said Nick Petrack, a fire management officer with the Superior National Forest. "It's a marathon, and we knew that going in. ... We're expecting this to go until snowfall."

Property owners and evacuated residents around McDougal Lake and the Hwy. 1 corridor, where the fire has already swept through, won't have to wait that long to check on their cabins, however.

"If we're able to get you in there, we'll try to get you in there," Lake County Sheriff Carey Johnson said at a community meeting in Finland on Thursday. Property owners should coordinate with the Sheriff's Office and aim to visit early in the day if possible.

The fire is still generating intense heat and suppression efforts are at the mercy of the weather, but incident commander Brian Pisarek said Thursday: "I feel way better than I did a few days ago."

The lightning-ignited fire started Aug. 15 and is burning southwest of Isabella in Lake County. The Forest Service says that "the communities of Ely, Babbitt, Finland, Toimi-Brimson and along the North Shore are not threatened by this fire," but some remote structures remain at risk.

An area along the upper Gunflint Trail is on pre-evacuation notice due to uncontrolled fires in the Boundary Waters Canoe Area Wilderness. Those fires saw "minimal activity" Wednesday.

Firefighters were expected to begin battling the 50-acre Whelp fire that is 5 miles northwest of Sawbill Lake on Thursday. The 1,500-acre John Ek fire is burning a few miles south of Little Saganaga Lake and could see personnel assigned Friday.

"It is kind of a time bomb, in our minds," Superior National Forest Supervisor Connie Cummins said Thursday about the John Ek fire. "Under the right conditions it could make a run on the Gunflint Trail."

Staff writer Jana Hollingsworth contributed to this report.

Brooks Johnson • 218-491-6496