Smoke from the largest outbreak of wildfires in Kentucky's Appalachian region in at least a decade is shutting schools, sending people to hospitals with breathing problems and forcing daytime motorists to drive with their headlights on.

Thousands of acres continued to burn on Tuesday, and about 1,500 firefighters from across the country were working to bring the fires under control, said Gwen Holt, spokeswoman for the Kentucky Division of Forestry.

"It's placing lots of people in danger. We're constantly working to protect homes," Holt said.

Wildfires — nearly all of them intentionally set — have burned across more than 150,000 acres of woodlands in Kentucky this year.

Aided by dry weather, fires in the past week have blackened 67,370 acres in Kentucky, North Carolina, Tennessee, West Virginia, Virginia and Georgia. Forecasters predict no chance of rain until next week.

In Kentucky's Appalachian region, the air hangs heavy with the acrid smell of burning wood.

"I don't go out there unless I have to," said 84-year-old Elsie Carter, who can hardly see the trees outside her Pikeville home because of the thick, gray haze. "It looks dangerous, and it is dangerous if people breathe it too long."

Smoke was so heavy outside Shelby Valley High School in eastern Kentucky that smoke alarms went off inside, said Superintendent Frank Welch, who called off classes at five schools until the air clears.

Highlands Regional Medical Center in Prestonsburg reported a sharp increase in emergency room visits.

"The smoke has become a health threat," spokeswoman Kathy Rubado said. "The smoke seems to be bothering people who are already sick the most."

In Knoxville, Tenn., firefighters working 18-hour shifts continued battling a northward spread of wildfires Tuesday night, as 911 operators were inundated with calls from residents worried about the spread of fires close to home.

The workers, hindered by shifting and gusting winds, appeared at least temporarily to gain an upper hand on a 3-week-old outbreak of wildfires in southeastern Tennessee. But state Department of Agriculture spokesman Tom Womack said he was continuing to receive reports of new fires to the north.

Fire crews also battled blazes in the Great Smoky Mountains National Park. The fires included one that covered some 2,200 acres, mostly in the Cherokee National Forest, and a fire north of Bryson City, N.C., that burned about 800 acres.

The Foothills Parkway from U.S 321 in Cosby to Interstate 40 remained closed due to smoke.

So far, at least two deaths have been linked to the fires.

Kentucky forestry officials believe about 80 percent of the wildfires in the state were set by arsonists.

"Some folks do it out of maliciousness," Holt said. "Some fires are set to get even with the government, some to get even with a neighbor. Some are set for no reason at all."

So far, 13 people have been arrested on arson charges in Kentucky and Tennessee.