"It was off the charts," Mark Shaffer of the Arizona Department of Environmental Quality said Friday night.
Calmer winds helped firefighters gain some ground, but also concentrated the sooty air, keeping it stagnate and raising pollution to levels officials hadn't seen yet since the blaze began several weeks ago.
"We've got a serious potential health problem on our hands," Shaffer said. "When you get levels like this, it's off the map."
Officials planned to bring in additional air quality monitoring equipment over the weekend, but warned people to just stay away.
Meanwhile, the blaze was poised to move into New Mexico on Saturday pushed by stronger winds, threatening more towns and possibly endangering two major power lines that bring electricity from Arizona to West Texas.
The fire has burned 639 square miles of forest, an increase of 114 square miles from a day earlier, officials said.
"It's getting very, very close to the New Mexico state line," Jim Whittington, spokesman for the teams battling the fire, said Friday night. "This is really rugged country. There is a lot of potential" for the fire to grow.
He said after a good day with calm winds, Saturday was expected to get dicey.
"The atmosphere will be unstable and very conducive to fire growth," Whittington said. "We're very concerned about the winds."
He said firefighters may eventually have to set ablaze 150,000 acres to burn off fuel in the forest in order to stamp out the flames.
"It's going to be really difficult," Whittington added.
Lighter winds Thursday and Friday helped the 4,400 firefighters make progress, but critical fire conditions remain.
Fire crews plan to try to strengthen what lines they've been able to establish and continue burning out forested areas in front of the main fire to try to stop its advance. It was officially just 6 percent contained Friday, but the actual numbers likely are higher, Whittington said.
The advances came on the fire's north side, near the working-class towns of Springerville and Eagar on the edge of the forest. Nearly 10,000 people have been evacuated from the two towns and from several mountain communities in the forest.
On Friday, fire officials gave reporters the first look at two of the mountain communities
- Alpine and Nutrioso - in nearly two weeks, driving them through the deserted resort towns and surrounding areas.