(CNN) -- Firefighters made inroads into the massive wildfire in eastern Arizona Sunday, as authorities said the threat of spreading flames had decreased enough to allow thousands of residents in some of the hardest-hit areas to return home.
"After consulting with fire officials, it has been determined that the fire risk has diminished enough that it will no longer be a threat to the citizens of the towns," said Cmdr. Webb Hogle of the Apache County Sheriff's Office.
But he said smoke from the wildfire "still poses serious health hazards" in Springerville, Eagar and South Fork, where authorities expect thousands to return home after evacuating.
"If they return, they do so at their own risk," he added.
The 15-day-old blaze in eastern Arizona was 10% contained, fire officials said Sunday evening, up from 6% late Saturday. Still, the wildfire inched closer to becoming the worst fire in the state's history.
The second-largest blaze had scorched 443,989 acres, firefighters said Sunday, an area bigger than most of the largest cities in the United States.
The so-called Wallow fire is 24,649 acres shy of matching the Rodeo/Chediski wildfire of 2002, Arizona's biggest.
Firefighters said they are making progress as they dig trenches, set their own fires to take away natural fuels from the advancing blaze and dump retardants from the air on the flames.
It "has been chasing us around, but after today we're feeling very optimistic," Jerome MacDonald, operations chief for the Southwest Interagency Incident Management team fighting the fire, told reporters late Saturday.
The fire broke out May 29 in the Apache National Forest.
Air quality for much of New Mexico and eastern Arizona was expected to worsen because of wildfires, the New Mexico health and environment officials said.
Heavy smoke could affect those in sensitive groups, including children, pregnant woman, asthma sufferers and people with lung and heart diseases, the New Mexico Environment Department said.
The National Weather Service said smoke plumes from Arizona wildfires will continue moving toward Santa Fe and Albuquerque, New Mexico.
Heavy smoke has permeated communities around the Arizona blaze.
"We have not seen the sun clearly for several days," said Rita Baysinger, a spokeswoman for the Southwest Interagency Incident Management team.
MacDonald said changing winds are likely to start clearing out the smoke.
"All the smoke from the fire has basically been funneling through Eagar in the valley and it settles there in the morning," MacDonald said. "I don't expect the same kind of smoky conditions over the next couple of days."
At least 9,114 people were evacuated from Springerville, Eagar, Nutrioso, Alpine, Greer and surrounding areas.
The wildfire has consumed 29 homes, 22 of them in the evacuated city of Greer.
CNN's Phil Gast and journalist Craig Johnson contributed to this report.