A relentless fire in a compost yard is keeping firefighters busy. Smoke from the South Valley fire is blanketing the metro area. 

While firefighters have most of the flames under control, it’s the smoke and air quality that has them concerned. 

Thursday morning, the smoke was so thick, residents say their throats were itching. 

The city put out a warning for the area about the air quality, urging people to stay inside. 

Bright orange lit up the sky Thursday morning as crews battled what seemed like a never-ending fire. 

“When crews arrived, they found a very large compost fire. It was about an acre upon our arrival and it’s grown to about five acres,” said Brian Rose, Bernalillo County Fire Department. 

KRQE News 13 is told the inferno sparked by heat and a chemical reaction in the decomposing compost at the Gro-Wells site, formerly known as Western Organics. 

Crews used heavy equipment and helicopters to put out the flames. 

Firefighters say these efforts are helping, but the heavy smoke is making them nervous about the air quality for people close by. 

“I’m a little more concerned with people with asthma or breathing difficulty,” Rose said. 

Firefighters say as the day progresses, they’re worried the smoke will blow into the South Valley homes, which prompted the city to issue a health alert. 

The whole situation has nearby residents Jim Romero and Angela Martines on edge.

“A little bit yeah,” said Martines. “I saw the smoke this morning from my house and it’s just kind of concerning what’s going on in the area.” 

“The smoke was pretty thick in this area,” said Romero. “It’s kind of thinned up now but it was actually causing kind of a raw throat you know. You could feel it in your throat obviously from breathing it.”

Firefighters are encouraging those in the South Valley to stay inside.

“I’m going to bring my pets in as well,” said Martines.

Romero hopes those battling the blaze won’t be affected.

“They’re right there, right in the middle of it,” said Romero. “We’re feeling it and we’re quite away from it. I just hope these guys watch out, that’s always a concern.” 

The health alert was extended until Friday.

That compost pile was about 25 feet hight and it’s still smoldering inside.

Firefighters said that compost fire could keep burning overnight.

PHOTOS: Fire erupts in South Valley

Sky News 13 South Valley fire video