The Garza Fire, which began in western Kings before crossing into Fresno County, grew to 41,709 acres Friday night, according to Cal Fire.
Better news was that containment had reached 60 percent.
More than 1,300 fire personnel are on scene, including some from the California National Guard, which also sent helicopters to assist.
Flames have been fueled by tall grass and brush after the fire initially broke out northwest of Tar Canyon Road.
A non-mandatory evacuation warning is in place west of Highway 33, north of the Fresno County line and south of Jacalitos Creek, according to Cal Fire.
The cause remains under investigation.
Residents, including some as far away as Atascadero, could see smoke rising out beyond the hills east of Paso Robles.
“We are aware of the visibility of the Garza Fire,” Cal Fire San Luis Obispo County wrote in a statement, “and are actively monitoring that situation and its proximity to San Luis Obispo.”
Air quality in the central San Joaquin Valley will take another dive because of the Garza and other fires burning in the state, the San Joaquin Air Pollution Control District reported.
Smoke from the Whittier Fire in Santa Barbara, the Schaeffer in Tulare County and the Alamo in San Luis Obispo are adding to particulate matter in the air.
A strong high pressure system is parked over the Valley, district spokeswoman Maricela Velasquez said, trapping the smoke. Spikes in particulate matter and ozone levels are possible.
A health alert will remain in place, and the district cautions that since ash from fire is larger than what its monitors detect, the Real-Time Air Advisory Network may not reflect exactly how much air quality has worsened.
“If you can smell smoke or see ash, that is an indication that you should be treating air quality conditions as unhealthy and remain indoors,” Velasquez said.
There were 13 wildfires in all burning in California as of Friday. The Alamo was at 28,687 acres but 90 percent contained.
The Schaeffer, which had burned 16,002 acres after it was triggered by lightning in Sequoia National Forest, was 61 percent contained as of Friday. The Whittier, at 52 percent containment, had burned more than 13,000 acres.