Others being treated include a county road worker and two sheriff's deputies, Fresno County Sheriff Margaret Mims said.
The exact cause of Friday's blast is under investigation, Mims said, but it happened at the spot where a county worker was operating a front loader.
The explosion sparked a fire that roared like a jet engine, Mims said.
The operator of the front loader was injured but able to walk to an ambulance, the sheriff said.
The most serious injuries were suffered by a group of inmates who were assigned to maintenance and cleaning work at the sheriff's firing range.
There were 10 inmates near the blast site who were also injured, officials said. Three other inmates at the site were not hurt. Earlier, the sheriff's office included them in the injury count.
After the blast, Mims said, two sheriff's deputies who were at the firing range ran toward the fire to move the injured.
CNN affiliate KFSN
posted a video of the scene that shows a tall ball of fire rising from near a highway.
One inmate was airlifted to the hospital, the sheriff said. That inmate and the county worker were undergoing emergency surgery, she said. The others suffered mostly burns.
The pair of deputies who provided aid also went for treatment for ringing ears and sunburn-like conditions, she said.
Firefighters put out the blaze, revealing a crater at the blast site and a blackened front loader, Mims said.