Miguel Ceja, 31, was making "honey oil," which is extracted from marijuana, in the apartment at 934 Clark St. around 8 p.m. when the explosion happened, Santa Rosa police Sgt. David Linscomb said.
Linscomb said the extremely combustible butane Ceja was using in the manufacturing process was ignited by a heat source, possibly a furnace pilot.
Ceja suffered severe burns over most of his body and was taken to a hospital by his girlfriend, Linscomb said. He was later flown to St. Francis Memorial Hospital in San Francisco for treatment of life-threatening burns.
Two women who were in the second-story apartment near downtown Santa Rosa were not injured, Linscomb said.
The Santa Rosa police and fire departments are investigating the incident.