The explosion happened at around 11:30 a.m. as work was being done to install a new floor in the basement of a home in the 1600 block of West Winona Street, Chicago police said.
The worker was taken to Stroger Hospital where he was last listed in critical condition. Other workers were on the scene but no one else was injured, police said.
"They were putting in a new floor in the basement, and they had a blow torch, and I don't know if there was some kind of chemical the torch hit," homeowner Mary Dellorto said.
Four people have been displaced, police said.
"If I had walked the dogs, I don't think my mother, she wouldn't have gotten out," homeowner Ellen Dellorto said. "I came in and was sitting on the couch with my dogs, and all of a sudden I felt the floor an explosion. I ran out to the back and I said, 'what happened?' and he was like, 'there's a fire.'"
Fire and smoke quickly filled the house the family has owned for nearly 25 years.
Ellen and her wife Mary, who was at work at the time, live on the duplex's first floor. Ellen's mother, Bea Dellorto, who sometimes uses a wheelchair, lives a second-floor unit and could have been trapped.
"I heard the explosion and then the smoke detector went off immediately, and the house was filled with smoke," Bea said. "We survived. We survived, except Lokee did not survive."
Lokee was the family's cat.
Police and fire officials continue to investigate the exact cause of the explosion and fire.