The blast occurred around 11 a.m. Monday. Two small babies and a teenager were among the injured, as was a contractor working on the water heater in the basement of one of the destroyed homes.
The contractor was taken to Temple University Hospital's burn unit in critical condition and treated for burns on 22 percent of his body.
"There was a fella who was all burned up, the firemen were hosing him down with a hose," said witness Michael McGraw. "He was really in bad shape."
Seven others were treated at Thomas Jefferson University Hospital with minor injuries.
Officials evacuated the 400 block of Daly street as a precaution, as well as another nearby street, totaling 70 homes.
The Philadelphia Gas Works shut off gas service to the block.