According to local television station KTVU, an employee at the Golden Gate Adult Superstore said the man was watching a video in the back of the store last Wednesday when he suddenly ran out screaming -- in flames.
Witnesses report seeing the man running down the street and collapsing.
"There was an ambulance crew that was in the area -- a private ambulance company -- that witnessed this person collapse, so they (extinguished the flames and) called it into the dispatch center," San Francisco Fire Department spokeswoman Mindy Talmadge told AOL News.
The man, whose name has not been released, is being treated for first, second and third-degree burns that reportedly cover 90 percent of his body.
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"He suffered life-threatening injuries," said Lt. Troy Dangerfield of the San Francisco Police Department. "He's still recovering."
Investigators have no idea what caused the man to catch fire. Oddly, Talmadge said investigators have found "no damage to the inside of the building at all."
One local pastor said he knows what caused it: God.
"I believe so. I definitely believe so," Roger Huang told AOL News. He has been protesting against sex shops for the past seven years. He sees this as a divine sign that his efforts are working.
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"I don't know if you believe in prayer," Huang said, "but ... the last seven years, every August, I go to every store and pray that it [will] be closed down. We have had seven or eight that have been closed down ever since."