A woman has been hospitalized with burns over 40 percent of her body after a freak fire at a Salt Lake gas station.

It happened about 1 a.m. Friday at a Chevron station at 502 S. West Temple. Salt Lake City police said the 25-year-old woman had gone into a restroom on the south side of the station.

"While attending to her business, she decided to light up a cigarette. The room instantly flashed, resulting in second- and third-degree burns over 40 percent of the victim's body," police said in a statement Monday.

Deputy Salt Lake City Fire Chief Devin Villa said Monday the flash fire ignited the restroom.

"The room was fairly well consumed," he said.

Arson investigators were called out to try to determine the cause of the fire, which has since been declared accidental. Authorities initially were focusing on either methane from the sewer or fumes from a truck refilling the station's gas tanks.

"The cause was determined to be fumes from filling the tanks," Villa said. "We're following up to see if there's any violations that might possibly be something faulty."