Doug Stanglin

USA TODAY

Air quality experts looking for the source of a "rotten egg" smell that has wafted over San Francisco for two days are investigating a wide-range of possibilities — including flaring incidents at a Chevron refinery, ships, landfills and wastewater treatment plants.

“Once we determine the source, we will pursue any and all enforcement actions available,” said Jack Broadbent, executive officer of the Bay Area Air Quality Management District.

The bad odor emerged early Wednesday, sending authorities scurrying to find the culprit. Fire trucks even responded to the luxury Fairmont hotel after complaints.

The odor, described as the smell of sulfur or rotten eggs, was reported around 2 a.m. Wednesday and again Thursday around midnight. It was not considered a health hazard.

A Chevron spokesman confirmed to the San Francisco Chronicle that there had been a flaring incident — a routine activity in which excess gas is burned off — but said its investigation at the Richmond facility showed it was not the source of the odor.

The air quality district said Thursday it had sent inspectors to the refinery to gather data. Investigators, who are analyzing wind and weather reports for the period, have also checked landfills, wastewater treatment plants and ships.

Initial reports of the smell poured into Pacific Gas and Electric for two days, but its officials ruled out a gas leak as a source of the bad odor.

"Even though we're saying this is not us, you should do the safest thing possible and make that call," PG&E spokesperson Teresa Jimenez said, ABC7 reported.