Several agencies are involved in the search.
The skies look hazy but it's the smell that sent fire trucks to Fairmont late Wednesday night after guests at the luxury hotel made a stink about a stink.
"Not a pleasant smell, rotten eggs and sulfur," one guest described. That's the most common description officials have heard in the dozens of calls from people in neighborhoods throughout the city over the last two days.
SF's swanky Fairmont hotel-one of the places firefighters visited searching for the source of a mysterious odor! pic.twitter.com/1LfbsFbL7x— carolyn tyler (@ctylerabc7) December 29, 2016
But the utility's inspectors have determined the odor is not caused by a gas leak. "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.
So what is fouling the air? The Chevron Refinery in Richmond could be the culprit, according to the Bay Area Air Quality Management District. Monitors around the refinery's fence line show there were flares over the past two days and at least one was a sulfur release.
ABC7 News Meteorologist Drew Tuma says wind fluctuations at the time could have sent smells wafting toward San Francisco.
"This is one of many potential sources. We're also looking at local wastewater treatment facilities, local landfills or it could be a combination of courses," Bay Area Air Quality Mgmt District's Kristine Roselius said.
MORE: At least 50 reports of rotten smell in San Francisco
If you smell an unusual odor and want to report it... you can call the Bay Area Air Quality Management District's Pollution Complaint Line: 1-800-334-ODOR (6367)