What to Know A strong gas-like odor led to the evacuation of Center City office buildings Monday.

The odor could be smelled from 15th and Market to 22nd and Chestnut.

While health officials said the additive Mercaptan was released during a cleaning at the former PES refinery in South Philadelphia, firefighters have not yet confirmed whether that was the source of the odor.

A strong smell similar to gas was reported in the heart of Center City Philadelphia Monday, leading to the evacuation of several office buildings.

Office workers gathered at 1601 Market, 22nd and Chestnut, near the Clothespin sculpture at 15th and Market streets and at other sites where the smell was detected after 11 a.m.

The smell was detected the same day cleaning took place at the PES refinery in South Philadelphia, which closed after a dramatic fire and explosion in June, 2019, health officials said Monday.

A spokesman for the Philadelphia Health Department said that the refinery had alerted the Philadelphia Fire Department that it would be cleaning tanks Monday as part of their closing process.

That cleaning included the release of Mercaptan, an additive used in the refining process, said health department spokesman James Garrow. "There is no threat to human health from this additive," Garrow said.

The city office of emergency management later confirmed that Mercaptan was released and an odor had wafted to other parts of the city.

Philadelphia Fire Dept has responded to Philadelphia Energy Solutions refinery to investigate citywide reports of an odor. PES reports there was a release of mercaptan, a non-hazardous substance, during equipment cleaning. If you suspect gas leak on your property, please call 911 — Philadelphia OEM (@PhilaOEM) March 2, 2020

The Fire Department and Office of Emergency Management are handling the investigation. Fire trucks could be seen at various locations in the busy corporate center of Philadelphia shortly before noon.

The Philadelphia Fire Department confirmed that it had received multiple calls about a strange odor Monday morning. They have not confirmed however whether the odor was caused by the Mercaptan or whether or not it originated from the refinery.

"We don't know the cause of these odors," Philadelphia Fire Commissioner Adam Thiel said Monday afternoon.

After an investigation at PES, the PFD Hazmat Task Force cannot confirm the initial report of a mercaptan release at the refinery. PES did clean equipment today, but the actual source of the smell remains undetermined. — Philadelphia Fire (@PhillyFireDept) March 2, 2020

PGW said it received more than 500 calls for the odor.

"We have not found any hazardous conditions on the incidents that we've investigated," Thiel said.

Center City buildings being evacuated because of smell of gas. Source of odor still unknown according to fire department. Edificios evacuado en el centro de la ciudad de Filadelfia. Todavía los bomberos no saben de dónde viene el olor de gas. @NBCPhiladelphia @Telemundo62 pic.twitter.com/zJAJ8GKWXS — Lucy Bustamante - News Anchor/Journalist (@LBustamanteNBC) March 2, 2020

This story is developing and will be updated.