RICHMOND (CBS SF) — Flaring on Saturday night at the Chevron refinery in Richmond is being investigated, the Bay Area Air Quality Management District said.

The flaring, which started about 6:25 p.m. and continued until 6:40 p.m., was “due to a processing unit upset,” the district said, noting that its inspection staff was on-site to investigate the incident.

READ MORE: Victims Of EDD Bank Of America Debit Card Fraud Could Be Taxed On Benefits They Never Received

“The flare is a safety device that burns pollutants during these types of upsets and prevents them from being released directly into the atmosphere,” the district said in a statement.

The refinery said the incident was listed as a Level 1 on the Community Warning System, the lowest on the scale and issued to keep residents informed.

READ MORE: 5 Charged In SF Corruption Probe, Temporarily Barred From Receiving City Contracts

“Flares are highly regulated safety devices designed to relieve pressure during the refining processes and help keep our equipment and plants operating safely,” the refinery.

The air district said its investigation is ongoing.

MORE NEWS: COVID: SF Restaurants Use High-Tech Tools To Make Indoor Dining Safe

Residents were advised to go to www.richmondairmonitoring.org to keep track of current air quality data.