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.

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“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.

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“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.

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