Chevron explains source of rotten egg smell that elevated hydrogen sulfide levels in Richmond last week
on October 27, 2026
Chevron said Monday the rotten egg odor that wafted through West Contra Costa County communities late last week was caused by a part of the Richmond Refinery’s flare system that was out of service, allowing gas containing hydrogen sulfide to reach the flare.
“The immediate issue has been resolved, and we are committed to working with the Air District and County Hazardous Materials officials to investigate the incident and implement appropriate corrective actions,” the company said in a Facebook post Monday.
The odor prompted 130 complaints to the Air District last Thursday and Friday, as the leak spewed hydrogen sulfide concentrations above established thresholds for six hours.
Wendy Allen was and Laura Haynes were driving from Oakland to Richmond on Interstate 80 Thursday evening when they smelled the unpleasant odor around Albany.
“It got to the point where the closer we got to Richmond, I had to take my scarf and put it over my face because I couldn’t stand it,” Allen, of El Sobrante, said. “When we got home, I had to run around and close all the windows because the smell in the house was horrible.”
Allen then checked Nextdoor.com to see if there was any information, but nobody seemed to know anything, she said.
Haynes, her housemate, received an alert from San Pablo police just before 6 p.m., saying the odor was coming from an unknown source and that Contra Costa County Fire Protection District and PG&E were investigating. She received a Nixle alert at 11:20 p.m. that said “strong odor reported in parts of Richmond. Local authorities continue assessing the area.”
San Pablo police posted an update on Facebook at 7:07 a.m. Friday, saying the odor had dissipated and there was no danger to the community at that time. It also noted the Air District was investigating.
In a Facebook post, Contra Costa County Supervisor John Gioia questioned the timing of Chevron’s notification, saying that at about 8:30 p.m. Thursday, Chevron said it thought it had identified the source. However, the company did not issue a community alert until after midnight. “I am very concerned about the 4-hour gap between when the odors were traced to the Refinery and when Chevron’s CWS alert went out,” Gioia wrote.
Tracking the odor
The Air District began receiving odor reports just after 6 p.m. Thursday and immediately sent staff to investigate, according to an Air District incident report.
At 7 p.m. the Air District’s air monitoring site at Seventh and Hensley streets showed peak one-hour average hydrogen sulfide concentrations above 200 parts per billion. That is more than the one-hour health benchmark of 30 parts per billion established by the California Office of Environmental Health Hazard Assessment.
At 8:30 p.m. the Air District traced odors to the Castro Street and Richmond Parkway area, with the strongest odors concentrated around Vernon Avenue and Seventh Street. Staff from the Air District and Chevron met at the refinery at 10:45 p.m. to trace the odor to an area “downwind of the refinery flares,” according to the report.
Monitors showed hydrogen sulfide concentrations remained above the standard for six hours between 5 p.m. Thursday and 1 a.m. Friday, the Air District report said.
During that time, Chevron quality monitors from North Richmond south to Atchison Village also measured elevated levels of hydrogen sulfide. Chevron has monitors at three locations around the refinery’s fence line, and three monitors in the community. After midnight, hydrogen sulfide concentrations rose in Point Richmond to 10 parts per billion.
Chevron sent out a Level 1 community alert at 12:19 a.m. Friday about a release of hydrogen sulfide that had migrated offsite. Level 1, the lowest alert category, is for a non-emergency hazardous materials release. It requires residents to take no action. A Level 2 alert is used when hazardous material goes off-site. It’s unclear why a Level 2 alert was not issued. Before 2025, Contra Costa County residents were not alerted to Level 1 events, which is how many refinery flares are classified. The change came out of a recommendation from a Contra Costa County grand jury.
On Monday, Contra Costa County Health told Richmond Confidential that because the peak hydrogen sulfide concentration did not reach 300 parts per billion over an eight-hour period, a Level 1 alert was appropriate. Spokesperson George Barahona aid in an email that Level 1 “is generally used to guide emergency response actions, such as health advisories or evacuations.” Asked if the Health Department had heard of any community health impacts, Barahona said, “We expect any health impacts to be reversible for most people.”
Levels return to normal
By 4 a.m. Friday, the hydrogen sulfide concentrations had returned to typical background levels at all air monitors.
Chevron spokesperson Caitlin Powell said in an email that the company worked in collaboration with the county and the Air District to investigate the source of odor and to determine when to alert the community.
In a Facebook post Friday afternoon Chevron said it had been reviewing operations and investigating conditions at the refinery since Thursday night.
According to the federal Occupational Safety and Health Administration, hydrogen sulfide is a flammable and highly toxic gas used in many industries including oil and gas refining.
Exposure to high concentrations of hydrogen sulfide can induce eye tearing and symptoms related to overstimulation of the sense of smell, including headache and nausea.
(Top photo of the Richmond Refinery by Kyle Sweasey)
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