Bay Area health agency report reveals what sparked complaints about refinery odor

After residents complained of odors from the refinery on last Monday, Contra Costa Health’s investigation was summarized in the 72-hour incident report, which found approximately 1,075 pounds of sulfur dioxide were released during flaring that stemmed from an electrical issue.

Though no injuries were reported from the incident, the release of chemicals has reignited scrutiny and frustration among Martinez and Benicia residents who have witnessed a gas leak explosion, the release of carcinogenic byproducts and skyscraper-sized smokestacks from the refinery in recent years.

“MRC received eight odor-related calls from the community related to the incident,” the report states. “All monitoring results for Sulfur Dioxide (SO2) and Volatile Organic Compounds (VOCs) were zero and no odors were detected. Affected units were stabilized or shut down, and flaring stopped.”

At 10:50 a.m. on Jan. 26, an “operational upset” related to an electrical issue caused loud and visible flaring at MRC that spread the smell of sulfur dioxide, according to the report. MRC officials reported the flaring to the National Response Center, Contra Costa Health, Bay Area Air District, Contra Costa County Fire Prevention Department and California Office of Emergency Services

CCH issued a Level 1 alert for Martinez and Benicia while it activated its Hazardous Materials Program to sweep the borders of the refinery and test the air for any contaminants. MRC stabilized the affected refinery units and flaring stopped within an hour, according to an incident summary in the CCH report.

A “brief increase” in sulfur dioxide was detected at MRC’s fenceline, according to the CCH Hazardous Materials Program, and the chemical reached a maximum concentration of 200 parts per billion, an amount under exposure limits laid out in the Office of Environmental Health Hazard Assessment, according to the report.

The U.S. Environmental Protection Agency considers sulfur dioxide a harmful chemical for human health because it can cause severe respiratory issues, particularly for young people and the elderly. MRC wrote last week that flaring and odors stopped at approximately 1 p.m. on Jan. 26, adding, “there is potential for intermittent flaring” as employees return the refinery to standard operation.

“Flares are an essential part of a refinery’s integrated, engineered safety systems, which are designed to safely manage excess gases and pressure through efficient and effective combustion,” MRC wrote in a statement on Jan. 26. “We will investigate the root cause of the incident and identify corrective actions.”

Martinez Refining Company has been at the center of numerous controversies involving public health over the past five years, including the accidental release of 20 tons of coke dust, a toxic byproduct of gasoline production that looks like soot and spread across local homes in 2023; the discharge of millions of gallons of partially treated wastewater into a nearby marsh in 2023 that resulted in a $4.5 million fine under the Clean Water Act; and, most recently, a gas leak led to an explosion and subsequent fire in February 2025.

MRC stated in its report to CCH that the incident is still under investigation and a 30-day follow-up report will be submitted to the agency. The 30-day report will include preventative measures the company will take to avoid future incidents, MRC said.