Unburned gas from stoves and ovens in California are leaking alarming levels of benzene into homes across the state, particularly in the San Fernando Valley, a new study shows.

In the study titled “Composition, Emissions, and Air Quality Impacts of Hazardous Air Pollutants in Unburned Natural Gas from Residential Stoves in California” — published in the science journal, Environmental Science and Technology — researchers measured the contents of unburned natural gas in stoves and ovens in 159 households across seven geographic regions, from the Northern San Fernando Valley to Sacramento.

The study covers the three major gas companies in the state: Pacific Gas and Electric (PG&E), Southern California Gas Company (SoCalGas) and San Diego Gas and Electric (SDG&E).

What the study shows is that samples from the Northern San Fernando Valley and Santa Clarita Valley contained several high benzene concentration areas — the maximum concentration observed was 66 times greater than the highest benzene level recorded in a similar study in Boston.

The study also shows that SoCalGas service areas had the highest variability and the highest overall concentrations of benzene. More than 22 percent of samples in SoCalGas stoves were above the maximum benzene concentrations observed from PG&E and SDG&E samples.

Additionally, the study found that natural gas leakage from stoves and ovens that are plugged in, but not in use, can result in indoor benzene concentrations that are comparable to environmental tobacco smoke, such as secondhand smoke.

The study indicates that there is an estimated 4,200 kg of statewide benzene emissions from leaks across California a year; a similar amount emitted by 58,800 cars annually.

One possible explanation for the high concentration of benzene in Southern California is due to the close proximity of the Aliso Canyon Natural Gas Storage Facility and the Honor Rancho Natural Gas Storage Facility. Fourteen samples taken near the Aliso Canyon facility exhibited some of the highest benzene concentration levels observed throughout the entire study.

“When you draw from those fields that are old, repurposed oil fields, it could be that the potential of benzene concentrations is higher because we know that the concentrations are high in oil residue,” explained Alexandra Nagy, California director of Sunstone Strategies.

“And what we’ve learned from the Aliso Canyon gas blowout is that the benzene concentrations were really high during that incident.”

Benzene is a non-methane volatile organic compound that is found in unprocessed natural gas that has been labeled a hazardous air pollutant under the Clean Air Act. It’s still unclear how much benzene is left in processed natural gas at the point of end use. It’s also a natural part of crude oil, gasoline and tobacco smoke.

Benzene is known as a human carcinogen that increases the risk of cancer through long-term exposure, such as leukemia, and is a cause of bone marrow failure.

The level of exposure to benzene without incurring adverse health effects, called a reference exposure level (REL), is 0.94 part per billion by volume over an 8-hour period. The study found that indoor benzene reached over 700 percent of the REL in the Northern San Fernando and Santa Clarita Valleys.

All stoves that samples were taken from were leaking natural gas, including recent models. Nagy also explained that leaks are found throughout the oil and gas system, from pipelines to compressor stations and storage facilities.

To check if your stove or oven is leaking, Nagy recommends calling your gas utility to make a home safety check for a possible leak. 

“Just on its own, sometimes the leaks are so small, but so consistent, that you maybe wouldn’t smell it or wouldn’t notice it, but it is leaking,” Nagy said.