It’s safe to breathe low levels of hydrogen sulfide for about an hour, according to state air regulators.

But that was also the case 53 years ago – despite national and state health and safety officials saying there are some consequences, such as headaches.

That’s something folks in El Segundo know all-too well. Since last summer, when the nearby Hyperion Water Reclamation Plant experienced nearly catastrophic flooding, El Segundo residents have dealt with a persistent rotten egg smell caused by hydrogen sulfide coming from the facility.

While officials at Los Angeles Sanitation & Environment, which operates Hyperion, have regularly said they are working hard to reduce odors from the plant – and have cited state and regional air regulators calling low-levels of hydrogen sulfide being safe to breathe –the odor has been so overwhelming at times that people couldn’t even open their doors and windows, and have experienced headaches, runny noses and other symptoms.

And now, USC’s Environmental Justice Research Lab is looking into how safe long-term exposure to hydrogen sulfide really is, even at low levels, and what impact the constant odor has on El Segundo residents’ mental and physical health.

“The problems aren’t daily, but when it stinks you’ll get a headache, runny eyes, (be) super tired, your skin will itch,” said El Segundo resident Corrie Zupo. “It’s acute and not a high priority, but if you’re exposed to it all the time, if you have a headache every day it’s going to drive you crazy.”

Hydrogen sulfide is a flammable, colorless gas that people can usually smell at low concentrations, ranging from 0.0005 to 0.3 parts per million, according to the Centers for Disease Control and Prevention.

And at those low levels, folks can experience effects from mild headaches or eye irritation, to unconsciousness and death, according to the U.S. Occupational Safety and Health Administration.

But one hour of exposure to that low-yet-potentially dangerous level of 0.03 parts per million is the California Air Resources Board’s standard exposure limit for hydrogen sulfide. That limit, called the Ambient Air Quality Standard, was adopted in 1969 – and remains unchanged.

The California Department of Public Health, meanwhile, reviewed the standard in 1981 and concluded that it was still adequate, according to the board.

When asked for a comment, CARB referred the Southern California News Group to the state’s Office of Environmental Health Hazard Assessment.

Amy Gilson, deputy director of External and Legislative Affairs for that latter agency, said in a Wednesday evening, Nov. 23, email that there isn’t currently enough information to know whether long-term exposure to hydrogen sulfide at currently acceptable levels has long-term health effects — but acknowledged it could.

“If people experienced 24/7 exposure at 0.03 ppm hydrogen sulfide over many years that would certainly raise concerns about long-term health effects,” Gilson said. “However, one hour a day for two years falls short of that duration.”

The U.S. Environmental Protection Agency, for its part, does not have a national ambient air quality standard for hydrogen sulfide.

USC’s goal in the study is to better understand and document how low levels of hydrogen sulfide impact residents’ mental and physical health. But Zupo, who reached out to USC for the study and has spearheaded several community efforts to seek relief from the odors, said that the potential outcome is bigger than Hyperion or El Segundo. Rather, Zupo said, she hopes the results will urge the California Air Resources Board to bring its hydrogen sulfide exposure limit up to date.

“You’d think the criteria would be updated based on new information,” said Zupo, who works in environmental management. “Any day where it’s 0.01 parts per million, everyone has a physical reaction; there’s nothing legally that can be done.”

Hydrogen sulfide is currently regulated as a nuisance, not as a toxicant, said Arbor Quist, a postdoctoral research fellow with USC’s Department of Population & Public Health Sciences. Many studies on the gas have shown that exposure to very high concentrations of it causes severe health effects among workers, she added, but community studies that examine lower levels have mixed results.

Hyperion recycles about 260 million gallons of wastewater every day for landscape irrigation, industrial processes and groundwater replenishment.

But in July 2021, Hyperion’s headworks facility became overwhelmed and the plant started to flood.

Plant officials faced a tough choice: Either release 17-million gallons of untreated sewage into the ocean or risk the sludge backing up into people’s homes and onto surrounding streets — and potentially having the entire facility go offline.

But more than 50% of the plant flooded that day anyway, causing equipment failure and a massive cleanup effort.

For three weeks afterward, sludge had nowhere to go as engineers tried to get a handle on the problem. The toilets kept flushing. And the sewers kept sending waste to Hyperion.

That’s what initially caused the stifling stench that overwhelmed El Segundo. Hyperion officials said the odor would go away with time.

And while the stench isn’t as bad as it was in the immediate aftermath of the flooding, El Segundo residents and officials have continued complaining about a lingering odor, saying community members have had health problems because of the air pollution.

The gas occurs naturally in crude petroleum, natural gas, volcanic gas and hot springs; it also comes from industrial sources like petroleum refineries, natural gas plants and food processing plants, per the CDC. And hydrogen sulfide can come from bacterial breakdown of organic matter, which happens during digestion – and ends up as waste.

Because it is heavier than air, hydrogen sulfide can collect in low-lying and enclosed spaces, such as manholes, sewers and underground telephone vaults, according to OSHA, making it potentially dangerous to work in confined spaces where it’s present.

Health effects, however, depend on how much hydrogen sulfide the person breathes and for how long.

The type of exposure El Segundo residents are experiencing doesn’t kill people or cause cancer, Zupo said, but can cause headaches and exhaustion.

But there is a caveat to the symptoms El Segundo residents have reported.

El Segundo is surrounded by the Los Angeles International AIrport and industrial companies such as Chevron.

So that makes it hard to know the exact cause of the community’s health issues, Zupo said, though she added that she’d never had any problems until the Hyperion spill happened.

USC researchers will begin their study by surveying residents to assess their experiences with the odors and health symptoms associated with them, Quist said.

They’ll assess how the odors and air pollution affect residents’ lung function, blood pressure, methemoglobin levels and sleep quality, Quist said. Researchers expect results from the study in a year or two.

The USC Environmental Justice Research Lab has also been working on understanding Carson residents’ health symptoms from elevated hydrogen sulfide levels near the Dominguez Channel.

The putrid aroma began there after a large fire broke out on a warehouse property in 2021. The blaze, which took several days to extinguish, resulted in chemicals contained in the stored products, including ethanol, passing through the sewer system into the local flood control waterway.

Organic materials in the channel began to anaerobically decay. Combined, those circumstances caused elevated levels of hydrogen sulfide to emanate from the channel — leading to the rotten egg smell that plagued locals.

The Dominguez Channel and El Segundo studies, Zupo said, will hopefully provide information for agencies to realize that health standards need to be updated.

Residents can take the survey at


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