South Bay Residents at Risk from Long-Term Toxic Gas Exposure

The toxic gas turns its victims’ blood green by chemically altering proteins. Hundreds of millions of years ago, after volcanic eruptions destroyed oxygen in the world’s oceans, bacteria took over and transferred so much of this lethal gas into the air that almost everything on the planet died. 

But it’s also something the human body produces. This gas relaxes blood vessels, which is partly why scientists think it could cure erectile dysfunction or aid in slowing down bodily functions to keep someone alive. 

Two molecules of hydrogen bound to one of sulfur create hydrogen sulfide gas, a stinky vapor that wafts from sewer grates, swamps and oil refineries. Decomposing organic matter release its widely identifiable odor of rotten eggs — the scent of death.  

People living next to the sewage-polluted Tijuana River knew it well, but not by name.  

That changed last summer when scientists and local politicians called the news media to a press conference and revealed hydrogen sulfide gas was spewing from the river water at alarming amounts.  

The “toxic soup,” emanating from pollution that spills over the U.S.-Mexico border via the river “impacts people for miles,” claimed atmospheric chemist Kim Prather from the University of California-San Diego’s Scripps Institution of Oceanography on X back in September of 2024. 

She discovered a gas “hot spot,” where the riverine waters tumble from a few concrete pipes. Whatever’s in the water mixes with the atmosphere. Air samples taken by her team showed hydrogen sulfide would, at times, reach concentrations almost 100 times above what California’s air quality standards say is OK.  

Prather called it a crisis, perhaps, worse than the infamous case of drinking water contamination in Flint, Michigan. The news inspired panic. San Diego County’s public health department sent their HAZMAT crews to investigate. Some South Bay schools cancelled outdoor activities.   

County politicians and public health officials sowed confusion by attempting to debunk the scientists’ claims. And all the while, reporters struggled to understand the science of hydrogen sulfide in real time.  

When should the presence of hydrogen sulfide trigger emergency actions like closing outdoor activities at schools? Was it even safe to breathe in South Bay?  

Since then, the county installed hydrogen sulfide-sniffing monitors in Tijuana River-adjacent communities. They’ve quietly been recording gas levels for the past year. Occasionally, usually overnight when Mexico’s wastewater is flowing over the border, the gas concentrations climb high enough to trigger an odor warning from air quality regulators, the San Diego Air Pollution Control District.  

“The problem here is this is not just an odor,” said Richard Gersberg, a now retired professor at San Diego State University who studied how chemicals and other pollutants pose risks to human health. “This is a health risk. And there’s health effects at levels perhaps well below where people can smell it.” 

A Voice of San Diego analysis of the Air Pollution Control District’s data shows South Bay’s hydrogen sulfide have stayed far below what Prather and her team detected last September. Even those concentrations wouldn’t instantly kill anyone – which hydrogen sulfide is more famously known to do.  

But the data shows that, on average, San Diegans from San Ysidro to Imperial Beach are breathing levels of the gas the U.S. Environmental Protection Agency says would cause harm to a person’s health in the long run.  

More science is needed to definitively link the presence of this gas to South Bay residents’ current health issues.  

The U.S. Centers for Disease Control rolled into South Bay a month after the hydrogen sulfide scare to take stock of residents’ health concerns. They talked to about 200 families, and 40 percent listed air quality as their greatest concern. 

“Many describe unpleasant odors and perceive the air quality as poor because of the contamination,” the CDC wrote in its final report. “Residents also report increased health concerns (including GI and respiratory issues), although there is limited evidence from syndromic surveillance.”  

The Air Pollution Control District continues to record gas levels but refers all of its data to state and federal agencies. They’ll make the call on what to do to protect public health, not us, the district spokesperson said.  

While South Bay waits for an answer, one thing is clear: People are living with a gas problem.   

Steady Stream of Gas in South Bay 

Following the revelations of the scientists last summer, the San Diego Air Pollution Control District set up three hydrogen sulfide monitors in South Bay: one at Berry Elementary School in Nestor and nearest to Prather’s riverine “hot spot;” another on a fire station in San Ysidro and another on the Imperial Beach Civic Center.  

Voice of San Diego calculated how much hydrogen sulfide gas those monitors picked up on average each month from all the readings taken since last summer.  

The results paint a grim picture for San Diegans living nearby.  

At Berry Elementary School, hydrogen sulfide stayed at an unsafe level, above what the EPA warns could cause long term health problems, for almost a full year — from October 2024 to September 2025.  

At the other two locations – San Ysidro and Imperial Beach – hydrogen sulfide rarely fell to a safe concentration.    

“That does indicate a risk existed chronically,” said Gersberg. “A risk which may have been both unappreciated and uncontrolled by both local and state regulators.”   

Nora Hernandez lives with it every day at her home near Berry Elementary School which three of her children attend. Two of her children have asthma and all three have complained of body aches which no doctor, she says, can explain.  

“It almost always smells bad,” Hernandez said. “But what are we supposed to do? It’s coming from another country but it’s affecting us here.”  

Antonio Cuevas lives across the street from Berry Elementary with his partner and two daughters who go to school there. They also smell the river’s stink daily and have to close their windows and run fans during hot days with little wind, which causes the smell to intensify. When the smell becomes unbearable, they sometimes sit in their car or drive to a shopping mall, he said. 

“We’re thinking of moving somewhere else, maybe Chula Vista,” Cuevas said. “We cannot stay here.”  

The South Bay Union School District purchased and installed over 400 air purifiers in Berry Elementary classrooms and office spaces in response to last fall’s gas scare, confirmed Amy Cooper, executive assistant to the superintendent.  

The district keeps tabs on gas levels recorded by the monitor attached to the school building. If concentrations are elevated, Cooper wrote, the school moves children to an indoor “rainy day” schedule and runs air purifiers in all indoor spaces to improve air quality.

“We are doing everything possible to protect students and staff from any potential elevation in those levels,” Cooper wrote. 

San Diego Air Pollution Control District’s community hydrogen sulfide guidance recommends school activities remain indoors when concentrations reach 30 hydrogen sulfide particles in 1 billion particles of air.  That’s only happened once or twice this past school year because most gas levels spike overnight, Cooper wrote. But there have been several close calls when levels dropped rapidly around 6 a.m. just before the school day began. 

But there’s wide disagreement among air quality and public health regulators over what amount of hydrogen sulfide should be tolerated. To some, the local air regulators’ guidance is, well, misguided.  

Hard to Say How Hydrogen Sulfide Impacts Health 

If you ask Gersberg, nobody was – or is – assessing the risk of the Tijuana River’s airborne pollution properly.   

“The only way to really ask, is this causing health effects, is to do a risk assessment,” Gersberg said.  

That’s done by monitoring the exposed population over a long period of time through an epidemiological study, which can take years and millions of dollars to complete. (San Diego County Supervisor Paloma Aguirre, the former mayor of polluted Imperial Beach, is pursuing money to do just that.)  

The next best thing is to look at the county’s gas data and compare it to what Gersberg referred to as “the bible”: the U.S. Environmental Protection Agency’s Integrated Risk Information System. The EPA says if out of one billion particles of air, 1.4 of them are hydrogen sulfide, then prolonged exposure to that air – many weeks to months – could make someone sick.   

This guideline is based on scientists exposing rats to hydrogen sulfide for six hours a day for 10 weeks. Those rats developed lesions in their nostrils. Few scientific studies, beyond the rat tests, have shown what could happen to the human body under such low-level gas exposure conditions over months to a year.    

The residents of Rotorua, New Zealand, living near geothermal pools along a volcanic fault zone are a rare case of a population living under the constant fog of hydrogen sulfide. Scientists discovered more Roturans live with cardiovascular diseases compared to other New Zealanders. But they couldn’t pin the cause on the gas. The data didn’t accurately account for the residents’ socioeconomic status or whether they also smoked. 

How H2S Can Kill 

When there’s a whole lot of hydrogen sulfide in an enclosed or confined space, the gas can kill a person pretty quickly. It’s called “knock down.” The gas paralyzes the nerve in the nose that allows us to smell its signature rotten egg scent, robbing its victims of the main warning sign that it’s there at all.  

Knock down happens when the gas reaches 500,000 parts per billion or more. It’s what scientists call “acute” exposure.   

People have died within minutes from a single, large and concentrated exposure to the gas — even just one breath. The respiratory system shuts down or seizes; skin turns a bruised color at the lips, ears or fingernails. Some people have involuntary muscle convulsions that look like seizures, the heart can stop and the gas can send people into a coma.   

Workers have died instantly from the gas after falling through a manhole into the sewer. Farm workers almost died after walking into a tank filled with pig manure. These dramatic deaths from mostly job-related exposures to a lot of the gas all at once are what color its reputation. It’s what most scientists study or what people in public health or emergency response know more about.   

Even so, it took a while for the U.S. government to take hydrogen sulfide seriously.  

Congress removed hydrogen sulfide from the EPA’s list of hazardous air pollutants (toxins that can cause death or serious, irreversible illness from routine exposure) in the early 1990s under pressure from the oil and gas industry, one of the largest man-made sources of the gas. Regulators slowly began compiling evidence on its toxicity after the rat study. ExxonMobil did its own in 2015 which showed low levels of the gas were associated with breathing impairment, eye and neurological effects.  

Still, hydrogen sulfide isn’t officially considered a pollutant the federal government requires industries to manage. Low levels of hydrogen sulfide from typical emitters – refineries, confined animal feeding operations or CAFOS, and landfills – mix with other contaminants in the air. So it’s hard for scientists to isolate whether hydrogen sulfide is the culprit for a specific health problem, researchers say. 

Even NASA isn’t totally sure how much hydrogen sulfide is safe. For its upcoming missions to the moon, NASA worries hydrogen sulfide trapped in moon rock ice taken from the lunar surface might sublime – turn from a solid to a gas – and release inside the spaceship. Almost all of the agency’s Spacecraft Maximum Allowable Concentrations for 24-hour to 180-day crew exposure to the gas during the upcoming Artemis III mission to send humans to the unexplored South Pole are based on those same rat studies.  

“Spaceflight presents a unique environment…continuous exposure for limited duration (seven days to six months), (in a) completely enclosed environment and lack of escape, emphasizing the need for Spacecraft Maximum Allowable Concentrations to be established for standard spaceflight duration exposures to hydrogen sulfide,” NASA’s 2024 report on the matter reads. 

At the Tijuana River “hot spot” where Prather was testing, the levels of hydrogen sulfide never reached a concentration that could cause sudden death or “knock-down.” State officials poo-pooed her alarms last fall.  

“Hydrogen sulfide levels would have to be much higher (5,000 parts per billion or more) to expect widespread health impacts,” Amy MacPherson, a CARB, spokesperson told Voice of San Diego, referring to the levels Prather found.   

Prather persisted.   

“It’s really important to listen to the community. So many eye, ear, nose, throat infections… skin infections… heart and lung issues, mental issues, the list goes on,” she told Voice in an email.  

Prather published her hydrogen sulfide data from the fall of 2024 in the journal Science. Gas levels peaked for minutes at a time at 4,500 parts per billion between Sept. 5 and Sept. 9 of last year. At least once, the gas reached an average hourly peak of around 2,100 parts per billion. That’s 70 times higher than the California Ambient Air Quality Standards say is detectable by the human nose. 

But those high levels of gas production by the Tijuana River pollution didn’t last. By Sept. 11, spills into the river and, therefore, its flows, dramatically halted. 

Since the installation of the San Diego Air Pollution Control District’s monitors shortly thereafter, hydrogen sulfide concentrations haven’t risen beyond around 500 parts per billion. 

Still, Gersberg agrees with Prather: That doesn’t mean people aren’t getting sick. 

 “If the data continues like it did last summer, I wouldn’t want to live there and I certainly wouldn’t want my kids going to school there,” Gersberg said. 

The post South Bay Residents at Risk from Long-Term Toxic Gas Exposure appeared first on Voice of San Diego.