Environment Report: How Much Toxic Gas Is Too Much? Nobody Seems to Agree.

I had a hell of a time wading through claims made over the last few years about hydrogen sulfide, a toxic but also prevalent and sometimes useful gas that scientists discovered hovering above the polluted Tijuana River.  

The initial shock that hydrogen sulfide was spewing from the river at high levels came from scientists revealing unverified measurements – an uncustomary thing for scientists to do. And it triggered local schools to keep students indoors and the county to roll out emergency hazardous waste teams to check it out.  

At the time, California and federal officials were telling me the gas levels the scientists found weren’t that big of a deal, health-wise. And it seemed like every government agency I talked to – be it the California Air Resources Board or the U.S. Centers for Disease Control and Prevention – had different guidance on hydrogen sulfide gas exposure. I found one, albeit dated, publication out of University of California-Berkeley showing almost every U.S. state that regulates the gas disagrees on how much hydrogen sulfide is dangerous in a given timeframe.  

Hydrogen sulfide isn’t a regulated or enforceable pollutant – a win by the oil and gas industry in the 1990s who influenced Congress to drop the gas from the U.S. Environmental Protection Agency’s list of hazardous air pollutants.  

I landed on a few facts: One breath of a lot of the gas, especially in an enclosed space, can kill. Enough of it – when we can smell it on a hot windless day in the South Bay – irritates our eyes, nose, throat, lungs and the stink can cause mental stress.  

But the newest revelation, for me at least, thanks to former San Diego State University researcher Richard Gersberg, was that breathing just a little bit of the gas consistently, months to a year, could cause long term health impacts. That’s based on guidance from the EPA: In one billion particles of air, if more than 1.4 of them are hydrogen sulfide, scientists would expect long-term health impacts.  

Even that guidance isn’t perfect. It’s based on a few studies in which scientists exposed rats to the gas six hours a day for 10 weeks. Yet these same rat studies are referenced again and again in the scientific literature about chronic exposure to hydrogen sulfide – chronic meaning a few months to a year.  

So that’s the limit I chose to measure against a year’s worth of hydrogen sulfide data from the San Diego County Air Pollution Control District.  

The district via spokesperson Melina Meza told me they’re not a health agency, meaning they wouldn’t be drawing any conclusions from their own gas data. They’re giving the data to state public health agencies and the CDC (specifically the Agency for Toxic Substances and Disease Control) which is supposed to make recommendations on how the public should protect themselves against gas exposure. 

But the CDC’s recommendations were different from the EPA’s. The CDC’s gas limit was almost 30 times higher. So I asked the CDC how they’d interpret the data in South Bay.  

“We intend to use EPA’s Reference Concentration (RfC) as a screening value for chronic inhalation hydrogen sulfide exposure as part of our ongoing Tijuana River Valley public health assessment,” wrote Sharleta Stamps, a spokesperson for the CDC, in an email. 

OK, so at least we’re all finally on the same page now. Sorta. 

For whatever reason I can’t yet explain, Stamps added that CDC would likely be using a different range for what’s considered safe and dangerous gas exposure. Since the EPA’s “North Star”threshold of 1.4 parts per billion is based on rat studies, Stamps said CDC would use human-level equivalents. The CDC will consider 460 parts per billion (quite a bit more gas), a level that won’tnegatively affect health. But at 1,380 parts per billion, that’s the level one might expect adverse health effects.  

“ATSDR will likely use those levels in our Tijuana River Valley health assessment to help determine if chronic exposures could harm people’s health,” Stamps wrote. 

Gersberg, from SDSU, said that’s wrong.  

“Only the (EPA value) is the important risk value that should be used,” he said.  

Clear as mud. 

In Other News 

• In other hydrogen sulfide news, South Bay residents complain the county’s system for distributing air purifiers to combat toxic gas is fraught with complications. (Union-Tribune) 
• More people are taking the bus in San Diego since Covid-19. Even so, the metro transit system faces a looming financial crisis. (Union-Tribune) 
• The city of San Diego produced more planet-warming emissions between 2022 and 2023, which is not ideal for a mayor who promised to drastically cut them under a revised Climate Action Plan. I reported that back in October when the city fired its Chief Sustainability Officer. (Union-Tribune and Voice of San Diego) 
• San Diego is finally cracking down on bad composting two years after the city rolled out new green bins to collect organics including food waste. (Union-Tribune) Earlier this August I reported that the city had hired teams of workers to pick trash out of the compost by hand at the landfill. (Voice of San Diego) 
• Bonsall farmers want San Diegans to “adopt” an avocado tree and get organic avocados shipped to their home via an annual membership. (Call me when I can adopt an agave plant and get tequila shipped to my house.) (KPBS) 

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