A new federal report confirms that exposure to hydrogen sulfide in the Tijuana River Valley causes short-term health problems, but found that the gas does not result in chronic, long-term illnesses.
On Sept. 23, the Agency for Toxic Substances and Disease Registry (ATSDR) released a report titled “Environmental Contamination in the Tijuana River Valley: Evaluation of Hydrogen Sulfide in Outdoor Air.” The report concludes that malodors caused by hydrogen sulfide – similar to the smell of rotten eggs – causes a public health hazard only during short-term exposures that can lead to health effects such as headaches, nausea and shortness of breath.
“ATSDR does not expect that intermediate (more than 14 to 364 days) exposures to hydrogen sulfide in the Tijuana River Valley will harm people’s health,” said the agency in the report. “… ATSDR does not expect that chronic (one year or more) exposures to hydrogen sulfide in the Tijuana River Valley will harm people’s health.”
The report comes after university researchers discovered alarming readings of hydrogen sulfide in the Tijuana River Valley in 2024.
That year the County of San Diego and the City of San Diego installed signs in the Tijuana River Valley to warn residents and visitors of hydrogen sulfide gas stemming from the untreated wastewater flows in the Tijuana River.
The county also partnered with the Centers for Disease Control and Prevention (CDC) and ATSDR to conduct community health surveys, a Community Assessment for Public Health Emergency Response and an Assessment of Chemical Exposures, gathering data of health concerns among people living and visiting the area.
Gov. Gavin Newsom signed two bills in September, Senate Bill 58 and Senate Bill 1046, that call for updating the state’s hydrogen sulfide air standard and workplace protections for lifeguards, park rangers and others working outdoors in the Tijuana River Valley by the new year, the San Diego Union-Tribune reports.
According to the county, this report is part of a public health assessment (PHA) by the CDC and ATSDR to evaluate whether exposure to chemicals in the Tijuana River Valley could harm the health of community members.
“For two years, families in Nestor, San Ysidro, Imperial Beach, and Coronado have said the air was making them sick. Headaches. Burning eyes. Kids who couldn’t sleep. Constant stress and anxiety from living with air that didn’t feel safe to breathe,” said Supervisor Paloma Aguirre in a statement on Sept. 23. “Today, a federal public health agency put it in the record: hydrogen sulfide in our air reached levels that can harm people’s health and wellbeing. For two years the answer was ‘it’s just a smell.’ Today the answer is ‘it’s a public health hazard.’ That’s the difference between being dismissed and being believed.”
ATSDR presents findings
According to the report, the agency began this work in January 2025, eight months after members of the California congressional delegation petitioned for an investigation. Six months later, ATSDR visited the Tijuana River Valley as part of its assessment.
Jona Johnson, the report’s lead author, said a future report will address potential exposures to the chemicals in surface water, sediment, and soil.
Johnson said ATSDR assessed the health impact of hydrogen sulfide using data provided by the San Diego County Air Pollution Control District (SDAPCD) and the University of California San Diego (UCSD).
The agency received air data collected at four monitoring locations in 2024 and 2025: San Diego Fire Station No. 29 in San Ysidro, Berry Elementary School, Leon Avenue, Imperial Beach Civic Center.
Johnson said hydrogen sulfide levels were measured above the level where people can begin to smell, 0.5 parts per billion, at all four sites up to 93% of the time.
According to Johnson, hydrogen sulfide affects people in two different ways: through symptoms experienced in response to malodors, and direct toxic health effects caused by the gas at much higher concentrations.
“Our noses can detect these odors at very low levels. And when you smell an odor, it activates the olfactory nerve. And stronger odors can stimulate the trigeminal nerve, which senses that irritation,” said Johnson. “Your brain processes these signals and can trigger physical responses, such as headache, nausea, irritation of the eyes, nose, or throat, coughing, chest tightness, or general feeling of discomfort or stress. And emotions associated with stress, such as anxiety, depression, anger, confusion. For people with asthma, odors may also trigger symptoms.”
Johnson said hydrogen sulfide at the Leon Avenue and the Berry Elementary School monitoring sites reached levels for short periods of time that could harm people’s health.
“Exposure to hydrogen sulfide at this level is a public health hazard,” said Johnson.
However, Johnson said the report’s third conclusion is that longer term hydrogen sulfide exposure (14 days or more) at all locations fail to pose a public health hazard or result in additional health effects beyond those linked to short-term elevated exposures.
“This means that while short-term spikes and hydrogen sulfide can occasionally reach harmful levels, and frequent odors can cause odor-related symptoms, the longer term average exposure levels are below levels that would cause additional health problems,” said Johnson.
Acknowledging the years-long crisis, Johnson said ATSDR did not have sufficient air monitoring data to make definitive public health conclusions about exposures prior to September 2024.
Limitations and uncertainties in the report include available monitoring, the lack of chronic hydrogen sulfide exposure studies and information available on possible health effects in children from hydrogen sulfide exposures.
“We don’t have evidence that hydrogen sulfide can lead to cancers,” said Dr. Michelle Zeager, ATSDR medical officer.
ATDSR called on multiple agencies and community members to continue addressing hydrogen sulfide concerns.
“We’ve funded a permanent redesign at Saturn Boulevard, where the river releases the most gas; we’ve expanded air monitoring and put a live dashboard in residents’ hands; and we’ve delivered free air purifiers to 14,500 homes,” added Aguirre. “This report explains why our Capture and Cleanup plan matters. The science is simple: fix the river and the air clears that night.”
The public can submit comments on the report by Nov. 21 by emailing OCHHAPublicComment@cdc.gov or by mail to: Agency for Toxic Substances and Disease Registry Attn: Records Center 4770 Buford Highway NE, MS S106-5 Atlanta, Georgia 30341.
“ATSDR will include comments received in our final report, along with the description of how we responded to the comments,” said Johnson.
For more information, visit: http://www.atsdr.cdc.gov/.
