New Federal Health Assessment of Tijuana River Valley Air Pollution Builds on UC San Diego Findings

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• Lauren Fimbres Wood - [email protected]

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The Agency for Toxic Substances and Disease Registry (ATSDR), a federal public health agency within the U.S. Department of Health and Human Services (HHS) that works jointly with the Centers for Disease Control and Prevention (CDC), released a new health consultation report on Sept. 23, 2026, that analyzed environmental contamination in the Tijuana River Valley and evaluated hydrogen sulfide levels in outdoor air.

The public health consultation report, which used data from scientists at the University of California San Diego in the assessment, released the following conclusions:

1. Environmental odors in the Tijuana River Valley can negatively affect people's health and well-being.
2. Hydrogen sulfide in outdoor air at the Leon Avenue monitoring site, and likely the Berry Elementary monitoring site, has reached a level for short periods of time that could harm people's health. Exposure to hydrogen sulfide at this level is a public health hazard.
3. Longer-term hydrogen sulfide exposures (14 days or more) at the four monitoring locations are not expected to pose a public health hazard or result in additional health effects beyond those linked to short-term elevated exposures. However, they may still be associated with odor-related symptoms.

The conclusions from the federal authorities align with the research findings of UC San Diego scientists, who began taking air measurements at a Leon Avenue (Nestor community) monitoring site, and raised alarm about the high levels of hydrogen sulfide in September 2024. These findings, led by atmospheric chemist Kimberly Prather, were ultimately published in the peer-reviewed journal Science in August 2025, revealing that the polluted Tijuana River releases large quantities of the toxic gas hydrogen sulfide, and peak concentrations reached 4,500 times what is typical for an urban area.

For decades, the Tijuana River has carried millions of gallons of untreated sewage and industrial waste across the U.S.-Mexico border. The river passes through San Diego's South Bay region before emptying into the ocean, leading to ongoing beach closures and water quality concerns. Residents have long voiced concerns about the foul smells emanating from the river, reporting a myriad of health issues including eye, nose and throat irritation, respiratory issues, fatigue and headaches.

"For two years, families in the San Diego communities of Nestor and San Ysidro, and in the cities of Imperial Beach and Coronado, have told us the air was making them sick. Today a federal health agency, working from the same measurements, has put that experience into the public health record," said Prather, distinguished professor and distinguished chair in atmospheric chemistry at UC San Diego's Scripps Institution of Oceanography and Department of Chemistry, and co-founding director of UC San Diego's Airborne Institute.

Added Prather: "Families in the local communities pointed us to the smelliest stretch of the river, at Saturn Boulevard. That is the community where we placed our instruments, and today a federal health agency has reached the same conclusion from the same data. Polluted water becomes polluted air, and air reaches everyone who breathes it, far beyond anyone who touches the water."

At the Leon Avenue monitoring site, where Prather's team continues to take measurements today, the lab's air quality instruments recorded hydrogen sulfide at a one-minute peak of 4,500 parts per billion (ppb) and a highest one-hour average of 2,100 ppb. That is roughly 70 times California's one-hour limit of 30 ppb, the state's odor-based air quality standard, and 50 times higher than the state's 42 ppb health-based acute reference exposure level.

The CDC/ATSDR assessment was initiated after the agency received a petition from members of the California congressional delegation to investigate the public health impacts of Tijuana wastewater contamination. In July 2024, the agency gathered air, surface water, sediment and biological data from federal, state, local and university partners, determining that there were enough data to accept the petition. On Jan. 7, 2025, the congressional delegation was notified that ATSDR would conduct a public health assessment of hydrogen sulfide in the air.

The assessment looked at environmental sampling data from the UC San Diego monitoring site, as well as three additional sites established by the San Diego County Air Pollution Control District (SDAPCD) at Berry Elementary School, San Diego Fire Station #29, and the Imperial Beach Civic Center. Between 139,719 and 262,373 measurements for each monitoring site were analyzed for this report.

The public health assessment also released recommendations for federal, state and local agencies, as well as for schools and community members. The list of recommendations range from infrastructure updates including to "improve and maintain wastewater treatment infrastructure to manage flows in the Tijuana River Valley," to actionable steps for community members such as obtaining a free air purifier available to eligible residents from the SDAPCD Air Improvement Relief Program.

"When wastewater flows were diverted away from the river back in September 2024, hydrogen sulfide levels dropped sharply that same night, which shows the air responds immediately when the water is managed," Prather said. "Since then, the Air Pollution Control District has expanded its monitoring, launched a public hydrogen sulfide dashboard and provided free air purifiers to residents. The County's economic impact study has begun to put a price on this crisis, and those costs should be read as a lower limit: air carries the pollution well beyond the river and the beaches, spreading the impact across a much broader region of San Diego."

Prather added: "This month, the County Board of Supervisors funded a permanent redesign at Saturn Boulevard, the exact spot where the river releases the most gases and aerosols. Our plan now is to keep measuring, so every decision rests on the best evidence and the gains become permanent."

This health consultation is released for a 60-day public comment period, ending Nov. 21. ATSDR will address all public comments and revise or append the document. The health consultation will then be reissued as a final document. The findings released today are one of two reports ATSDR will produce. A second report will address other potential exposure pathways, which ATSDR has described as surface water, sediment and soil.

The Prather Lab's research in the Tijuana River Valley is supported through Community Project Funding executed through the National Oceanic and Atmospheric Administration (NOAA) and support from the Balvi Filantropic Fund.