New research has found that the Tijuana River, which carries untreated sewage and industrial waste from Mexico into the United States and onto the Pacific Ocean, is not just a water problem – it’s also a major source of air pollution.
A study by scientists at UC San Diego’s Scripps Institution of Oceanography, UC Riverside, San Diego State University and the National Center for Atmospheric Research found that the river releases large amounts of hydrogen sulfide, of rotten-egg smell fame.
Researchers measured hydrogen sulfide levels up to 4,500 parts per billion (ppb), more than 4,500 times higher than what is typical in urban areas and nearly 70 times higher than California’s one-hour standard. These peaks frequently occurred at night, when winds die down and pollution lingers.
Benjamin Rico, a UC San Diego PhD candidate and lead author of the study. said: ‘Our results validate the community voices that have been saying that air quality near the Tijuana River has been a problem for many years.’
Hydrogen sulfide exposure can cause headaches, nausea, respiratory problems, and fatigue. The California Office of Environmental Health Hazard Assessment recommends chronic exposures remain below 7.3 ppb, but residents near the river were exposed to levels far exceeding that limit for hours at a time.
Paula Stigler Granados, an environmental health scientist at San Diego State University, said: ‘This level is too high for chronic exposures as 30 parts per billion is already associated with headaches, nausea, respiratory symptoms and other adverse health effects. Framing this as merely an odour issue dangerously understates the real public health risks.’
In addition to hydrogen sulfide, the team detected hundreds of other gases, some with known health risks. Atmospheric modeling and odor complaint records confirmed the Tijuana River as the pollution source. When a wastewater pump station in Mexico diverted flows on September 10, 2024, pollution levels dropped dramatically — further evidence linking the river to the toxic emissions.
Kimberly Prather, an atmospheric chemist at UC San Diego who led the project. said: ‘This study reveals a direct airborne pollutant exposure pathway – from contaminated rivers into the air we breathe.’
As short-term relief, San Diego County is providing free air purifiers to local residents, and the Air Pollution Control District has created an online dashboard with real-time hydrogen sulfide data and alerts. Long-term solutions, researchers argue, must include upgrading wastewater treatment infrastructure on both sides of the border.
Representative Scott Peters, who helped secure federal funding for the study, said the findings should spur action: ‘This peer-reviewed study links water pollution to worse air quality with clear and convincing data. I’ll keep working toward binational infrastructure solutions which strengthen public health and coastal resilience.’
Photo: Beatriz Klimeck/UC San Diego
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