More than 100 billion gallons of raw sewage and industrial waste have flowed into the Tijuana River since 2018, creating a cross-border environmental crisis that has exposed tens of thousands of Southern California residents to toxic gases and widespread health problems, according to the International Boundary and Water Commission.

The 120-mile river originates in Tijuana, Mexico, crosses into California, and empties into the Pacific Ocean. Research conducted by University of California, San Diego scientists found hydrogen sulfide concentrations in affected neighborhoods reached levels 4,500 times higher than typical urban areas and 150 times higher than California air quality standards during peak river flows.

A 2024 survey by San Diego County and the Centers for Disease Control and Prevention of approximately 40,000 households near the river found that 71% could smell sewage inside their homes and 69% had at least one family member become ill from exposure. Local doctors report treating patients for migraines, nausea, respiratory problems, and eye infections that correlate with increased river flows.

Environmental Protection Agency Administrator Lee Zeldin said during a February visit that resolving the crisis will take approximately two years. The United States and Mexico signed an agreement in 2024 to upgrade wastewater treatment plants to handle Tijuana's population growth and industrial waste from factories. The sewage contains hydrogen sulfide, a toxic gas that can cause headaches, nausea, respiratory problems, and neurological damage, according to the CDC.

San Diego County has distributed over 10,000 air filters to affected homes, while some residents have installed hospital-grade filtration systems. The contamination has forced the closure of nearby beaches for years and has affected military personnel training in the area. Just since January 2026, the river has carried 10 billion gallons of mostly raw sewage and industrial waste across the border.