Monitoring data reveal a significant increase in hydrogen sulfide exceedances near the U.S.-Mexico border, prompting legislative action and highlighting lingering health risks from the Tijuana River wastewater crisis.
Hydrogen sulfide measurements in South Bay exceeded the county's safety limit more frequently in 2026, indicating an upward trend in the presence of this toxic gas despite ongoing efforts to address the wastewater crisis and Tijuana River pollution along the U.S.-Mexico border.
Hourly readings at a hydrogen sulfide monitoring station in Nestor, near Berry Elementary School, surpassed the threshold more than 10 percent of the time so far this year-a rise from less than 2 percent recorded in 2024-according to monitoring data obtained from the San Diego County Air Pollution Control District. This represents the steepest increase among the three control points the district oversees; by comparison, since October 2024, only about 1 percent of hourly readings exceeded the threshold at Imperial Beach Civic Center, and around 0.5 percent in San Ysidro.
This trend aligns with state Senator Steve Padilla's (D-San Diego) push for Senate Bill 58. That measure would require the state Air Resources Board to develop a response framework and assess whether hydrogen sulfide standards, originally adopted in 1969 to address odors rather than health effects, are adequate. Hydrogen sulfide is a colorless gas with a strong rotten egg smell produced during the decomposition of wastewater and organic waste.
Scripps atmospheric chemist Kimberly Prather, who has been monitoring the gas near the border for nearly a decade, said her own data corroborates what county figures now show: the problem is not improving. Researchers have linked much of the region's worst hydrogen sulfide pollution to a specific stretch of the river known as the Saturn hotspot, near Saturn Boulevard in the Nestor community.
Prather's team identified this site after observing that hydrogen sulfide levels spiked when wind blew from inland rather than from the ocean, which was the opposite of what they initially expected. South Bay residents "are breathing elevated hydrogen sulfide-don't get me wrong-but they're also breathing quite high levels of other important pollutants that they shouldn't be breathing," Prather said, adding that hydrogen sulfide "attacks mitochondria, the brain, and the nervous system," raising particular concern for children.
The gas is released from water due to turbulence. As untreated wastewater and industrial debris churn and bubble at that hotspot, bubbles burst and release hydrogen sulfide and other pollutants into the air. When Mexican authorities briefly diverted wastewater from that location in September 2024, hydrogen sulfide levels dropped dramatically-though not to zero-confirming that turbulence at that specific spot was the primary source. The county has been measuring hydrogen sulfide levels since about October 2024.
Before that, residents complained about the odor and perceived health effects-complaints that lacked scientific backing and were largely ignored. Prather's team compared years of neighborhood odor complaints with their own hydrogen sulfide measurements taken around September 2024 and found the two sources of information matched almost perfectly.
"The people were acting as sensors," Prather said. "Basically, they were being psychologically manipulated, being told it's nothing and not to worry about it. Their concerns were completely dismissed.
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