SAN DIEGO (CNS) — This week's high heat, combined with untreated wastewater flows from Mexico in the Tijuana River Valley, has led to elevated hydrogen sulfide emissions, county officials announced Friday.
The gas, often recognized by a strong rotten egg smell, is being tracked by the San Diego Air Pollution Control District's network of monitors.
According to the county, work to repair a broken pump in Tijuana's sewage infrastructure may also be contributing to the foul smells. The US International Boundary and Water Commission reported a pump failure at Mexican Pump Station PB1 caused transboundary flows at Stewart's Drain on Saturday and Sunday. While odor reports tend to rise regionwide during heat events, the Tijuana River Valley experiences dry-weather flows of untreated wastewater.
"We are committed to supporting the South Bay community," said Dr. Sayone Thihalolipavan, county public health officer. "We will continue to coordinate to expand resources and find long-term solutions to the unacceptable conditions in the Tijuana River Valley."
The USIBWC is in contact with construction and project managers in Mexico and has been "working extensively with Mexico to mitigate transboundary flows as much as possible," a county statement read.
People in South Bay communities can take steps to reduce the odor by limiting outdoor activities, keeping doors and windows closed, increasing air circulation indoors using fans or air conditioning on recirculation mode and running an air purifier with a high-efficiency particulate air filter. Those more sensitive to air pollution, like those with asthma, chronic obstructive pulmonary disease, or other chronic lung conditions, should keep rescue medication(s) readily available, the county advises.
Anyone with persistent, worrisome, or worsening symptoms should reach out to their medical care provider or seek emergency care if symptoms feel life- threatening.
Imperial Beach residents expressed worries about the air quality.
"As a physician who both lives and practices in the community, I have observed an increase in patients presenting with respiratory symptoms and headaches consistent with elevated hydrogen sulfide exposure," said Dr. Kimberly Dickson. "I am deeply concerned about both the acute and long-term health impacts of exposure to pollution from the Tijuana River. The situation is unacceptable and demands urgent intervention."
The odors of hydrogen sulfide and other toxic gases are not new to the southern part of the county near the Tijuana River.
In research published last year, scientists from UC San Diego's Scripps Institution of Oceanography, UC Riverside, San Diego State University and the National Science Foundation's National Center for Atmospheric Research found that extremely high levels of the toxic gas hydrogen sulfide -- among others -- are making its way into the homes and lungs of many in the county.
"This study reveals a direct airborne pollutant exposure pathway -- from contaminated rivers into the air we breathe," said Kimberly Prather, the lead investigator of the study, a UCSD professor and director of the Center for Aerosol Impacts on Chemistry of the Environment. "For the first time, we've shown that poor water quality can profoundly degrade air quality, exposing entire communities to toxic gases and other pollutants.
"These findings validate the experiences of residents who have endured this crisis for decades, and also underscore the urgent need for action to protect public health in San Diego and in vulnerable communities worldwide."
In 2024, the number of foul odor complaints spiked precipitously among residents near the Tijuana River. Enough so that Prather told her team to pull out of the area until the gas levels dropped. The readings the team took found levels of hydrogen sulfide 4,500 times higher than what is typical for an urban area.
Strangely, though, the highest readings were not coming from the river itself or even the ocean, but in an inland area in the Nestor neighborhood.
What the scientists found was that a wastewater discharge pipe was expelling thousands of gallons of sewage into a confined area, creating turbulence and foam. When the bubbles in the foam burst, they released gases into the air -- more than 1,000 different kinds, including many that are toxic. These aerosolized pollutants then produced the noxious odors residents had noted for years.
"The community was acting as sensors, but felt they were being dismissed," Prather said.
Once the source of much of the smell was located, the UCSD team began monitoring gas levels at the site off Saturn Boulevard, which became known as the "hot spot." Elevated gas levels corresponded to increase odor complaints at an almost perfect correlation. The residents of the area had clocked the noxious gas better than scientific measurements up to that point.
Symptoms reported to that point mirrored those associated with exposure to hydrogen sulfide. According to UCSD, the health effects of long- term exposure to hydrogen sulfide are not fully understood, but the California Office of Environmental Health Hazard Assessment sets a chronic exposure limit of 7.3 parts per billion, while the California Air Resources Board's one-hour standard is 30 ppb, a level based on odor, not health.
The teams monitoring the South Bay found peaks as high as 4,500 ppb and up to an average of 2,100 ppb for one hour -- peaking at night, when winds die down.
"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, particularly among vulnerable populations," said Paula Stigler Granados, an environmental health scientist at SDSU's School of Public Health. "Framing this as merely an odor issue dangerously understates the real public health risks of repeated exposure to toxic gases at such concentrations."
The San Diego Air Pollution Control District has an online air quality dashboard showing current levels of hydrogen sulfide to help residents limit their exposure when concentrations are high. The SDAPCD also sends out alerts to all residents when hydrogen sulfide levels go above 30 ppb.
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