Exposures to some levels of hydrogen sulfide in outdoor air near the sewage-laced Tijuana River are a public health hazard, according to a new report by the Agency for Toxic Substances and Disease Registry (ATSDR).
In a 72-page report released Wednesday, the agency concluded that short-term exposure to the sewer gas can lead to health effects such as headaches, cough, nausea and shortness of breath. But longer-term exposures, defined as two weeks or more, and chronic exposures, or at least one year, are not expected to cause harm to people’s health beyond those symptoms.
The federal public agency, which works alongside the Centers for Disease Control and Prevention, analyzed more than 800,000 air measurements collected between September 2024 and May 2025 at four monitoring sites in South County.
Its findings were based primarily on continuous air monitoring data from monitoring sites maintained by the San Diego County Air Pollution Control District and UC San Diego, as well as weather data, odor complaints submitted to the air district, interviews with people who worked in the area and health symptoms reported in public health surveys.
The findings are limited to data analyzed between September 2024 and May 2025 because the agency said it did not have sufficient data before then.
“ATSDR cannot conclude whether breathing hydrogen sulfide in outdoor air in the Tijuana River Valley before September 2024 could have harmed people’s health,” the report said. “ATSDR understands that the Tijuana River sewage issue has persisted for many years and exposures to hydrogen sulfide in outdoor air have almost certainly occurred as a result.”
Hydrogen sulfide, or H2S, is a colorless gas that smells like rotten eggs. It’s a byproduct of sewage pollution that frequently spills over the U.S.-Mexico border from Tijuana by way of the Tijuana River before emptying into the Pacific Ocean.
People can begin to smell H2S in the air at levels as low as 0.5 parts per billion. According to the nonprofit Environmental Working Group, one drop of water in an Olympic-sized swimming pool equals 1 ppb.
The report said most daily average concentrations of the gas exceeded the 0.5 ppb odor threshold at all four monitoring sites.
“Specifically, 1-minute hydrogen sulfide readings exceeded the odor threshold at the San Diego Fire Station in San Ysidro, Leon Avenue, Imperial Beach Civic Center, and Berry Elementary School monitoring sites 70%, 82%, 89%, and 96% of the time, respectively,” the report said.
County Supervisor Paloma Aguirre, whose District 1 includes the communities affected by the pollution, said the federal agency’s report confirms what her constituents have long reported.
“Today, a federal public health agency put it in the record: hydrogen sulfide in our air reached levels that can harm people's health and well-being,” she said in a statement. “For two years the answer was 'it's just a smell.' Today the answer is 'it's a public health hazard.' That's the difference between being dismissed and being believed.”
The report comes after the county Board of Supervisors agreed earlier this month that the county should explore treating pollution in the Tijuana River, reforming how the region manages the decades-long pollution crisis.
ATSDR said Wednesday’s report focuses exclusively on exposure to hydrogen sulfide in the air.
A future report is expected to address potential exposures to chemicals in surface water, sediment, and soil.
The agency plans to discuss the report’s findings and answer questions during a virtual meeting on September 30th.
