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 conclusions in the report mark the first time a federal agency publicly affirms what residents have been saying for years.

Sewage pollution in the Tijuana River has been flowing for decades. But it was only about two years ago that scientists and others officially started monitoring air around the river. At the time, county officials tried to ease concerns about risks associated with exposure to hydrogen sulfide.

“The levels are not posing a public health hazard. I wanna make sure I emphasize that,” Former County Board Chair Nora Vargas said at a 2024 news conference.

Since that time, there’s been continuous air monitoring and a growing understanding of exposure to sewer gas.

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.

The report found levels of hydrogen sulfide were highest at Leon Avenue, across the street from Berry Elementary School.

Air monitoring data has influenced how South Bay Union School District, which includes Berry Elementary, has responded to exposure. That included spending half a million dollars on air purifiers in classrooms, said Jose Espinoza, the district superintendent.

“Whenever the readings are elevated during the school day, we have protocols in place to, you know, bring students indoors for recess for lunch,” he said. “It’s altered life in our community, not just in our schools.”

Virginia Castellanos is one of the district’s school nurses who lives in Nestor, near the river. She said she’s noticed her daughter’s asthma worsens when pollution spikes.

“Either that same day or a couple days later, she starts having, like, a runny nose, a cough, and then she ended up having to go to the emergency room,” she 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.

In a statement, the county said that while ATSDR’s assessment “does not capture some of the significant hydrogen sulfide elevations seen in 2026, it does further validate the community’s long-held concerns about Tijuana River Valley pollution, in particular the hydrogen sulfide in the air, that while most concentrated around the Saturn Boulevard river crossing, regularly impacts the entire community.”

The report was released just a week after the county completed, in partnership with the CDC, a second survey of community households to check in on how they are faring and learn of any new needs. Results are expected next month.

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 Sept. 30.