A new study has found that pollutants in the Tijuana River—which carries untreated sewage from Tijuana into the Pacific Ocean, where it ends up on San Diego beaches—are also being released in the air as fine particles that can travel for miles.

Samples from air and water along the coast from Tijuana, Mexico, to La Jolla, California, revealed that ocean aerosols contained illicit drugs (including enough to make a fish addicted and experience withdrawal), drug byproducts produced from urine, and chemicals from tires and personal care products, researchers from the University of California–San Diego concluded in a study published on May 28 in the journal Science Advances.

The 120-mile-long Tijuana River in the Mexican state of Baja California empties millions of gallons per day of raw sewage, industrial waste, and runoff into the Pacific Ocean on the U.S. side of the border, making its way up San Diego beaches.

The pollution has persisted for decades, but researchers say particles get released into the air when the river crashes into ocean waves, creating a toxic spray that’s inhaled in the tens of nanograms per hour by those closest to the river in Imperial Beach, California.

“The global surge of untreated wastewater entering lakes, rivers and oceans poses a growing health threat,” one of the study authors, distinguished professor of atmospheric chemistry Kimberly Prather, said in a May 28 statement published on UC San Diego Today. “Aerosolization of this polluted water exposes billions of people through airborne transmission, reaching far beyond those in direct contact and impacting countless others who inhale contaminated air that can travel for many miles.”

Beaches in San Diego, including Imperial Beach and Coronado, have been closed periodically for years because of dangerous levels of bacteria in the water from the river. Researchers said the implications for health need to be studied further.

Researchers collected samples along the U.S.–Mexico border, Imperial Beach, and Scripps Institution of Oceanography in La Jolla between January 2020 and March 2020. And because nothing has changed since then regarding how the raw sewage is processed, released, and managed, researchers say the findings are still relevant.

Researchers used benzoylecgonine (BZG), a cocaine byproduct found in urine, to trace sewage origins. After rainfall, BZG levels spiked in Imperial Beach ocean water alongside increased Tijuana River flows, with corresponding spikes in aerosols tied to sea spray emissions. The researchers found high correlations among BZG, methamphetamine, octinoxate from sunscreen, and dibenzylamine, a chemical found in tires.

“It’s been shown that octinoxate can degrade DNA when exposed to light,” Jonathan Slade, a co-author and associate professor of chemistry at UC–San Diego, said in a statement. “And if it’s in these tiny aerosols we’re breathing in, it can get deep into our lungs and pass into our bloodstream. That’s very concerning, especially considering the high levels at which we found it in the air.”

Researchers said air pollutants were significantly higher near the river mouth along the U.S.–Mexico border and in Imperial Beach than they were further up the coast in La Jolla.

“We simply don’t know the human health implications of chronic exposure to these chemical pollutants,” Slade said.

Residents of Imperial Beach, one of San Diego’s southernmost beaches on the frontlines of the sewage crisis, have long complained of headaches, insomnia, respiratory illness, and foul smells. The river was recently designated as the second-most endangered U.S. river by nonprofit environmental group American Rivers.

“Often the sewage crisis is considered a water issue—and it is—but we show that it’s in the air, too. Truthfully, we don’t yet know the acute health effects,” Slade said. “But the numbers we report can be incorporated into models to help us better understand what we’re breathing in and how much we’re exposed to.”

During a visit to the Tijuana River Estuary in San Diego in April, EPA Administrator Lee Zeldin said the Trump administration is working with Mexico to end the sewage crisis.

Zeldin said that “a lot of good, insightful ideas” aimed at resolving the crisis had come out of a roundtable discussion with bipartisan leaders.

“We don’t want the 70 percent solution or the 90 percent solution, but we all need to be on the same page on the 100 percent solution from the U.S. side,” Zeldin said at a press conference at the time.