How Raw Sewage Is Polluting the Air Along California’s Beaches

Researchers uncover the invisible threat of aerosolized sewage carried by sea spray onto California’s coast.

By Medha deb

Created on Sep 26, 2025

Researchers uncover the invisible threat of aerosolized sewage carried by sea spray onto California’s coast.

In recent years, communities along California’s southern coast, particularly near the U.S.-Mexico border, have faced not just polluted waters but also polluted air as a result of untreated sewage released into the Tijuana River. As scientists now reveal, toxic chemicals and bacteria from wastewater are becoming airborne, carried inland by sea spray aerosols generated by crashing waves. This invisible threat raises new public health concerns and calls for urgent cross-border action.

California’s Ongoing Struggle with Water Pollution

For decades, the Tijuana River has transported millions of gallons of untreated sewage and industrial waste from Tijuana, Mexico, across the border into southern California. This relentless flow has led to chronic water contamination in the region, resulting in more than 1,300 consecutive days of beach closures in places like Imperial Beach and long-standing complaints about foul odors and health issues among residents.

Sewage spills overwhelm the river, especially after rainfall, when urban runoff is highest.
Pollutants include not just human waste but also industrial chemicals, drug residues, and personal care products.
These recurrent pollution events prompt repeated beach closures, harming local recreation, tourism, and wildlife.

The Hidden Impact: From Water to Air

Until recently, attention focused mainly on the risk of entering contaminated water. However, new research from UC San Diego and partner institutions reveals that the hazardous material in the Tijuana River does not end at the shoreline. Through a complex process, sea spray aerosols—tiny airborne droplets generated when breaking waves burst bubbles at the beach—are transferring a cocktail of harmful chemicals and microbes from surf to sky, reaching communities inland near the border.

This means that even people who never go near the water are potentially exposed to contaminants from the river every time the wind blows inland from the ocean.

How Pollutants Become Airborne

Ocean water pollution can become aerosolized via the following process:

Stormwater runoff and sewage outflows transport pollutants—bacteria, viruses, industrial chemicals—into coastal waters.
Breaking waves and churning surf generate bubbles that trap these contaminants.
As the bubbles burst, they eject microscopic droplets (aerosols) into the air, which are then carried by coastal winds inland.

Researchers found that this transfer can carry pollutants for miles, exposing many more people than those simply swimming, surfing, or fishing near the pollution source.

Key Pollutants Detected in the Air

Sewage-associated bacteria and viruses
Hydrogen sulfide (“sewer gas”) at concentrations up to 4,500 times higher than urban baselines
Illicit drug residues and their byproducts, such as benzoylecgonine—a marker of cocaine use in human urine
Personal care product chemicals (e.g., octinoxate, a sunscreen component known to degrade DNA under light exposure)
Compounds from tire manufacturing and urban runoff

Some of these compounds, though present in tiny amounts, pose risks because they can penetrate deep into human lungs and even pass into the bloodstream when aerosolized.

Evidence Linking Pollution to the Tijuana River

Researchers used a combination of air and water sampling, chemical tracing, and atmospheric modeling to pinpoint the river as the source of airborne contaminants:

A sudden drop in river flow—following activation of a pump station in Mexico—was accompanied by a steep reduction in airborne chemicals like hydrogen sulfide upwind of local communities.
Atmospheric models only reproduced observed pollution patterns when the Tijuana River was used as the pollutant source.
Public complaints about odor and health symptoms, like headaches and respiratory irritation, surged on days with the highest concentrations of hydrogen sulfide.

Geographic Reach of the Pollution

Sampling across multiple coastal sites showed:

Pollutant concentrations were highest at Imperial Beach, close to the river’s mouth, but detectable as far north as La Jolla.
Inland communities several miles from the beach were affected by airborne contaminants, depending on wind and weather patterns.

Potential Health Concerns

Fine aerosols transported from the ocean can be breathed deep into the lungs, where hazardous chemicals and microbes may cause health effects. Reported symptoms in impacted communities include:

Eye, nose, and throat irritation
Headaches and fatigue
Respiratory complaints

Research confirms that hydrogen sulfide, in particular, is an irritant that can cause headaches and dizziness at high enough concentrations. Long-term exposure to a mix of sewage bacteria, chemical residues, and industrial pollutants may pose more severe risks, especially for sensitive populations like children and those with respiratory illnesses.

Knowledge Gaps Remain

Researchers stress that the full spectrum of health impacts is still not fully understood, as many chemicals detected in sea spray have poorly characterized effects when inhaled. Ongoing studies aim to close these knowledge gaps and inform health guidelines for coastal populations.

Beyond the Beach: Why This Matters

The findings reframe the issue of water pollution as one of public air quality, not just water safety. Impacts extend well beyond surfers and swimmers to anyone living, working, or visiting regions near polluted rivers and coastlines.

Direct Contact	Indirect (Aerosolized) Exposure
Swimming or wading in contaminated water	Breathing air containing sea spray aerosols with sewage pollutants
Gastrointestinal and skin infections risk	Respiratory irritation, headaches, potential long-term effects
Affects limited recreation groups	Affects entire communities, even far from the shore

Local and International Response

The persistence of transboundary water pollution underscores the complexity of fixing the problem. Solutions require collaboration between U.S. and Mexican authorities, investment in wastewater infrastructure, and ongoing environmental monitoring.

Recent upgrades to pump stations on the Mexican side have temporarily reduced pollution, but have not solved the root problem.
San Diego County and California state officials are campaigning for stronger federal involvement and international agreements to curb upstream sewage discharges before they reach the river.
Communities, scientists, and environmental advocates are calling for more robust systematic monitoring of both water and air quality to guide policy and protect public health.

Frequently Asked Questions (FAQs)

Q: What is sea spray aerosol and how does it form?

A: Sea spray aerosol consists of tiny droplets ejected into the air when bubbles at the ocean surface burst, typically at the shore where waves break. These droplets can carry dissolved and particulate pollutants from the water into the coastal atmosphere.

Q: How far can airborne sewage pollution travel inland?

A: Studies show that pollutants can be detected several miles inland, depending on the wind direction and strength. Communities near affected beaches, such as Imperial Beach and parts of San Diego, have reported airborne odors and symptoms linked to these aerosols.

Q: What chemicals have scientists found in the air near polluted beaches?

A: Chemicals detected include residues from illicit drugs (like benzoylecgonine), sunscreen ingredients (octinoxate), industrial byproducts, and gases like hydrogen sulfide. The mix depends on what is present in the river’s sewage and runoff.

Q: Can people get sick from simply breathing the air at these beaches?

A: Some residents have reported symptoms such as respiratory irritation, headaches, and fatigue. While more research is needed to establish long-term health outcomes, scientists caution that inhaling contaminated aerosols is likely riskier for vulnerable groups.

Q: What steps are being taken to address airborne pollution from the Tijuana River?

A: Authorities are working to improve sewage collection and treatment upriver, activate pump stations to divert polluted water, and ramp up air and water quality monitoring. Permanent solutions will require further cross-border collaboration and investment in infrastructure.

The Path Forward: Protecting Public Health

The emerging evidence from the Tijuana River region spotlights a pressing need for coordinated action to prevent sewage from polluting both waters and skies. Moving forward requires:

Modernizing and expanding sewage treatment plants in both Tijuana and southern San Diego County.
Enhancing binational agreements to hold polluters accountable regardless of which side of the border they operate on.
Prioritizing scientific research to track how pollutants move from water to air and deepen understanding of their health effects.
Keeping affected communities informed and empowered to demand protective measures.

Once seen as just a water quality issue, the story of the Tijuana River reminds us that what flows downstream can quickly become everyone’s problem, in ways both visible and invisible.

References

Author

medha deb

Medha Deb is an editor with a master's degree in Applied Linguistics from the University of Hyderabad. She believes that her qualification has helped her develop a deep understanding of language and its application in various contexts.

Read full bio of medha deb