The Tijuana River’s polluted waters don’t just contaminate Southern California’s beaches – they also release toxic gases and aerosols that travel far beyond the riverbanks, threatening the health of nearby communities, according to a new study. The Tijuana River Valley, straddling the US-Mexico border, faces a severe and worsening pollution crisis as untreated sewage, industrial waste, and toxic runoff flow into the Pacific, causing prolonged beach closures and persistent environmental health risks. While most concern has centered on direct contact with contaminated water, mounting evidence shows pollutants can aerosolize, becoming airborne and dispersing far beyond the riverbanks. This overlooked pathway means communities may face greater exposure through inhalation than through direct contact with contaminated water. With over half the global population living near waterways, understanding the impact of water pollution on air quality is an urgent yet understudied public health priority.

Building on previous research that traced airborne bacteria and chemical pollutants near the mouth of the Tijuana River in San Diego, California, and guided by community reports of foul odors and health symptoms of those who live near the river, Benjamin Rico and colleagues identified a turbulent stretch of the river as a likely hotspot for gas and aerosol emissions. This prompted the authors to use a mobile air quality lab to measure hydrogen sulfide (H₂S) – a toxic gas produced by the breakdown of sewage – as an airborne tracer of water pollution. They found that the record high dry-season flows of 2024 led to a significant spike in H₂S emissions, with nighttime peaks reaching 4500 parts per billion (ppb) – thousands of times above typical urban levels (<1 ppb). According to Rico et al., the findings highlight the impact that turbulent portions of contaminated rivers have on regional air quality. Because existing air quality models omit emissions from polluted rivers and estuaries, incorporating these pathways is critical for accurately predicting health impacts, addressing inhalation risks, and guiding mitigation. Moreover, the exceedingly high H₂S concentrations confirmed the validity of long-dismissed community observations, highlighting the disproportionate burden borne by marginalized communities. “Sustained monitoring, coordinated cross-border efforts, and leadership from federal, state, and local authorities are crucial to finally provide the protection and justice long denied to communities affected by this ongoing environmental and public health crisis,” Rico et al. write.

A segment of Science's weekly podcast related to this research will be available on the Science.org podcast landing page [http://www.science.org/podcasts] after the embargo lifts. Reporters are free to make use of the segments for broadcast purposes and/or quote from them – with appropriate attribution (i.e., cite "Science podcast"). Please note that the file itself should not be posted to any other Web site.

Researchers at Washington University in St. Louis find sea spray concentrations, used in measuring cloud formation, can vary widely and estimates made from shore should not apply to more lower levels of spray on the open ocean.