Heavily polluted Tijuana River drives regional air quality crisis

Industrial chemicals and untreated sewage have polluted the Tijuana River for decades, recently causing >1300 consecutive days of California beach closures. In summer 2024, wastewater flows surged to millions of gallons per day despite no rain, enhancing water-to-air transfer of hydrogen sulfide (H 2 S) and other toxic gases at a turbulent hotspot. High wastewater flows and low winds led to nighttime H 2 S peaks, reaching 4500 parts per billion (ppb)—exceeding typical urban levels of <1 ppb. H 2 S levels and community malodor reports were strongly correlated (correlation coefficient r = 0.92), validating long-dismissed community voices and highlighting an environmental injustice. This study demonstrates that poor water quality can substantially affect air quality—although rarely included in air quality models and health assessments—with far-reaching implications as polluted waterways increase globally.