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 (H2S) and other toxic gases at a turbulent hotspot. High wastewater flows and low winds led to nighttime H2S peaks, reaching 4500 parts per billion (ppb)-exceeding typical urban levels of <1 ppb. H2S 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.