For decades, billions of gallons of sewage and industrial waste have been dumped into the Tijuana River. Recent research shows this chronically contaminated watershed in Southern California is also polluting the air. The river releases large amounts of hydrogen sulfide into the air. The toxic gas that smells like rotten eggs can cause headaches, nausea and worsen asthma. Its long-term health problems are only starting to be understood. The United States and Mexico signed an agreement last year to clean up the longstanding problem. However, neither country has outlined plans to address the public health impact in the meantime to more than 100,000 people directly affected on both sides of the border.