The polluted air of Mexico City and Santiago, Chile, has many similarities to the smog that plagued Los Angeles about 20 yearsago, according to recent studies. "It's a message of hope," says Tom Cahill, director of the Air Quality Group at UC Davis, a research team that specializes in monitoring aerosols in the United States and around the world. "These cities' problems are difficult problems, but they are not unprecedented," Cahill says. "California had similar problems, and the solutions of California are not irrelevant to these cities." Cahill's comparative study of the air quality of Los Angeles, Mexico City and Santiago also shows that a significant amount of the particles obscuring vision and getting into people's lungs come from dust in the surrounding countryside, suggesting that solutions to city air problems should be regional in nature. "It would be extremely useful to have comparative studies on every supercity, since air quality problems are a major factor for health and welfare," says Cahill, whose research team also acts as a consultant to the United Nations World Meteorological Organization Global Atmospheric Watch network.