Air quality policies based on scientific information have proved to be effective for controlling air pollution and protecting public health. Intensive field studies provide knowledge that combined to data from emission inventories and air quality monitoring allows to understand the causes that trigger air pollution and catalyze the design of effective control measures. We review the case of Mexico City, where past international collaborative studies were fundamental to improve air quality, but a null progress and a possible reversal to high air pollution levels in recent years suggest that a new dedicated field measurement campaign is urgently needed.