As in many areas of the world, in Mexico ambient air pollution is a pervasive component of the lived experience. Most conspicuous in large urban centers, air pollution flows across the diverse Mexican terrain, unifying the country’s political geography while also routinely permeating international boundaries. In Mexico’s capital, air pollution is unyieldingly stagnant and often lingers in the valley for days during winter temperature inversions and periods of low wind activity. Although Mexico City has long suffered from seasonal dust pollution, a consequence of the slow, human-engineered desiccation of the lakes that once surrounded the city, as well as from pollution naturally generated by the relatively more sporadic volcanic eruptions known to afflict the city and its environs, the mid-20th century spawned an altogether different, more human pollution problem. Driven by state-sponsored industrialization, population growth, and a rise in the use of motorized transportation, a phase collectively known as the “Mexican Miracle,” from approximately the 1940s to the 1990s, Mexico City transformed into an industrial powerhouse and the most polluted city in the world, the latter status officially recognized by the United Nations during the Earth Summit in 1992. The state, dedicated to carrying out its comprehensive modernization project, had left Mexico City’s air pollution to fester for decades, framing the legal protection of the environment—atmosphere included—as antithetical to economic growth. This rhetoric pervaded the ways that antipollution laws, passed in the 1970s and 1980s, were enforced. Though they set into motion important classification and monitoring efforts, for the most part air pollution control laws were poorly executed due to bureaucratic inefficiencies and the collapse of the economy, which halted spending on environmental protection programs. Other spheres such as science and environmental activism were also important in the history of Mexico City’s experience with air pollution, as actors within these realms contributed to the creation of air pollution knowledge throughout the second half of the 20th century. In their own ways, scientists and activists discursively rendered air pollution a threat to human life and the ecological future of Mexico City. From the 1940s to the 1990s, then, dirty air connected politics, science, and environmentally minded citizens in important and intriguing ways.