Mexico City is prone to extreme environmental risks. Such hazards shape the lives of the valley's inhabitants. Yet, since the 1960s, its residents have grappled with changing conceptions of urban-environmental risk, particularly related to the air they breathed. Three decades of dedicated urban-industrial growth, combined with a geography hostile to the free flow of air into and out of the valley, resulted in a new disaster: a killer air. Defining this air as a danger to the city’s prosperity and the lives of its inhabitants was a deeply political and power-laden process, marked by economic, rather than health, concerns.