Improving air quality has become a major issue for many large cities. A large literature documents the social cost of air pollution; airbone pollutants have been shown to have adverse effects on health, causing respiratory diseases and aggravating cardio-vascular diseases. While technological improvements have contributed to a reduction of emissions in certain areas, large cities still have to cope with record levels of emissions and are thus prompted to implement local regulations. The regulation of traffic in the city can influence air quality, by reducing congestion, allowing only the cleanest vehicles around town or limiting the total number of kilometers driven. Some major cities have implemented urban tolls to manage traffic externalities, in most cases to reduce congestion, and sometimes with a focus on emission reduction. Given the growing number of such policy initiatives, it is crucial to provide reliable assessments of their impacts in terms of air quality. We use the recent developments in the empirics of comparative case studies to analyze the effect of tolls on urban air quality. We study both a case of urban toll to reduce congestion (London) and a case to improve air quality (Milan) by employing a synthetic control approach. In this approach, the counterfactual for each "treated" by a city is constructed by a weighted average of "non-treated" cities such that the actual city and its synthetic counterpart coincide as much as possible with respect to fine particles emissions before the "treatment" (adoption of the urban toll). In the case of Milan, we exploit in addition a quasi-experiment based on a transition period of a few weeks without toll between the program Ecopass and the program Area C