Since over half of the world's population now live in cities, it has become important to monitor the inequities in health that exist within cities worldwide. This month the PLOS Medicine Editors discuss how to generate useful health inequity indicators, and some hazards specific to urban environments—both the obvious and the subtle—with a focus on air quality. The Editors examine the lack of correlation between the size of a city and its air quality, noting that the strength of environmental laws and the accountability of the country's government have a greater impact on urban air quality than a city's size. As the association between hazardous air particulates and adverse effects on health is now well established, the Editors argue that we need to ensure air quality data are truly representative of the quality of air breathed by all city-dwellers, including those working in industrial “hot spots.” The Editors also express their concern about the effects on the quality and transparency of data that national embarrassment over air pollution can produce. Air quality data must be made available transparently if the imbalance in urban health equity is to be redressed, they argue.

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