caption: Air quality in Mexico City is influenced by the bowl-like valley it sits within, surrounded by pollution-trapping mountains like those in this 47-story view (photo 1). Heavy traffic accounts for most of the air pollution in this megacity of 22 million residents (photo 2). Yet it also has the largest bike-sharing program (photo 3) in Latin America -- one of many attempts to improve the city's environment and quality of life.
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