We investigate the relationship between the opening of a city's subway network and its air quality. We find that particulate concentrations drop by 4% in a 10km radius disk surrounding a city center following a subway system opening. The effect is larger near the city center and persists over the longest time horizon that we can measure with our data, about eight years. We estimate that a new subway system provides an external mortality benefit of about $594m per year. Although available subway capital cost estimates are crude, the estimated external mortality effects represent a significant fraction of construction costs.
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