In this paper the authors find that on average, particulate concentrations are unchanged by subway openings. However, for cities with higher initial pollution levels, subway openings reduce particulates by 4% in the 10km radius area surrounding a city center. The effect decays with distance to city center up to 25 km from the center and persists over the longest time horizon that we can measure with our data, about four years. For highly polluted cities, they estimate that a new subway system provides an external mortality benefit of about $1b per year. For less polluted cities, the effect is indistinguishable from zero. Back-of-the-envelope cost estimates suggest that reduced mortality due to lower air pollution offsets a substantial share of the construction costs of subways.