This study meticulously reviews the impact of subway development on air pollution, focusing on China as a case study. Amidst rapid urbanization and escalating private vehicle usage, China's air pollution has soared, prompting significant investments in public transportation as a mitigation strategy. This article examines how subway expansions correlate with changes in air quality across multiple Chinese cities, drawing from high-frequency data on air pollutants like PM10, PM2.5, NOx, SO2, and CO. A notable finding is that new subway openings generally lead to a decrease in PM10 and PM2.5 levels, with reductions averaging 17% across different cities. However, the study also identifies complexities in the relationship between subway development and air pollution reduction. While short-term impacts occasionally show increased pollutant levels, likely due to construction activities, long-term analysis reveals more consistent improvements. For example, after subway line inaugurations, PM10 levels initially rise by approximately 5% but subsequently decrease by around 9% over more extended periods. These dynamics underscore the effectiveness of subways in reducing air pollution, particularly in high-density urban areas, though the benefits may vary based on local conditions and the scale of the subway network. By integrating comprehensive spatial econometric analyses, this research not only highlights the direct benefits of subway infrastructure on environmental health but also enriches the policy dialogue concerning sustainable urban transport solutions in mega-urban settings.