What is the impact of cycling infrastructure on air quality in cities? This paper leverages the staggered rollout of New York City’s bike share program to estimate the effect of cycling infrastructure on air pollution concentrations. I combine the universe of bike share trips with ground-level, high-resolution observational air pollution measures between 2010 and 2019. Through a routing algorithm, I use the location of bike share stations to map areas where road traffic is expected to decrease after the introduction of bike share. I compare these areas with others where traffic was likely unaffected using a staggered difference-in-differences strategy to retrieve causal estimates. I find that the deployment of bike share is associated with a 3% reduction in black carbon and 13% reduction in nitric oxide concentrations, both pollutants associated with road traffic. Back-of-the-envelope valuation of the health and mortality benefits associated with the reduction in nitric oxide concentrations suggests that bike share prevented up to $327 million in social damages. In addition, I investigate potential mechanisms and show that the introduction of bike share is associated with a decrease in short taxi trips in areas served by bike share, which I interpret as suggestive evidence that bike share substitutes road traffic