We examine the role of social capital in explaining the highly unequal regional distribution of firms’ carcinogenic releases. Our model predicts that social capital, by enabling information sharing and coordination among community members, decreases carcinogenic releases. Our analysis, based on the US county-level releases derived from around 2 million chemical-facility-level reports during 1998-2019 and the instrumental variable approach, confirms our prediction. However, the impact is reduced when counties rely on waste-releasing firms for economic opportunities. An important policy implication of our study is that the efficacy of initiatives to alleviate environmental injustice is likely to depend on communities’ social capital.