To promote a justice-oriented approach to science education, we formed a research-practice partnership between middle school science teachers, their students, curriculum designers, learning scientists, and experts in social justice to co-design and test an environmental justice unit for middle school instruction. We examine teacher perspectives on the challenges and possibilities of integrating social justice into their standards-aligned science teaching as they participate in co-design and teach the unit. The unit supports students to investigate racially disparate rates of asthma in their community by examining pollution maps and historical redlining maps. We analyze interviews and co-design artifacts from two teachers who participated in the co-design and taught the unit in their classrooms. Our findings point to the benefits of a shared pedagogical framework and an initial unit featuring local historical content to structure co-design. Findings also reveal that teachers can share similar goals for empowering students to use science knowledge for civic action while framing the local socio-political factors contributing to the injustice differently, due in part to different institutional supports and constraints. Student interviews and a pre/postassessment illustrate how the unit facilitated students' progress in connecting socio-political and science ideas to explain the impacts of particulate matter pollution and who is impacted most. Analyses illuminate how teachers' pedagogical choices may influence whether and how students discuss the impact of systemic racism in their explanations. The findings inform refinement of the unit and suggest supports needed for co-design partnerships focused on integrating social justice and science.