Air pollution is the leading risk factor for premature human mortality and poses major challenges for air-quality management because emissions, transport, and health impacts often do not align with political and regulatory boundaries. This study introduces the concept of an airshed, as a reduced-complexity, data-driven approach, to quantify transboundary influences and attribute responsibility for premature deaths across jurisdictional boundaries. Applied to PM2.5 concentrations in the contiguous U.S., the airshed approach reveals that over half the states are net exporters of annual premature deaths, while roughly one third have zero net cross-state contribution. Although premature mortality has declined by ∼35% from 1998 to 2022, the cross-state contribution to premature mortality has remained steady at 40% nationally. This approach highlights the unequal distribution of both health risk and responsibility across state lines. The findings underscore the need for further regulatory strategies and cooperative mechanisms to address interstate air pollution.