Minimizing the environmental impact of a leaked item requires minimizing both mass and environmental lifetime . The MIs for each objective were combined into a compound MI of material efficiency and minimal environmental lifetime as the product of the two individual MIs. For stiffness-limited design, the compound MI was 𝜌 𝑘 𝑑 𝐸 2/3 and for strength-limited design, the compound MI was 𝜌 𝑘 𝑑 𝜎 𝑦 . Optimizing these MIs results in selecting the material that best reduces the social cost of plastic pollution , which depends on the initial mass and the environmental lifetime of the item. Social costs reflect the external costs (e.g., air pollution on human health) to society associated with a good, service, or outcome (e.g., combustion engines). Whereas foaming proved no benefit in optimizing these MIs for PS, it yielded increased material performance (lowering the social cost of plastic pollution) by 10-20 times for CDA (Figure ). At equivalent densities, CDA optimized these MIs compared to PS. The data plotted are available in Supplementary Tables . Data for solid PS was referenced from .