Mining the exotic metals used in modern electronics inflicts a lot of damage on the environment, especially by polluting the atmosphere with carbon dioxide. But overall, the global warming impact of metals is still dominated by the old-school elements iron and aluminum. That’s one of the bottom lines from a comprehensive environmental analysis of 63 metals, published this week in PLOS ONE. Iron (Fe), and its alloy steel, are the dirtiest of the bunch, responsible for 30% of all industrial CO2 emissions. The runner up is aluminum (Al) at 2% of CO2. In third place—perhaps a surprise to some readers—is calcium (Ca), an alkaline earth metal. It’s mined for quicklime, an ingredient in cement. (What typically consumes the most energy is not getting the rock out of the ground, but refining the ore. All told, producing metals takes 9.5% of world energy.) Other environmental damage related to production, such as acid-mine drainage, is also dominated by these common metals. A unique aspect of the study is that it teases apart the environmental impact of each element, even though many are mined or processed in combinations. The detailed analysis could help engineers design products that substitute more benign metals.