Marginal value of steel (DD) Marginal value of clean water forsaken (WW) $6 Optimal (X) Unpolluted water Very polluted water Steel produced (clean water used up →) Marginal value ($) At X: marginal value of steel = $6, marginal value of the clean water it costs = $6. The two marginal values are equal. This is the wealth-maximizing amount of pollution, and it is not zero.

Drag the vertical marker left or right (or focus it and use the arrow keys). It snaps to the optimum X.