Marginal Damages and Marginal Abatement Costs The marginal damage curve (MD) shows the change in damages that come from a given change in emission levels. The horizontal axis measures pollution. Note that E₀ is the maximum pollution that can occur without environmental cleanup. The vertical axis measures the environmental costs of this pollution. Marginal abatement costs curve (MAC) begins at zero at E₀, then rises as emission levels are reduced (moving leftward from E₀). Socially optimal pollution is E_S, at a cost to society of C_e (point e).