The MBA curve represents marginal benefits of pollution reduction. Needless to say, such benefits are extremely difficult to measure or estimate.

Suppose new estimates show pollution to be more harmful that previously thought. This is like saying the benefits of removing pollution are greater or that the MBA curve has shifted to the right to MBA₂. As can be seen on the graph to the right, this means that the optimal amount of pollution reduction has increased to A*, and the amount that should be spent on the last unit removed has risen to MCA*.