The Socially Optimal Quantity of Pollution Pollution yields both costs and benefits. Here the curve MSC shows how the marginal cost to society as a whole from emitting one more unit of pollution emissions depends on the quantity of emissions. The MSC curve is upward sloping, so the marginal social cost increases as pollution increases. The curve MSB shows how the marginal benefit to society as a whole of emitting an additional unit of pollution emissions depends on the quantity of pollution emissions. The MSB curve is downward sloping, so the marginal social benefit falls as pollution increases. The socially optimal quantity of pollution is QOPT; at that quantity, the marginal social benefit of pollution is equal to the marginal social cost, corresponding to $200.