Figure 1.4, a well-known graph, illustrates this statement. As long as the marginal social damage from an additional unit of particulate and GHG emissions is higher than the marginal economic benefit to the generator, emissions should be reduced, i.e. from any point to the right of the optimal point Q* one should move left-wards. Conversely as long as the marginal benefit (avoiding the marginal cost of abatement) to the generator from an additional unit of emissions is higher than its marginal damage, then more emission should be allowed to, i.e. from a point to the left of the optimal point Q* one should move rightwards. The optimal level of emission is reached where the marginal social costs equate to the marginal private costs of abatement.