marginal benefit of consumption is just equal to social marginal cost of
production.
The socially optimal level of output is such that the social marginal cost equals the social
marginal benefit.In order to internalize the externality successfully, it is necessary to
measure its size accurately. Looking at Figure 17-1 , we must be able to
measure the magnitude of the marginal external cost, MEC. In practice,
however, external costs are quite difficult to measure. This measurement
is especially difficult in the case of air pollution, where the damage is
often spread over hundreds of thousands of square kilometres and can
affect millions of people.