We're studying negative externalities. Externalities are an example of market failure and in the case of negative externalities are situations where a market creates a cost that is paid by someone else. The outcome is inefficient: society would be better off if less of these goods were produced.
In class I mentioned pollution from automobiles as an example and suggested that the "real cost" of a gallon of gasoline is more than just the production and distribution costs, but includes higher mortality and morbidity, loss of productivity, congestion and resource depletion.
Here is some reading on the topic:
An interview at NPR regarding a study by the IMF
A summary of a study from Duke University at ecomento (an electric car blog)
More on the Duke U. study here at Forbes
Here is the full study at Climatic Change
Are electric cars better? It depends on where you live.