I think it just counts externalities that are already priced in via taxes/subsidies.
It's not really possible to price global warming or clean air anyway.
Plus, remember the story about primary school parents who were fined for being late to pick up their kids?
The effect of trying to price global warming or clean air is harmful in and of itself. We shouldn't be trying to accurately assess the cost of destroying our environment to determine the exact tax to levy on the fossil fuel companies. We should be taxing them into oblivion now that we have alternatives.