> GDP does not account for the externalities that the study took into account.

Externality estimates and valuations are notorious for reflecting the biases of the estimators.

Estimates that are a large fraction of GDP are very suspicious.

Do you seriously believe that the clean air act "saved" 1/6th of the US economy?

I hear what you are saying, but I think you're mixing units. Since the study is accounting for things that are not accounted for in GDP, It doesn't seem to make much sense to compare the dollar figure reached in terms of GDP. If GDP devalues human health as part of its basic assumptions, an analysis that does not devalue human health will come to different numbers. It's almost a tautology.