The MN PUC held a hearing on September 4, 2014, to address the impending investigation of social costs of pollutants under Minn. Stat. § 216B.2422 subd. 3 (known as externalities values).  First established in the late 1990s, the MN PUC re-opened the investigation in a February 2014 Order, focusing exclusively on PM_2.5,…