http://www.exploreshale.psu.edu/ http://www.nytimes.com/interactive/2011/03/01/us/chemicals-and-toxic- materials-in-hydrofracking.html https://www.osha.gov/dte/grant_materials/fy10/sh-20839-10/hierarchy_of_controls.pdf https://www.fractracker.org/2014/05/worker-safety/http:/www.marcellus.psu.edu
