More than 12 years in the making, U.S. EPA recently issued two new technical guides for assessing and mitigating the vapor intrusion pathway: Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air and Technical Guide for Addressing Petroleum Contamination at Leaking Underground Storage Tank Sites. You can read a summary of some of the key issues in the guides here.