For decades, scientists have propounded on the risks of vapor intrusion, the process by which contaminants present in soil and groundwater migrate, via volatilization, into buildings and affect indoor air quality. After years of deliberation, the United States Environmental Protection Agency (USEPA) has now added a vapor intrusion component to the Hazard Ranking System (HRS). The USEPA uses the HRS to evaluate whether a site warrants inclusion on the agency's National Priorities List (NPL) for Superfund sites. While the inclusion of vapor intrusion on the HRS is a positive, albeit overdue, development in Superfund law, how USEPA structures and implements the vapor intrusion component into the existing HRS will determine whether this new enhancement will actually prove to be protective of human health. First, this note will examine the concept of vapor intrusion. Included in this discussion will be a description of the contaminants at play, how vapors enter buildings, where vapor intrusion sites are located, what the human health risks are and, lastly what is done to mitigate the risks that vapor intrusion presents. The note will then discuss the importance that vapor intrusion plays in the Brownfields, Superfund and the LEED-certified building discussion and why vapor intrusion is often identified as an environmental justice issue. Next, the note will discuss many of the difficulties facing vapor intrusion regulation and the roadblocks that may have inhibited the promulgation of federal regulation earlier. I will then describe the different ways states have regulated vapor intrusion by assessing two enforcement mechanisms used by states in regulating sites where vapor intrusion may be an issue. First, I will evaluate how states initially assess vapor intrusion versus how states assess other more regulated exposure pathways (such as ingestion of contaminated drinking water). If there is some sort of ranking or prioritization of sites, what weight, if any, is given to sites where vapor intrusion is an issue? Secondly, I will look at what steps states take, or require potentially responsible parties or responsible parties to take, when vapor intrusion has been identified. Lastly, based on components of state regulation, I will propose a regulatory framework for addressing vapor intrusion.

"This report presents an evaluation of empirical data modeling studies of the behavior of petroleum hydrocarbons (PHC) vapors in subsurface soils at petroleum release sites and how these vapors can affect subsurface-to-indoor air vapor intrusion. Specifically, the report develops an inclusion distance approach for screening petroleum release sites for vapor intrusion that should improve the efficiency of petroleum release site investigations and help focus resources on the sites of most concern for petroleum vapor intrusion (PVI)." -- ES-1.