The Hazard Ranking System (HRS), required by the Superfund statute, is the primary mechanism used by the EPA to assess the relative threat associated with actual or potential releases of hazardous substances. As a matter of agency policy, those sites that score 28.50 or greater under the HRS are eligible for inclusion on the National Priorities List (NPL). The NPL is intended primarily to guide the EPA in determining which sites warrant further investigation. A score of 28.50 does not represent a specified level of risk but is a cutoff point that serves as a screening-level indicator of the highest priority releases or threatened releases. The HRS includes four scoring pathways - ground water, surface water, air and soil exposure. Additional pathways have been identified as posing significant threats to human health and the environment that should be considered when evaluating sites for the NPL. One such pathway is subsurface intrusion. Subsurface intrusion occurs when contaminants are released, enter the subsurface environment and move into occupied structures (e.g., residences, workplaces and other buildings) as a gas, vapor or liquid. Over the past decade the EPA and state environmental programs have learned significantly more information regarding the risk that this pathway poses to human health. Historically, the EPA's Superfund program has responded to vapor intrusion contamination by two mechanisms: (1) through its emergency response program at sites not on the NPL, or (2) through sites placed on the NPL because of other pathway-related risks. In a May 2010 report, the Government Accountability Office (GAO) concluded that if vapor intrusion sites are not assessed and, if needed, listed on the NPL, some seriously contaminated hazardous waste sites with unacceptable human exposure may not otherwise be cleaned up. Thus, the EPA is considering adding a new screening component to the HRS that would allow sites with vapor intrusion contamination to be evaluated for placement on the NPL. This addition would enable the HRS to directly consider the human exposure to contaminants that enter building structures through the subsurface environment.