The U.S. Environmental Protection Agency (EPA) is adding a subsurface intrusion (SsI) component to the Hazard Ranking System (HRS), which is the principal mechanism that EPA uses to evaluate sites for placement on the National Priorities List (NPL). The NPL is a list of national priorities among the known or threatened releases of hazardous substances, pollutants or contaminants throughout the United States. Sites on the NPL are priorities for further investigation to determine if further response actions are warranted. The subsurface intrusion component (this addition) expands the number of available options for EPA and state and tribal organizations performing work on behalf of EPA to evaluate actual and potential threats to public health from releases of hazardous substances, pollutants, or contaminants. This addition enables EPA to directly consider human exposure to hazardous substances, pollutants, or contaminants that enter regularly occupied structures through subsurface intrusion in assessing a site's relative risk, and thus, enable sites with subsurface intrusion contamination to be evaluated for placement on the NPL.