Hazard Ranking System (HRS)

The Hazard Ranking System (HRS) is the principal mechanism that the EPA uses to place uncontrolled waste sites on the National Priorities List (NPL). It is a numerically based scoring system that uses information from initial, limited investigations - the preliminary assessment (PA), the site inspection (SI), and the expanded site inspection (ESI) if necessary - to assess the relative potential of sites to pose a threat to human health or the environment. Any person or organization can report spills and environmental violations and petition the EPA to conduct a preliminary assessment.

HRS scores do not determine the priority in funding EPA remedial response actions, because the information collected to develop HRS scores is not sufficient to determine either the extent of contamination or the appropriate response for a particular site. The sites with the highest scores do not necessarily come to the EPA's attention first - this would require stopping work at sites where response actions were already underway. The EPA relies on more detailed studies in the remedial investigation/feasibility study which typically follows listing.

The HRS uses a structured analysis approach to scoring sites. This approach assigns numerical values to factors that relate to risk based on conditions at the site. The factors are grouped into three categories:

• likelihood that a site has released or has the potential to release hazardous substances into the environment;
• characteristics of the waste (e.g. toxicity and waste quantity); and
• people or sensitive environments (targets) affected by the release.

Four pathways can be scored under the HRS:

• ground water migration (drinking water);
• surface water migration (drinking water, human food chain, sensitive environments);
• soil exposure and subsurface intrusion (population, sensitive environments); and
• air migration (population, sensitive environments).

After scores are calculated for one or more pathways, they are combined using a root-mean-square equation to determine the overall site score.