The Hazard Ranking System (HRS) is the principal mechanism EPA uses to evaluate uncontrolled waste sites for possible inclusion on the National Priorities List (NPL). It is a numerically based screening system that uses information from initial, limited investigations - the preliminary assessment and the site inspection - to assess the relative potential of sites to pose a threat to human health or the environment. The HRS uses data that can be collected relatively quickly and inexpensively, thus allowing most Superfund resources to be directed to remedial actions at sites on the NPL. The HRS assigns each site a score ranging from 0 to 100. Sites receiving HRS scores of 28.50 and above are eligible for the NPL, though EPA generally also considers other factors (most notably the views of the state) before actually listing a site on the NPL.
Note: The Hazard Ranking System is a screening tool, based on initial, limited investigations. While not a risk assessment, the HRS does provide a measure of relative risk among the universe of potential NPL sites. EPA has never attached significance to the cutoff score as an indicator of a specific level of risk from a site, nor has EPA intended to imply that "risky" and "nonrisky" sites can be precisely distinguished. Nevertheless, the cutoff score has been a successful screening tool that has allowed EPA to set priorities and to move forward with studying and, where appropriate, to clean up hazardous waste sites. The vast majority of sites scoring above 28.50 in the past have been shown to present risks.
HOW DOES THE HRS SCORE HAZARDOUS WASTE SITES?
The Hazard Ranking System 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 (resident population, nearby 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. If all pathway scores are low, the site score is low. However, the site score can be relatively high even if only one pathway score is high. This is an important requirement for HRS scoring, because some extremely dangerous sites pose threats through only one pathway.
THE REVISION OF THE HRS
The original HRS was revised to incorporate changes mandated by the 1986 amendments to the Superfund Act, as well as improvements identified as necessary by EPA and the public. Sites were first scored under the revised HRS in early 1991. Key provisions of the revised HRS make it more comprehensive and accurate than the original HRS. The revised HRS retains the same cutoff score and basic approach as the original HRS. It retains the ground water, surface water, and air pathways, drops the direct contact and fire/explosion pathways, and adds a fourth pathway, soil exposure. Some of the most important changes in the revised HRS include:
- A fourth pathway, the soil exposure pathway was added to address direct contact problems.
- The food chain threat was added to the surface water pathway.
- Extra emphasis is placed on those sites that result in actual human exposure, as opposed to potential exposure.
- The toxicity factors were revised to include consideration of chronic noncarcinogenic, carcinogenic, and acute effects. (The original HRS considered only acute toxicity.)
- Targets are now weighted according to their distance from a site or the amount of dilution likely to occur.
- Environmental targets are given a more comprehensive evaluation and greater weight.
- The air pathway can be scored for potential release. (The original HRS scored only observed releases.)
See The Revised Hazard Ranking System: Background Information for more detail.