The Hazard ranking system (HRS) is a numerical scoring procedure that the federal Environmental Protection Agency (EPA) uses to place and prioritize waste sites on the National Priorities List. Only these priority sites can be cleaned up through the Superfund Trust Fund program.

The HRS score is based on an evaluation of threats related to the release or potential release of hazardous substances. The HRS assessment of a site ranks public health factors such as threats to drinking water, the food chain, and populations exposed through occupational and ambient environments. Also evaluated are environmental threats like the effect of substances on air quality, resources, and sensitive ecosystems.

Federal investigators score a site by evaluating four pathways that could be affected by hazardous releases. The pathways are ground water migration, surface water migration, air migration, and soil exposure.

The pathway scores are combined using a root-mean-square equation. This calculation produces the overall score for a site. A high HRS score does not guarantee immediate action because clean-up work may be going on at other sites. The decision to take action on a site is based on additional research that includes a remedial investigation of what corrective action is needed.