Threshold quantity criteria for risk management programs: recommendations for toxic releases

Abstract

Facilities using hazardous substances are required to comply with risk management programs that aim to reduce the frequency of chemical accidents and the severity of consequences in the event of an accident. Both the European Union and the United States use chemical-specific weight thresholds for toxic substances to determine those facilities and processes that must comply with such programs. This study evaluates whether the establishment and use of these ‘threshold quantities’ is consistent and protective of public health. The chemical footprint or hazard zone length is calculated using current threshold quantities and ‘level of concern’ (LOC) concentrations for 77 toxic chemicals in the US regulations. Using the worst-case scenario and the recommended procedure involving the Risk Management Program (RMP) ∗Comp, footprint lengths range up to 40 km. However, the RMP ∗Comp program provides inconsistent results. Threshold quantities are then calculated using an atmospheric dispersion model and several meteorological and land-use scenarios. In the base scenario (winds at 4.3 m/s, neutral stability, urban conditions, and distances of 100, 250, and 1000 m), distance-based weight thresholds are considerably smaller than current listings for most toxic substances. Distance-based and current thresholds have low correlation (e.g. r=0.34) and large discrepancies (e.g. differences up to three orders of magnitude). Alternative scenarios evaluated for distance-based threshold quantities, which used using stable atmospheres and rural settings further reduce the distance-based weight thresholds and increase discrepancies. Linear relationships are shown between threshold quantities and level of concerns for each scenario and dispersion mode (neutral or dense) that allow simple calculation of threshold quantities. The current thresholds may exclude facilities that could pose significant off-site risks, and the thresholds are inconsistent with the off-site consequence analysis (OCA). Recommendations include revising the threshold quantities that determine covered facilities/processes; modifying RMP ∗Comp to eliminate errors; establishing threshold quantities using a more rational approach, e.g. based on hazard zones or distances using credible scenarios; and using the same health-based level of concern in both initial screening and subsequent off-site consequence analyses.