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

Area of Science:

• Environmental Science
• Chemical Engineering
• Public Health

Background:

• Facilities using hazardous substances must comply with risk management programs to prevent chemical accidents.
• Current regulations in the EU and US use chemical-specific weight thresholds for toxic substances to determine compliance.
• These 'threshold quantities' aim to ensure facilities manage risks and protect public health.

Purpose of the Study:

• To evaluate the consistency and public health protectiveness of current 'threshold quantities' for hazardous substances.
• To assess the adequacy of existing regulations in identifying facilities that pose significant off-site risks.

Main Methods:

• Calculated chemical footprint/hazard zone length using current US threshold quantities and 'level of concern' (LOC) concentrations for 77 toxic chemicals.
• Utilized the Risk Management Program (RMP)*Comp model for worst-case scenario analysis.
• Calculated alternative distance-based weight thresholds using an atmospheric dispersion model with various meteorological and land-use scenarios.

Main Results:

• RMP*Comp yielded inconsistent results, with hazard zone lengths up to 40 km.
• Distance-based thresholds were significantly smaller than current listings, showing low correlation (r=0.34) and large discrepancies (up to three orders of magnitude).
• Alternative scenarios further reduced distance-based thresholds, increasing discrepancies and highlighting potential underestimation of risks.

Conclusions:

• Current threshold quantities may exclude facilities posing significant off-site risks and are inconsistent with off-site consequence analysis.
• Recommendations include revising threshold quantities, improving RMP*Comp, and establishing thresholds based on hazard zones or credible scenarios.
• Using consistent health-based LOC in screening and analysis is crucial for effective risk management.