Offsite Consequence Analysis (OCA) is the part of EPA's Risk Management Program that asks: if a worst-case release happened tomorrow, how far would it travel and who would it reach? Required under 40 CFR Part 68 Subpart B (sections 68.20 through 68.36), OCA combines air dispersion modeling, explosion modeling, public receptor identification, and population estimation to characterize the offsite hazard from each covered process. The results feed the Risk Management Plan, drive emergency response coordination with local responders, and are heavily reviewed by EPA inspectors. Because OCA inputs and outputs become public information, OCA is also the most visible part of an RMP — what gets modeled, how, and with what assumptions matters not just for compliance but for facility reputation.
Two Required Scenarios
Worst-case release scenario
For each regulated toxic substance above its TQ, model the release of the largest quantity in a single vessel or pipe over 10 minutes. For flammables, model the explosion of the entire quantity. Document the resulting distance to the toxic endpoint or 1 psi overpressure and identify any public or environmental receptors within that distance. The worst-case scenario is required for all program levels (1, 2, and 3).
Alternative release scenario
For Program 2 and Program 3 processes, model at least one more likely release scenario per regulated toxic substance and one collective alternative scenario for flammables. Alternative scenarios reflect realistic upset events — transfer hose ruptures, vessel ruptures with active mitigation, process upsets — rather than maximum-credible events. Active mitigation systems can be credited.
Required Modeling Assumptions
EPA prescribes specific input assumptions to standardize worst-case modeling across facilities. Some can be relaxed for alternative scenarios.
| Assumption | Worst-Case | Alternative |
|---|---|---|
| Quantity released | Maximum quantity in a single vessel or pipe | Realistic release amount given the scenario |
| Release duration (toxics) | 10 minutes (gases); evaporation rate from pool (liquids) | Determined by scenario (cause, mitigation) |
| Active mitigation | Cannot be credited | Can be credited if reliable |
| Passive mitigation | Can be credited (containment dikes, building enclosures) | Can be credited |
| Wind speed (toxics) | 1.5 m/s (3.4 mph) | Typical for area (e.g., 3 m/s) |
| Atmospheric stability | F stability class | D stability class typical |
| Temperature/humidity | 25°C and 50% humidity | Typical for area |
| Terrain | Urban or rural based on actual surroundings | Same |
Endpoint Criteria
The "endpoint" is the concentration or pressure at which the modeled release stops being relevant for OCA purposes — the boundary of the impact zone.
Toxic endpoints (Appendix A to 40 CFR 68)
Each regulated toxic substance has a specified toxic endpoint, expressed as a concentration in the air. EPA derived endpoints from ERPG-2 (Emergency Response Planning Guideline level 2), AEGL-2, or other authoritative values. Examples: chlorine 0.0087 mg/L (3 ppm), anhydrous ammonia 0.14 mg/L (200 ppm), hydrogen fluoride 0.016 mg/L (20 ppm), sulfur dioxide 0.0078 mg/L (3 ppm).
Flammable endpoint
For vapor cloud explosion (VCE) modeling, the endpoint is 1 psi overpressure — the pressure level associated with significant property damage and potential injury to people from secondary effects (broken glass, structural damage). EPA's RMP*Comp uses the TNT equivalency method with a yield factor of 10%.
Pool fire and BLEVE endpoints
For pool fires, the endpoint is 5 kW/m² radiant heat (sufficient for second-degree burns over 40 seconds). For BLEVE (boiling liquid expanding vapor explosion), endpoint criteria depend on the specific scenario but typically use 5 kW/m² radiant heat or a probit injury criterion.
Common Modeling Tools
EPA accepts any reasonable air dispersion or explosion model whose inputs and assumptions are documented. The choice of tool typically depends on facility complexity, in-house expertise, and the desired level of refinement.
RMP*Comp (free, EPA)
EPA's free screening-level tool. Built-in databases for all RMP-listed substances, simple input requirements, runs in a web browser. Sufficient for many small to mid-size facilities — particularly water/wastewater chlorine, ammonia refrigeration, and ag retail facilities. Limitations: only standard release types, simplified terrain handling, limited mitigation crediting.
ALOHA (free, NOAA/EPA)
The Areal Locations of Hazardous Atmospheres tool, a free desktop air dispersion model jointly developed by NOAA and EPA. More flexible than RMP*Comp; handles a wide range of release types, terrain, and weather conditions. Often used for emergency response planning as well as RMP OCA. Standard tool for refining RMP*Comp screening results.
PHAST (commercial, DNV)
Process Hazard Analysis Software Tool. Industry-standard commercial dispersion and explosion model used widely in petroleum refining, petrochemical, and chemical manufacturing. Handles complex release types, jet dispersion, dense gas effects, BLEVEs, and vapor cloud explosions with refined methods. Substantial licensing cost; typically run by experienced consultants.
SAFER, TRACE, and other commercial tools
Several other commercial dispersion modeling packages are accepted by EPA when used appropriately: SAFER (Trinity Consultants), TRACE (DNV), CIRRUS (Quest Consultants), and others. Selection depends on facility needs, available expertise, and integration with existing systems.
SLAB and DEGADIS (free, public domain)
Public domain dense gas dispersion models. Widely accepted for RMP modeling but require more user expertise than RMP*Comp or ALOHA. Used for refined analysis when commercial tools are not available.
Public Receptor Identification
"Public receptor" includes any offsite location where members of the public could be exposed to a release. Identification is a deliberate, document-everything exercise — gaps here are one of the most common findings in EPA inspections.
Categories of public receptors
- Residences — single-family homes, multi-family housing, apartments, condominiums, dormitories
- Schools — primary, secondary, colleges, universities, daycares
- Hospitals and medical facilities — hospitals, nursing homes, assisted living, urgent care
- Prisons and correctional facilities
- Public recreational areas — parks, playgrounds, sports fields, beaches, hiking trails
- Commercial buildings — office buildings, retail centers, restaurants
- Industrial facilities — other industrial sites, warehouses, manufacturing
- Major roadways and transportation — highways, train stations, airports, bus terminals
How to identify public receptors
- Aerial imagery (Google Earth, Bing Maps, county GIS portals) within the worst-case distance
- USGS topographic maps for terrain context
- County zoning maps and assessor data
- Site reconnaissance to verify and identify receptors not visible in imagery
- Coordination with the LEPC and local planning departments
Population Estimation
Census block methodology
EPA's preferred method for estimating residential population within the worst-case endpoint distance is to identify all U.S. Census blocks intersecting the impact zone, sum the residential population, and prorate boundary blocks where the zone covers only part. EPA's RMP Comp documentation and the OCA Guidance document include detailed instructions.
Tools that automate the calculation
EPA's LandView CD-ROM (legacy) and various commercial GIS applications calculate population within a polygon or circle. Many facilities use Census Bureau's online TIGER/Line and American FactFinder data combined with desktop GIS (QGIS, ArcGIS) to compute the estimate.
Reporting in the RMP
Report residential population to the nearest 100 (e.g., 1,200; 4,500; 12,300). Round up — conservative estimates are appropriate. Document the data source, vintage of the census data, and the boundary distance used.
Documenting OCA
What to keep on file
- Modeling tool name and version
- Substance, quantity, and release type for each scenario
- All input parameters (weather, terrain, mitigation)
- Model output reports with endpoint distances
- Map showing impact zone overlaid on aerial imagery with public receptors marked
- Population calculation worksheet
- Justification for any deviations from EPA's prescribed assumptions
- Date of analysis and analyst name
How long to keep records
Maintain OCA documentation for the life of the covered process. EPA inspectors routinely request the underlying calculations supporting the data submitted in RMP*eSubmit. Inability to produce supporting documentation can lead to findings even when the submitted numbers are technically defensible.
Common Pitfalls
Ecesis Software for OCA Documentation
EPA RMP Software
OCA inputs, outputs, and public receptor records linked to processes
Hazard Analysis
PHA findings tied to alternative release scenarios
Document Management
Model output reports, maps, and population worksheets
Chemical Management
Maximum quantity tracking driving worst-case inputs
Management of Change
Catch process changes that invalidate OCA results
Emergency Planning
OCA results inform emergency response and LEPC coordination
