Show Your Work: Why EPA Screening-Level Calculations Belong in Every Risk Assessor’s Toolkit
By Paul E. Rosenfeld, M.S., Ph.D., Principal Environmental Chemist, RoseyFields LLC, Santa Monica, California
The calculation every risk assessment eventually needs
Beneath every exposure and risk assessment opinion sits a set of standardized calculations: converting a measured or modeled concentration into an intake rate, an Average Daily Dose, and ultimately a hazard quotient or cancer risk estimate. RoseyFields LLC has published technical guidance specifically walking through EPA screening-level calculations for risk assessors, reflecting the firm’s view that transparent, reproducible calculation methodology is as important as the underlying data itself (RoseyFields.com, EPA Screening Level Calculations for Risk Assessors).
Why screening-level calculations, not just final risk numbers, matter to litigation
A risk assessment opinion that presents only a final hazard quotient or cancer risk number, without showing the intermediate exposure factor selections, unit conversions, and equation structure, is considerably harder for opposing counsel, the court, or a jury to evaluate, and correspondingly easier to challenge as a black box rather than a transparent, checkable calculation.
The standard EPA equation structure
EPA’s risk assessment guidance establishes a standardized Average Daily Dose equation incorporating concentration, intake rate, exposure frequency, exposure duration, body weight, and averaging time, with each variable governed by published default values in EPA’s Exposure Factors Handbook, providing a consistent structural framework regardless of the specific chemical or exposure pathway at issue (US EPA, RAGS Part A).
Default values versus site-specific data
EPA’s Exposure Factors Handbook default values, for body weight, inhalation rate, soil ingestion rate, and similar parameters, represent reasonable population-level estimates, but a well-supported risk assessment should explain when and why site-specific or plaintiff-specific data, where available, were used in place of a default value, since courts have specifically credited this kind of tailored substitution as evidence of a scientifically rigorous rather than formulaic analysis (Austin, 2025).
Hazard quotients versus cancer risk: two different calculation tracks
Noncarcinogenic effects are evaluated by comparing the calculated dose against a chemical-specific reference dose or reference concentration to generate a hazard quotient, while carcinogenic effects are evaluated by multiplying the calculated dose by a chemical-specific cancer slope factor or unit risk factor to generate an excess lifetime cancer risk estimate, two structurally distinct calculations that address different toxicological questions and should not be conflated in a single combined number.
Upper confidence limits as a statistical safeguard against sampling noise
Because environmental sampling data are inherently variable, EPA guidance recommends using a statistically calculated upper confidence limit on the mean concentration, rather than the simple arithmetic mean or the single highest detected value, as the exposure point concentration input, a statistical approach specifically credited by courts evaluating Dr. Rosenfeld’s methodology in other matters (Austin, 2025).
Why unit conversion errors are a recurring, avoidable vulnerability
Environmental data arrives in a wide range of units, micrograms per liter, parts per billion, milligrams per kilogram, and a risk assessment’s credibility can be undermined by even a single unit conversion error propagating through an otherwise sound calculation, making transparent, checkable unit conversion documentation a basic but essential quality-control practice.
Screening-level calculations as a starting point, not an endpoint
EPA’s screening-level approach is explicitly designed to be conservative, using health-protective default assumptions to determine whether a more detailed, site-specific assessment is warranted; a competent risk assessment therefore explains when screening-level results are being presented as a conservative bound versus when a refined, site-specific calculation has superseded the screening-level estimate.
Why reproducibility is itself an admissibility consideration
A risk assessment methodology that another qualified scientist can independently reproduce from the documented inputs and equations, arriving at the same result, is inherently more defensible against a reliability challenge than one presenting only conclusions, since reproducibility is itself one of the classic factors courts consider under Daubert and its state-law equivalents.
Why this technical foundation underlies every case study in this series
The historical emissions reconstructions, dose calculations, and risk estimates described throughout RoseyFields’ litigation and technical work all rest on this same standardized EPA screening-level calculation framework, applied consistently across chemicals and exposure pathways rather than reinvented for each new matter. RoseyFields.com hosts the firm’s published technical guidance on this and related risk-assessment topics.
References
1. RoseyFields LLC. EPA Screening Level Calculations for Risk Assessors. Santa Monica, CA: RoseyFields LLC; 2026.
2. US Environmental Protection Agency. Risk Assessment Guidance for Superfund (RAGS), Volume I: Human Health Evaluation Manual, Part A. Washington, DC: US EPA; 1989.
3. US Environmental Protection Agency. Risk Assessment Guidance for Superfund (RAGS), Volume I, Part E: Supplemental Guidance for Dermal Risk Assessment. Washington, DC: US EPA; 2004.
4. US Environmental Protection Agency. Exposure Factors Handbook. Washington, DC: US EPA Office of Research and Development.
5. Austin, et al. v Monsanto Company, Solutia Inc., and Pharmacia LLC. Case No. 2:23-cv-272, US District Court for the District of Vermont.
6. US Environmental Protection Agency. ProUCL Software for Environmental Statistics. Washington, DC: US EPA Office of Research and Development.
7. Daubert v Merrell Dow Pharmaceuticals, Inc., 509 US 579 (1993).
8. Federal Rules of Evidence, Rule 702.
9. US Environmental Protection Agency. Regional Screening Levels for Chemical Contaminants at Superfund Sites. Washington, DC: US EPA.
10. US Environmental Protection Agency. Toxicological Review of Benzene (Noncancer Effects). Integrated Risk Information System (IRIS). Washington, DC: US EPA; 2003.
11. US Environmental Protection Agency. Revisions to the Guideline on Air Quality Models. 40 CFR Part 51, Appendix W. Fed Regist. 2017;82(10):5182-5235.
12. Rosenfeld PE. RoseyFields LLC: Human Health Risk Assessment Methodology. Santa Monica, CA: RoseyFields LLC; 2026.
13. US Environmental Protection Agency. Integrated Risk Information System (IRIS) Program Overview. Washington, DC: US EPA.
14. Cannon K, Donovan E, Ramirez G, Sassler C, Walton H, et al. v BP Products North America, Inc. Case No. 3:10-cv-00622 (SD Tex).