EPA Reference Values: Regulatory Context

1. George M. Woodall, PhD NCEA Toxicologist Leland Urban Air Toxics Research Center October 18, 2005 EPA Reference Values:Regulatory Context

2. National Emission Standards for Hazardous Air Pollutants (NESHAP) • Program Mandated in 1990 CAA Amendments • Maximum Achievable Control Technology (MACT) • Assumption - Reducing emissions will reduce risk • However - No characterization of risk • Residual Risk Assessments • Risk remaining after NESHAPs enactment – usually 8 years • Assessment of risk

3. Residual Risk • Characterization of emissions • Annual • Hourly (generally, 10 x apportioned annual emissions) • Modeling of emission dispersion • Emissions • Current Reported • MACT Limit • Meteorology • Worst-case for Hourly • 5-year Average for Cancer • Worst year for Chronic Non-cancer • Calculations of health-based risk • Using modeled receptors (often highest exposed receptor) • Both Cancer and Non-cancer Effects (acute and chronic durations)

4. What is a Reference Value? Reference Values = Guidelines & Standards • Guidelines are recommendations for safe exposure levels • Integrated Risk Information System (IRIS) • Reference Concentration (RfC) • Standards are enforceable legal limits • National Ambient Air Quality Standards (NAAQS)

5. Risk Assessment Paradigm (NAS, 1994)

6. Purpose of Health Effects Reference Values • Each reference value system has a specific reason for existence • Protection for specific populations • Workers • General population (Public Health) • Susceptible sub-populations • Defined exposure scenarios • Peak vs. Repeated vs. Continuous exposures • Duration, schedule, etc. • Organizational Mandate

7. Reference Valuesand HAP Chemicals • Two Durations Modeled in Residual Risk • Chronic • Continuous (24-hour/day; 7-days/week; potentially for a lifetime) • Low concentrations • Acute • Short-term (<= 24-hour, single events; potentially repeated) • High concentrations

8. Cancer Reference Values • US EPA (http://www.epa.gov/iris/subst/index.html)-or- Cal EPA (http://www.oehha.ca.gov/air/cancer_guide/index.html) • Inhalation – Unit Risk (IUR) • Oral – Cancer Slope Factor (CSF) • Chronic Exposure Durations Assumed

9. Chronic Non-Cancer Reference Values • US EPA –Reference Concentration (Chronic RfC) • http://www.epa.gov/iris/subst/index.html • ATSDR – Minimal Risk Level (Chronic MRL) • http://www.atsdr.cdc.gov/mrls.html • California EPA – Reference Exposure Level (Chronic REL) • http://www.oehha.ca.gov/air/chronic_rels/index.html OAQPS Hierarchy: RfC > MRL > REL

10. Categories of Acute Health Standards and Guideline Levels • Occupational • Healthy worker population • Exposures for average workday/workweek and short-term peaks • Emergency Response • General population – not necessarily the “most susceptible” • Rare, short-term exposures • Adverse effects, not “safe” exposure levels (not re-entry) • “Safe” Public Health Values • All susceptible subgroups (generally more conservative) • Longer-term, potentially repeated exposures

11. Acute Reference Value Definitions (Woodall, 2005)

12. Acute Reference Values • No hierarchy chosen • Arrays of all chemical-specific values used to determine “Safe” exposure level • Occupational values NOT used in Residual Risk • Comparisons to Chronic reference values also performed.

13. No Public Health nor low-level Emergency Response values available for Ethylene Oxide Occupational values below the AEGL-2 and ERPG-2 levels indicate a potential problem. The Draft Acute RfC is not yet ready for use in the regulatory setting. So, what do you use?

14. Analysis to Support Residual Risk Assessment • Characterize the Acute Reference Values for HAPS • Best value to use in individual Residual Risk assessments • Understand the basis for differences between values • Determine best course when critical Acute Reference Values are missing

15. Reference Values Database(Air Toxics Health Effects Database: http://www.epa.gov/ttn/atw/toxsource/summary.html) Database 854 Chemicals (2,275 Values ) Acute Inhalation 243 Chemicals (696 values) Comparable Values126 Chemicals

16. Acute Chronic n =

17. Comparison between Acute and Chronic Reference Values • Determine which acute values may be more critical for Residual Risk Assessments • Simple comparison (ratios) of acute to chronic values for single chemicals • A Priori Assumption – Concern if Acute values (mostly 1-hr) within 3 orders of magnitude of their corresponding Chronic value

18. Ratio of Acute to Chronic Non-Cancer Inhalation Reference Values by HAP Chemical

19. * ** ***

20. Acute to Chronic Comparisons • 92 chemicals had ratios calculated: • 25 had a lowest ratio value ≤ 10 • 16 had a lowest ratio value > 10 and ≤ 100 • 19had a lowest ratio value > 100 and ≤ 1000 • 32 had a lowest ratio value > 1000

21. Summary • Health Reference Values are developed for specific purposes and use outside those purposes should be done judiciously, if at all • Comparisons between Health Reference Values are more valid: • Within certain categories (occupational, emergency releases, public health protection) and • For comparable time frames • Acute reference values for some chemicals may be more critical for residual risk analysis than their corresponding chronic values.

22. Acknowledgements • Roy L. Smith, PhD (US EPA/OAQPS) • Robert Hetes, PhD (US EPA/ORD) • Mark Corrales, PhD(US EPA/OPEI)

23. References National Academies of Science (1994) Science and judgment in risk assessment. Washington, DC: National Academy Press Woodall, G.M. (2005) Acute health reference values: Overview, perspective, and current forecast of needs. Journal of Toxicology and Environmental Health, Part A, 68:901-926