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George M. Woodall, PhD
NCEA Toxicologist
Leland Urban Air Toxics Research Center
October 18, 2005
EPA Reference Values:Regulatory Context
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
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)
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)
Risk Assessment Paradigm (NAS, 1994)
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
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
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
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
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
Reference Value Organization Legal Standing
Type Value TWA (Yes/No)
Exposure Duration
PEL - Permissible Exposure Limit
OSHA Standard Occupational Yes 8-hour
Ceiling OSHA Standard Occupational No Up to10-minute
REL - Recommended Exposure Limit
NIOSH Guideline Occupational Yes 8-hour
IDLH - Immediately Dangerous to Life and Health
NIOSH Guideline Occupational No Up to 30-minute
STEL - Short Term Exposure Limit
NIOSH Guideline Occupational Yes 15-minute
TLV - Threshold Limit Value ACGIH Guideline Occupational Yes 8-hour
TLV-STEL - TLV Short Term Exposure Limit
ACGIH Guideline Occupational Yes 15-minute
AEGL - Acute Exposure Guideline Level
NAC/AEGL; NRC/AEGL
Guideline Emergency Response
10- and 30-minute; 1-, 4- and 8-hour
ERPG – Emergency Response Planning Guideline
AIHA Guideline Emergency Response
1-hour
TEEL – Temporary Emergency Exposure Level
DOE Guideline Emergency Response
1-hour
ERG – Emergency Response Guidebook
DOT Guideline Emergency Response
Specialized application
MRL - Minimal Risk Level ATSDR Guideline Public Health 1-14 days (acute); 15-364 days (intermed.); >365 days (chronic)
REL - Reference Exposure Level
Cal-EPA OEHHA
Guideline Public Health 1-8 hours
EPA – Acute RfC US EPA Draft Guideline
Public Health 1-, 4-, 8-, and 24-hours
Acute Reference Value Definitions (Woodall, 2005)
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.
Ethylene Oxide Acute Reference Values
0.01
0.1
1
10
100
1000
0 60 120 180 240 300 360 420 480 540
Minutes
pp
m -
EtO
AEGL-3 (Interim)
AEGL-2 (Interim)
ERPG-3
ERPG-2
NIOSH REL
OSHA PEL
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.
No Public Health nor low-level Emergency Response values available for Ethylene Oxide
So, what do you use?
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
Reference Values Database(Air Toxics Health Effects Database:
http://www.epa.gov/ttn/atw/toxsource/summary.html)
Database854 Chemicals(2,275 Values )
Acute Inhalation 243 Chemicals (696 values)
Comparable Values126 Chemicals
ChronicAcute
n =
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
Ratio of Acute to Chronic Non-CancerInhalation Reference Values by HAP Chemical
***
**
*
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 19 had a lowest ratio value > 100 and ≤ 1000 32 had a lowest ratio value > 1000
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.
Acknowledgements
• Roy L. Smith, PhD (US EPA/OAQPS)
• Robert Hetes, PhD (US EPA/ORD)
• Mark Corrales, PhD(US EPA/OPEI)
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