Issues with Use of Toxicity Values For Emergency Response by Timothy Bauer

1. Issues with Use of Toxicity Values For Emergency Response by Timothy Bauer Naval Surface Warfare Center Dahlgren Building 1480 Room 227 4045 Higley Road Suite 346 Dahlgren, VA 22448-5162 540-653-3091 Fax: 540-653-8747 8th Symposium on the Urban Environment AMS 89th Annual Meeting 11 - 15 January 2009 Phoenix Convention Center, Phoenix, AZ

2. Introduction • Emergency responders must have a reasonable estimate of the location and size of the hazard area resulting from a TIC incident • Modern hazard assessment models provide comparable concentration versus location and time estimates • Includes both open terrain and urban models • Current approach in applying model output to estimating human toxicity effects is not appropriate • Many different toxicity values • Some values are for occupational or lifetime exposure or for chronic effects • Most values are for the most sensitive sub-population • Concentrations are normally for an assumed 1 hour exposure at constant concentration • Expected value toxicity estimates are needed for proper emergency response support

3. Toxicity Concentrations • REL = Reference Exposure Level for no effects  lifetime • GPL = General Population Limit  lifetime • TLV-TWA = Threshold Limit Value, Time-Weighted Average  8 hours • WPL = Worker Population Limit, equivalent to TLV-TWA  8 hours • EEGL = Emergency Exposure Guideline Level  1 – 24 hours • TLV-STEL = Threshold Limit Value, Short Term Exposure Limit  15 min • TEEL-0, 1, 2, 3 = Temporary Emergency Exposure Limit  1 hour • ERPG-1, 2, 3 = Emergency Response Planning Guideline  1 hour • AEGL-1, 2, 3 = Acute Exposure Guideline Level  10 min - 8 hours • IDLH = Immediately Dangerous to Life and Health  10 min • LCLO = Lowest Lethal Concentration  1 hour • LC50 = Median Lethal Concentration  1 hour

4. Current Hazard Estimation • Estimates for emergency planning and response are normally based on ERPG-2 or 1-hour AEGL-2 concentrations • ERPG-2: The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects, or symptoms that could impair an individual’s ability to take protective action. • AEGL-2: The airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects, or an impaired ability to escape. • Approach seems reasonable, but ends up being impractical when applied to real-world incidents

5. Example Scenario • Baltimore, MD  population = 631,000 at 2800 persons/km2 • Incident involving release of 2500 lb HCN from a rupture in a tanker truck located near city center • Could be a terrorist attack or just a transportation accident • 1-hour AEGL-2 = 8.0 mg/m3 = 7.1 ppm • 3 m/s wind speed, 30 C air temperature, neutral stability, and urban terrain • Hazard assessment models (e.g., ALOHA, DEGADIS, HPAC) predict maximum distance to which ERPG-2/AEGL-2 is exceeded • Area is displayed as circle, 60 degree angle fan, or contour

6. 22,182 people 3697 people 1288 people 1588 m length/radius Baltimore Incident Hazard Areas Typical concentration hazard area estimates

7. 1288 people 1588 m length, 345 m width Baltimore Incident Conc. Contour Typical concentration contour estimates

8. Dosage Output • Toxic effects are a result of concentration plus exposure duration • Threshold effects may be just a function of concentration • Constant concentration: D = C t • Dosage is actually the integral of concentration versus time • Does not require a constant concentration • Frequency should not be less than human breathing cycle of ~ 5 seconds • HCN 1-hour AEGL-2 dosage = 480 mg-min/m3

9. 223 people 730 m length, 131 m width Baltimore Incident Dosage Toxic area represented by dosage

10. Toxic Load Output • Toxic effects are actually more complicated than just dosage • Human and animal systems are able to process or remove almost all toxic substances • A low concentration over a long period of time is handled better than the same dosage from a high concentration over a short period of time • Dosage is then a function of exposure duration with longer durations requiring higher dosage values • Represented by toxic load equation • K = CN t • As with dosage, can integrate toxic load over time • Toxic load constant is independent of duration • HCN toxic load exponent is 2.0, so AEGL-2 toxic load constant is 3840 mg2-min/m6

11. 393 people 945 m length, 179 m width Baltimore Incident AEGL-2 Toxic Load AEGL-2 toxic load area

12. Expected Value Toxicity • AEGL-2 does not represent adverse health effects for average person • Safe-sided for most sensitive sub-population • Young, old, immune compromised, pregnant • Need toxic load parameters to represent average person • Median effective toxic load represents where 50% of exposed persons will experience adverse health effects • Reanalysis of existing toxicity data being conducted to determine expected values • HCN expected severe effects toxic load values: EC50 = 128 mg/m3 = 114 ppm, N = 2.0, t = 60 min, K = 983,000 mg2-min/m6

13. 34 people 301 m length, 36 m width Baltimore Incident EC50 Toxic Load EC50 toxic load area

14. Probability of Effect • EC50 toxic load parameters only provide area within which 50% of persons will experience severe health effects • What about persons further inside or outside of area? • Probit slope is final toxicity parameter needed • Determines percent of population affected as toxic load increases or decreases away from median effective value • 84% and 16% effects represent 1 standard deviation • 2.5% and 97.5% represent 2 standard deviations • HCN probit slope is 12 • Casualties can now be estimated • Simple approach: Differential contour area times population density times percent affected; sum for all contours • Integral approach: Compute percent affected at each grid location, multiply by grid element area and population density, and sum for all grid locations

15. 32 casualties Baltimore Incident Casualty Estimate Toxic load areas for 1 and 2 standard deviations

16. Conclusions • Current approach of using concentrations results in areas too large for effective emergency planning and response • Dosage provides a better hazard area representation, but toxic load is even better • Use of AEGL-2 or ERPG-2, even with toxic load, is not appropriate because of safe-sided interpretation • New expected value toxic load parameters will significantly improve area estimates • Addition of probit slope to calculations allows generation of areas by percent of population expected to have toxic response • Realistic areas are much smaller and allow effective emergency planning and response • Evacuation versus sheltering-in-place planning guidance • Search and rescue for casualties during response