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Issues with Use of Toxicity ValuesFor Emergency Response

by Timothy BauerNaval Surface Warfare Center Dahlgren

Building 1480 Room 2274045 Higley Road Suite 346Dahlgren, VA 22448-5162

540-653-3091 Fax: 540-653-8747

8th Symposium on the Urban EnvironmentAMS 89th Annual Meeting

11 - 15 January 2009Phoenix Convention Center, Phoenix, AZ

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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

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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

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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

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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

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Baltimore Incident Hazard Areas

Typical concentration hazard area estimates

22,182 people3697 people1288 people

1588 m length/radius

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Baltimore Incident Conc. Contour

Typical concentration contour estimates

1288 people

1588 m length, 345 m width

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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

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Baltimore Incident Dosage

Toxic area represented by dosage

223 people

730 m length, 131 m width

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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

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Baltimore Incident AEGL-2 Toxic Load

AEGL-2 toxic load area

393 people

945 m length, 179 m width

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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

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Baltimore Incident EC50 Toxic Load

EC50 toxic load area

34 people

301 m length, 36 m width

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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

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Baltimore Incident Casualty Estimate

Toxic load areas for 1 and 2 standard deviations

32 casualties

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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