Issues with Use of Toxicity Values

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• 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 [phone]
• 8th Symposium on the Urban Environment
• AMS 89th Annual Meeting
• 11 - 15 January 2009
• Phoenix 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 individuals 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

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Baltimore Incident Conc. Contour

• Typical concentration contour estimates

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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
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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

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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

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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

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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