MODELING PROTECTIVE ACTION DISTANCES

FOR CHEMICAL WARFARE AGENTS

Hypothetical Situation

As part of your training, during a written exam question, you are asked to estimate a public evacuation zone for a chemical warfare agent (let us say Sarin) believed contained in a large package attached to what appears to be an explosive device.   You don’t know the amount of the warfare agent in the package, but you do know roughly the package size and there appears to be a small amount of leakage which evaporates.  The package is outdoors at an urban location.  A few people nearby have complained of nausea and difficulty seeing in dim light.  The belief that the package may contain Sarin is based on an examination of the eyes of the people exposed, and pinpoint pupils were noted.  The area has been blocked off preventing access, but a much wider public evacuation zone needs to be done.  The public evacuation is based on an assumption that all of the package contents may become airborne as the result of an explosion before it can be disarmed.  

When your estimate for public evacuation is compared with other estimates of your team you find that the estimates differ considerably.   Why is this?    The reason for the differences goes beyond differences in estimates of amount of Sarin released or weather conditions.  The methodologies for estimating a Protective Action Distance (PAD) are different, and there is an uncertainty of what value to use as a level of concern.

Let us look at different methods of estimating a Protection Action Distance as applied to chemical warfare agents.

2004 Emergency Response Guidebook (2004 ERG)

The Emergency Response Guidebook is published jointly by Transport Canada, U.S. Department of Transportation, and SCT (Mexico) at four-year intervals.   It is designed to give information to emergency responders in case of a transportation accident involving hazardous chemicals.

The 2004 ERG gives a sketch showing an initial isolation zone, a protective action zone, and a protection action distance in the case of a spill of a hazardous chemical.  The initial isolation zone represents the distance in all directions all persons must be removed from the spill.  The Protective Action Distance (PAD) is the distance downwind for which protective action should be considered, whether this is an evacuation or shelter in place or a combination thereof.   The Protective Action Zone is defined by a square whose length and width are the same as the downwind distance represented by the PAD.  Anyone within this Zone is at risk for harmful exposure.  A copy of the 2004 ERG can be obtained off the Internet at http://hazmat.dot.gov/pubs/erg/erg2004.pdf.    The PEAC tool marketed by AristaTek also contains a copy of the 2004 ERG.  To access the 2004 ERG information for Sarin via the PEAC tool, select “Sarin (when used as a weapon)” as a chemical choice on the left side of the PEAC tool, then select 2004 ERG at the upper right.  The information pertaining to Sarin (when used as a weapon) is then displayed.

The Initial Isolation and PADs can then be read for Sarin (when used as a weapon)

“When used as a weapon” means that the chemical is aerosolized or gasified as in an explosion or using an aerosol device as opposed to “when spilled on land” which means a simple spill of the sarin liquid which puddles and evaporates.   The initial isolation distance for small spills is 500 feet, and 3000 ft for large spills.   The PAD is further subdivided into daytime and nighttime spills.   The user can display the PADs in either English units (as shown) or metric units.    Nighttime releases display a greater PAD than daytime releases because, on the average, the air is more stable and the chemical is less likely to be dispersed. 

What is a “small spill”  and what is a “large spill”?   According to the instructions provided with the 2004 ERG, a small spill is from a 55-gallon drum or smaller, and a large spill is greater than 55 gallons.   But chemical warfare agents are not normally shipped on the highways or by rail.  There was a large public outcry when the military proposed to destroy old and possibly leaking stockpiles of chemical warfare agents by incineration; one of the options considered was shipping to a centralized location (such as Tooele, Utah) for destruction  (don’t ship the stuff past my neighborhood, and don’t incinerate the CWA here).   So what size container was the 2004 ERG modeling based?   The answer is in Table 2.4 of the following document,

D.F. Brown, W.A. Freeman, R.A. Carhart, and M. Krumpolc.  “Development of the Table of Initial Isolation and Protective Action Distances for the 2004 Emergency Response Guidebook”,  Argonne National Laboratory, May 2005.  A copy of this report can be downloaded from the Internet at http://hazmat.dot.gov/pubs/erg/Argonne_Report08042005.pdf .

Small release:   2 kg  (Sarin, VX, Soman, Tabun, Lewisite, or Mustard HD).

Large Release:  100 kg  (Sarin, VX, Soman, Tabun, Lewisite, or Mustard HD).

The 2004 ERG offers the user only two choices, ether a small spill of 2 kg  (4.4 lbs) or a large spill of 100 kg (220 lbs) .  If the user assumed that a small “spill” was 55 gallons or less, which is implied by a quick read of the 2004 ERG, a wrong answer would be obtained.   A 55 gallon container of Sarin if full would contain 227 kg of Sarin.  The 2004 ERG considers a release of 100 kg as a large spill.

Incidently, Brown’s developmental document cited above uses 60 gallons or less for a small spill of industrial chemicals whereas the 2004 ERG lists the cutoff as 55 gallons or less for a small spill.

Levels of Concern

The distances listed in the 2004 ERG for the initial isolation zone and protective action distance is based on a level of concern.  The 2004 ERG has adopted the American Industrial Hygiene Association’s Emergency Response Planning Guideline Level 2 (ERPG-2) as the Level of Concern (LOC) for the PAD.   If an ERPG-2 value has not been published, then the 1-hour LC₅₀ /100  value is used as an approximation to ERPG-2.   The Initial Isolation Zone distance is based on 0.05 times the 1-hour LC₅₀ value.  Earlier editions of the ERG used the Emergency Response Planning Guideline Level 3 (ERPG-3), the NIOSH IDLH, or other LOC criteria for the calculation of the Initial Isolation Zone distance. 

Table 1.  1-hr LC₅₀ Values for Chemical Warfare Agents used in the 2004 ERG

Definitions of these Levels of Concern are as follows:

ERPG-2:  The maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects, or symptoms that could impair an individual’s ability to take protective action.

ERPG-3:  The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects.

1-hour LC₅₀ :  The lethal concentration in air that 50% of the test animals (rats preferred) will die after exposure to the chemical for 1 hour.

These ERPG-2, ERPG-3, and LC₅₀ concentrations only consider acute toxic effects and do not consider carcinogens nor explosive environments.

NIOSH IDLH:  The National Institute for Occupational Safety and Health has established “Immediately Dangerous to Life and Health” concentrations for workers exposed to airborne chemicals.   The IDLH value is the maximum concentration that a worker can be exposed to for up to 30 minutes without loss of life or irreversible health effects or severe eye or respiratory irritation that would prevent his/her escape.  The IDLH value does consider explosive effects (either 10% of the Lower Explosive Limit concentration or toxic effects, or impairment of senses that might prevent escape).

This may seem straightforward, but every four years when a new ERG is published the criteria historically has changed.   Also the numbers for ERPG and LC₅₀  can change as more information becomes available.   If a user has a 2000 ERG and compares initial isolation distances and PADs listed with the 2004 ERG,   the distances are different for the same chemical.   More changes can be expected for the 2008 ERG if the Acute Exposure Guideline Levels (AEGLs) developed by the Federal Advisory Committee  (a collaborate effort of public and private sectors) are adopted for PAD and initial isolation distance criteria.  The AEGLs are used by the U.S. Environmental Protection Agency.

As of 2005, no ERPGs have been established for chemical warfare agents.   The 1 hour LC₅₀ values used in the 2004 ERG for seleced chemical warfare agents are listed in Table 1.   For example, the 1-hour LC₅₀ is listed as 0.1 ppm for Sarin.    The 2004 ERG PAD distance is based on 0.001 ppm Sarin as the Level of Concern.

The U.S. Department of Energy (DOE) has published [see http://www.eh.doe.gov/chem_safety/teel.html ]  what they call Temporary Emergency Exposure Limits (TEELs) to serve as surrogates until the American Industrial Hygiene Association publishes their ERPGs.    ERPGs have been published for about 115 chemicals to date, but DOE has published TEELs for over 2000 chemicals.   The 2004 ERG did not consider TEELs as Levels of Concern.  ERPGs are also available at this website.

Table 2.  DOE Published TEELs as Levels of Concern

TEEL-2 may be considered as a surrogate for ERPG-2

TEEL-3 may be considered as a surrogate for ERPG-3

Acute Exposure Guideline Levels (AEGLs) are published at the EPA website, http://www.epa.gov/oppt/aegl/ .   They are intended to describe the risk to humans resulting from rare or once-in-a-lifetime exposure to airborne chemicals.   AEGLs are not now considered in the 2004 ERG, but the developmental document cited above hints that they may be considered in the future 2008 ERG.   Three levels or considered:

AEGL-1:  Airborne concentration of a substance at or above which it is predicted that the general population including “susceptible” but excluding “hypersusceptible” individuals could experience noticeable discomfort.

AEGL-2:  Airborne concentration of a substance at or above which it is predicted that the general population including “susceptible” but excluding “hypersusceptible” individuals could experience irreversible or other serious, long-lasting effects or impared ability to escape.

AEGL-3:  Airborne concentration of a substance at or above which it is predicted that the general population including “susceptible” but excluding “hypersusceptible” individuals could experience life-threatening effects or death  

The AEGLs are exposure time dependent.  Therefore AEGL values are given for various exposure times (5 minute, 10 minute, 30 minute, 60 minute, 4 hours, and 8 hours).

The U.S. Military with some exceptions has adopted the AEGLs for their Military Exposure Limits for Chemical Warfare Agents.  The listings are in

 “TG 230 Chemical Exposure Guidelines for Deployed Military Personnel”, published by the U.S. Army Center for Health Promotion and Preventive Medicine (USACHPPM), and can be obtained at

http://chppm-www.apgea.army.mil/desp/pages/samp_doc.htm   (2003 edition) or at http://chppm-www.apgea.army.mil/imo/ddb/dmd/DMD/TG/TECHGUID/Tg230.pdf  (1999 edition).

Table 3  One-hour AEGLs for Chemical Warfare Agents (in ppm or mg/m³)

The values listed in Tables 3 and 4 are the same but under categories of “Minimal”, “Significant”, and “Severe” or under AEGL levels.   The PEAC tool lists the Military Exposure Guidelines, not just for one-hour exposure but for 5 minute, 10 minute, 30 minute, 60 minute, 4 hours, and 8 hours, which are also the same as the AEGLs.

Table 4.  One-hour Military Exposure Guidelines for Chemical Warfare Agents

For comparison, a ten minute exposure limit for the same conditions are listed in Table 5.   These are the same numbers as the corresponding AEGL-1, AEGL-2, and AEGL-3 listings for the same exposure time.

Table 5.  Ten-minute Military Exposure Guidelines for Chemical Warfare Agents