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OCR for page 11
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holding of run-off water with more treatment until accept- Quantity and Persistence
able levels of both the initial contaminant and any added de-
contaminants (i.e., chlorine and soap) are reached. The quantity of a chemical released and the chemical type
and toxicity determine the potential human health effects.
The quantity and persistence of the released chemical deter-
2.1.2 Emergency Response mine the duration of the human health risk. An upper range
Information Needs estimate of the quantity of a chemical release from an indus-
trial storage or transport container can be determined easily
Regardless of the cause of a chemical release, from an
on the basis of container size and can assist in developing a
emergency response perspective, the primary considerations
conservative estimate of the area that may receive concen-
for response management are the chemical type and toxicity,
trations of concern. The larger volumes of chemicals in trans-
quantity and persistence, exposure route; and dispersion and
port are in railcars, with amounts up to 90 tons per car, and
population density in the area at risk. These factors and their
on inland and coastal barges in amounts up to 2,500 tons per
interrelationship are discussed below. Table 2-3 delineates
barge. These containers may be particularly attractive from a
some of the information needed in a chemical event to decide
terrorist perspective because of the possibility of very
the appropriateness of transportation goals for isolation, shel-
quickly releasing large quantities of a toxic chemical. Toxic
ter-in-place, evacuation, and checkpoint establishment.
gases (transported as both gases and as liquids under pres-
sure) are generally not very persistent, so typically present
Chemical Type and Toxicity only a short-term threat.
Estimating the release quantity and associated area with
Chemical type and toxicity are key factors in assessing the concentrations of concern may be more problematic from a
threat posed to human health, thus is important for determin- chemical weapons release, particularly if there is a delay be-
ing appropriate protective wear for first-responders and tween exposure and observable symptoms. Particularly
clean-up personnel, and population risks. The health risks to among blister and nerve agents, the persistence of chemical
responders and the general population are also affected by toxicity after release varies greatly (i.e., from minutes to
the contaminant concentration (derived for the initial quan- weeks). The identification of a chemical weapons release
tity and area of dispersion) and the exposure route. In popu- agent needs to be confirmed with laboratory tests to estimate
lated areas, the general type of a released chemical can often confidently its persistence and the protective wear needed
be assessed within minutes of release because of the rapid during clean up.
onset of symptoms. Most industrial transport chemicals are
choking agents or asphyxiants. In non-industrial events, mul-
tiple agents may be released, and a more common agent, such Exposure Route
as tear gas, initially may mask the presence of another agent,
Possible exposure routes are determined by whether the
such as the blister agent mustard gas, for which symptoms
chemical is a gas, liquid, or solid. If it is one of the latter two
other than those similar to tear gas may be delayed for several
forms, aerosol or particle size and evaporation rate must also
hours.
be considered. Inhalation is the primary exposure route
of concern for choking agents and for some nerve agents
TABLE 2-3 Determination of Chemical (e.g., sarin). In a severe release, inhalation may also be an im-
Event Emergency Response portant exposure path for some blister agents. The time re-
Chemical Event quired for an agent suspended in air to be diluted, and for par-
Information Needed to Determine Appropriate ticles and aerosols to settle to the ground and evaporate,
Emergency Response determines how long inhalation is a concern. The size of the
Estimated population exposed to levels of concern as particles is critical in that if the particles are too large, they
determined from: quickly fall to the ground and no longer pose a threat through
Affected area information (see below)
Population data
the inhalation exposure route. Table 2-4 provides estimates
Estimated affected area as determined from:
Quantity and toxicity of material
Release parameters (density, temperature, TABLE 2-4 Particle Size and
momentum, etc.) Sedimentation Rate in Stagnant Air
Location of release
Wind direction and speed Particle Diameter Time to Fall 100 Ft.
Topography, urban or rural environment 100 microns 2 minutes
Levels of concern (i.e., toxicity) 10 microns 3 hours
Possible exposure pathways as determined from: 3 microns 30 hours
Chemical identity 1 micron 240 hours
Physical form (gas, liquid, solid) 0.5 microns 820 hours
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of how long it takes various particle sizes to settle to the water within the transportation system is possible, but prob-
ground when there are no wind currents. ably would not be a very large event in terms of injuries or
The smaller the particle, the further it may travel within the casualties.
respiratory tract. In general, particles greater than 10 m (mi- Food contamination with toxic chemicals is also possible.
crometers, also called microns) in diameter are trapped by Although only isolated cases of intentional food poisoning
nasal hairs and released with exhaled air or sneezing. Parti- have occurred, several single accidental food poisoning
cles less than 10 m in diameter are referred to as "inhalable" events within the last decade have sickened thousands of peo-
because they may pass into the upper portions of the lungs, ple, suggesting the potential effects that could be achieved
which contain many branched passageways. These passages with intentional food poisoning.
(bronchi and terminal bronchioles) are lined with mucus and
cilia. The mucus traps particles, and the cilia gradually push
the mucus and its contents up and out of the lungs within Dispersion, and Population Density
about a day. Chemical agents trapped in the mucus are then
swallowed, entering the digestive tract. The smallest parti- The density of the population in the area at risk affects the
cles or aerosols (e.g., less than 2.5 m diameter) are referred means for communicating instructions and the choices of
to as "fine" or "respirable" particles. Fine particles may pass transportation-related responses. The number of individuals
into the deepest portions of the lungs (i.e., respiratory bron- at risk during a chemical event depends on the population
chioles, alveolar ducts, and alveolar sacs) where they may density and the area over which health-threatening levels of
dissolve or be removed by macrophages over many days. contamination are dispersed. Health-threatening levels are
The unaided human eye can discriminate individual par- determined by the type and toxicity of the chemical and the
ticles down to the size of 30 microns. These particles will exposure route. Dispersion is determined from the physical
be trapped by nose hairs and mucous and blown out or form of the chemical (i.e., gas, liquid, solid, or particles),
swallowed before reaching the lungs. To put particle sizes topology and meteorology (i.e., rain and wind currents), and
in perspective, particle size of commonly used substances the quantity released.
are presented in Table 2-5. One of the more common worst-case scenarios of a chem-
Absorption through skin is the primary exposure route for ical incident involves the release of a chemical that is a gas
some chemical weapons agents (i.e., essentially all blister under ambient conditions, but is transported as a liquid under
agents and some nerve agents). Many of the agents for which pressure in 90-ton railroad tank cars. A release from one of
the skin is a primary exposure route are oily liquids. Aerosol these cars carrying chlorine or ammonia has been projected
droplets too large to be readily respired are a common means to create a danger zone in the range of 14 miles--in a highly
for dispersal of these agents. Droplets of some of these agents urbanized area, such a release could affect millions of peo-
(e.g., VX) can take weeks to evaporate or break down to less ple. When pressurized liquids are released, part of the liquid
toxic forms. Many common fabrics provide protection from immediately flashes to a gas, forming a vapor cloud. The re-
skin absorption of some agents (e.g., the blister agent phos- mainder may be fragmented into fine liquid droplets (i.e., an
gene oxime), but provide little protection from other agents aerosol), which will soon evaporate. Depending on the re-
(e.g., sulfur mustard). lease conditions, some of the liquid may form an evaporat-
Ingestion of chemical toxins may occur with tainted drink- ing liquid pool on the ground. The highly turbulent nature of
ing water or food. An effective chemical toxin release in these releases facilitates mixing with air. Within a minute,
the drinking water supply of a city-sized population gener- the cloud typically consists of air with a few percent of the
ally is not viewed as a credible threat by terrorism experts be- released chemical. The vapor cloud is heavier than air, caus-
cause of the quantity of an agent needed and the degradation ing it to slump to the ground, becoming a broad cloud that
of chemicals that results from standard water disinfection backs up against the wind. Within a few minutes, a release
processes. However, a chemical attack on post-purification of common bulk transport quantities of a liquefied gas
drinking water storage in a small municipality or a specific (i.e., 20 to 90 tons) can cause a vapor cloud with a backup of
building is viewed as more credible, but difficult to get several hundred meters and a width of a kilometer or more.
without site-specific knowledge and access. By extension, Depending on atmospheric conditions, the size of the release,
intentional chemical contamination of passenger drinking and the toxicity of the released gas, the area of concern may
TABLE 2-5 Individual Particle Size of Commonly Used Substances
Substance Individual Particle Size (Microns) Inhalable / Respirable
Beach sand 74 to 187 No
Table salt crystal 100 No
Powdered confectioner's sugar 10 Inhalable
Talc powder 1.5 to 37 Inhalable and Respirable
Tobacco Smoke 0.5 Inhalable and Respirable
