**CHAPTER 5**

PRINCIPAL HAZARDS IN THE UNITED STATES

This chapter describes the principal environmental hazards that are of
greatest concern to emergency managers in communities throughout the
United States. Each of these hazards will be described in terms of the
physical processes that generate them, the geographical areas that are
most commonly at risk, the types of impacts and typical magnitude of
hazard events, and hazard-specific issues of emergency response.

##### Introduction

Most of the hazards that concern emergency managers are environmental
hazards, which are commonly classified as natural or technological.
Natural hazards are extreme events that originate in the natural
environment, whereas the technological hazards of concern to emergency
managers originate in human controlled processes (e.g., factories,
warehouses) but are transmitted through the air and water. The natural
hazards are commonly categorized as meteorological, hydrological, or
geophysical. The most important technological hazards are toxic
chemicals, radiological and nuclear materials, flammable materials, and
explosives.

The list of natural and technological hazards that could occur in the
United States is much larger than can be addressed here. Accordingly,
this chapter focuses on the hazard agents that most commonly confront
local emergency managers. The first section addresses four
meteorological hazards---severe storms (including blizzards), severe
summer weather, tornadoes, and hurricanes. It also includes wildfires
because these are significantly influenced by lack of rainfall. The
second section describes three hydrological hazards---floods, storm
surges, and tsunamis. The third section addresses geophysical
hazards---volcanic eruptions, earthquakes, and landslides. The material
in these three sections is drawn primarily from Alexander (1993), Bryant
(1997), Ebert (1988), Federal Emergency Management Agency (1997),
Hyndman and Hyndman (2005), Meyer (1977), Noji (1997), Scientific
Assessment and Strategy Team (1994), and Smith (2001). The fourth
section covers technological hazards, primarily toxic, flammable,
explosive, and radiological materials. The material in these three
sections is drawn primarily from Edwards (1994), FEMA (no date, a),
Goetsch (1996), Kramer and Porch (1990), and Meyer (1977). The last
section summarizes information on biological hazards. The material in
this section is drawn primarily from World Health Organization (2004),
World Health Organization/Pan American Health Organization (2004), and
Chin (2000).

The chapter does not address emergencies caused by large, unexpected
resource shortages, energy shortages being a prime example. Nor does it
address slow onset disasters such as ozone depletion, greenhouse gas
accumulation, deforestation, desertification, drought, loss of
biodiversity, and chronic environmental pollution. For information on
these long term hazards, see sources such as Kontratyev, Grigoryev and
Varotsos (2002).

##### Meteorological Hazards

The principal meteorological hazards of concern to emergency managers
are severe storms (including blizzards), severe summer weather,
tornadoes, hurricanes, and wildfires.

####### *Severe Storms*

####### The National Weather Service (NWS) defines a severe storm as one whose wind speed exceeds 58 mph, that produces a tornado, or that releases hail with a 3/4 inch diameter or greater. The principal threats from these storms are lightning strikes, downbursts and microbursts, hail, and flash floods. Lightning strikes can cause casualties, but these tend to be few in number and widely dispersed so they are easily handled by local emergency medical services units. However, lightning strikes also can initiate wildfires that threaten entire communities---especially during droughts (see the discussion of wildfires below). Downbursts (up to 125 mph) and microbursts (up to 150 mph) are threats to aircraft as they take off or land. This creates a potential for mass casualty incidents. Large hail generally causes few casualties and the associated damage rarely causes significant social or economic disruption. The areas with the greatest thunderstorm hazard are in the desert southwest (northwest Arizona), the plains states (centered on Kansas) and the southeast (Florida), but only the latter two areas have high population densities. 

Severe winter storms pose a greater threat than those at other times of
year because freezing temperatures produce substantial amounts of
snow---whose volume exceeds that of an equivalent amount of rain by a
factor of 7-10. A severe winter storm is classified as a blizzard if its
wind speed exceeds 38 mph and its temperature is less than 21°F (degrees
Fahrenheit). These conditions can produce significant wind chill effects
on the human body. Table 5-1 shows increasing wind speed significantly
accelerates the rate at which a low temperature causes frostbite. It is
important to recognize that a temperature of 40°F and wind speed of 20
mph will not freeze water, even though the wind chill is 30°F.

######## These storms can immobilize travel, isolate residents of remote areas, and deposit enormous loads of snow on buildings---collapsing the long-span roofs of gymnasiums, theaters, and arenas. In addition, the weight of ice deposits can bring down telephone and electric power lines. The hazard of winter storms is most pronounced in the northern tier of states from Minnesota to northern New England, but also can be extremely disruptive farther south where cities have less snow removal equipment. 

**Table 5-1.** Wind Chill Index.

<table style="width:100%;">
<colgroup>
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 4%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
</colgroup>
<tbody>
<tr class="odd">
<td colspan="2" rowspan="2"></td>
<td colspan="18">Temperature (°F)</td>
</tr>
<tr class="even">
<td>40</td>
<td>35</td>
<td>30</td>
<td>25</td>
<td>20</td>
<td>15</td>
<td>10</td>
<td>5</td>
<td>0</td>
<td>-5</td>
<td>-10</td>
<td>-15</td>
<td>-20</td>
<td>-25</td>
<td>-30</td>
<td>-35</td>
<td>-40</td>
<td>-45</td>
</tr>
<tr class="odd">
<td rowspan="12"><blockquote>
<p>Wind speed (mph)</p>
</blockquote></td>
<td>5</td>
<td>36</td>
<td>31</td>
<td>25</td>
<td>19</td>
<td>13</td>
<td>7</td>
<td>1</td>
<td>-5</td>
<td>-11</td>
<td>-16</td>
<td>-22</td>
<td>-28</td>
<td>-34</td>
<td>-40</td>
<td>-46</td>
<td>-52</td>
<td>-57</td>
<td>-63</td>
</tr>
<tr class="even">
<td>10</td>
<td>34</td>
<td>27</td>
<td>21</td>
<td>15</td>
<td>9</td>
<td>3</td>
<td>-4</td>
<td>-10</td>
<td>-16</td>
<td>-22</td>
<td>-28</td>
<td>-35</td>
<td>-41</td>
<td>-47</td>
<td>-53</td>
<td>-59</td>
<td>-66</td>
<td>-72</td>
</tr>
<tr class="odd">
<td>15</td>
<td>32</td>
<td>25</td>
<td>19</td>
<td>13</td>
<td>6</td>
<td>0</td>
<td>-7</td>
<td>-13</td>
<td>-19</td>
<td>-26</td>
<td>-32</td>
<td>-39</td>
<td>-45</td>
<td>-51</td>
<td>-58</td>
<td>-64</td>
<td>-71</td>
<td>-77</td>
</tr>
<tr class="even">
<td>20</td>
<td>30</td>
<td>24</td>
<td>17</td>
<td>11</td>
<td>4</td>
<td>-2</td>
<td>-9</td>
<td>-15</td>
<td>-22</td>
<td>-29</td>
<td>-35</td>
<td>-42</td>
<td>-48</td>
<td>-55</td>
<td>-61</td>
<td>-68</td>
<td>-74</td>
<td>-81</td>
</tr>
<tr class="odd">
<td>25</td>
<td>29</td>
<td>23</td>
<td>16</td>
<td>9</td>
<td>3</td>
<td>-4</td>
<td>-11</td>
<td>-17</td>
<td>-24</td>
<td>-31</td>
<td>-37</td>
<td>-44</td>
<td>-51</td>
<td>-58</td>
<td>-64</td>
<td>-71</td>
<td>-78</td>
<td>-84</td>
</tr>
<tr class="even">
<td>30</td>
<td>28</td>
<td>22</td>
<td>15</td>
<td>8</td>
<td>1</td>
<td>-5</td>
<td>-12</td>
<td>-19</td>
<td>-26</td>
<td>-33</td>
<td>-39</td>
<td>-46</td>
<td>-53</td>
<td>-60</td>
<td>-67</td>
<td>-73</td>
<td>-80</td>
<td>-87</td>
</tr>
<tr class="odd">
<td>35</td>
<td>28</td>
<td>21</td>
<td>14</td>
<td>7</td>
<td>0</td>
<td>-7</td>
<td>-14</td>
<td>-21</td>
<td>-27</td>
<td>-34</td>
<td>-41</td>
<td>-48</td>
<td>-55</td>
<td>-62</td>
<td>-69</td>
<td>-76</td>
<td>-82</td>
<td>-89</td>
</tr>
<tr class="even">
<td>40</td>
<td>27</td>
<td>20</td>
<td>13</td>
<td>6</td>
<td>-1</td>
<td>-8</td>
<td>-15</td>
<td>-22</td>
<td>-29</td>
<td>-36</td>
<td>-43</td>
<td>-50</td>
<td>-57</td>
<td>-64</td>
<td>-71</td>
<td>-78</td>
<td>-84</td>
<td>-91</td>
</tr>
<tr class="odd">
<td>45</td>
<td>26</td>
<td>19</td>
<td>12</td>
<td>5</td>
<td>-2</td>
<td>-9</td>
<td>-16</td>
<td>-23</td>
<td>-30</td>
<td>-37</td>
<td>-44</td>
<td>-51</td>
<td>-58</td>
<td>-65</td>
<td>-72</td>
<td>-79</td>
<td>-86</td>
<td>-93</td>
</tr>
<tr class="even">
<td>50</td>
<td>26</td>
<td>19</td>
<td>12</td>
<td>4</td>
<td>-3</td>
<td>-10</td>
<td>-17</td>
<td>-24</td>
<td>-31</td>
<td>-38</td>
<td>-45</td>
<td>-52</td>
<td>-60</td>
<td>-67</td>
<td>-74</td>
<td>-81</td>
<td>-88</td>
<td>-95</td>
</tr>
<tr class="odd">
<td>55</td>
<td>25</td>
<td>18</td>
<td>11</td>
<td>4</td>
<td>-3</td>
<td>-11</td>
<td>-18</td>
<td>-25</td>
<td>-32</td>
<td>-39</td>
<td>-46</td>
<td>-54</td>
<td>-61</td>
<td>-68</td>
<td>-75</td>
<td>-82</td>
<td>-89</td>
<td>-97</td>
</tr>
<tr class="even">
<td>60</td>
<td>25</td>
<td>17</td>
<td>10</td>
<td>3</td>
<td>-4</td>
<td>-11</td>
<td>-19</td>
<td>-26</td>
<td>-33</td>
<td>-40</td>
<td>-48</td>
<td>-55</td>
<td>-62</td>
<td>-69</td>
<td>-76</td>
<td>-84</td>
<td>-91</td>
<td>-98</td>
</tr>
</tbody>
</table>

Source: National Weather Service \<http://www.noaa.nws.gov\>

Note: Wind Chill Temperature is defined only for temperatures less than
or equal to 50°F and wind speeds greater than 3 mph. Bright sunshine may
increase the wind chill temperature by 10-18°F.

*Extreme Summer Weather*

####### Emergency managers should be concerned about extreme heat because this can be a silent killer within the community. The body responds to high heat by using evaporating sweat to cool itself. However, high humidity decreases the efficiency with which perspiration can discharge heat, so the body's core (internal) temperature rises. When the heat gain exceeds the amount the body can remove, extreme core temperatures can cause a series of heat-related disorders. The least serious condition is *heat cramp*, which is characterized by mild fluid and electrolyte imbalances. Next in severity is *heat syncope*, which causes sudden loss of consciousness that disappears when the victim lies down. *Heat exhaustion* produces symptoms of weakness or dizziness and *heat stroke* is a condition in which the victim might be delirious or comatose. Unless treated effectively by rapid cooling, heat stroke can produce neurological damage and fatalities in about 15% of those affected. 

####### Temperature and humidity are combined into a heat index of *apparent temperature* the National Weather Service uses for weather advisories (see Table 5-2). Apparent temperatures of 80-90°F warrant *caution* because prolonged exposure and physical activity can cause fatigue. *Extreme caution* should be taken when apparent temperatures reach 90-105F because prolonged exposure and physical activity can cause heat cramp and heat exhaustion. *Danger* exists when apparent temperatures reach 105-130F because prolonged exposure and physical activity can cause heat stroke. *Extreme danger* exists when apparent temperatures exceed 130F because heat stroke is imminent.

Hazard maps show the most severely exposed areas are in the desert
Southwest, Mississippi Valley, and Southeastern states. Demographic
groups at greatest risk are outdoor laborers, the very old (particularly
those over 75), the very young, and those who have chronic diseases. The
problem can be especially severe in the inner cities where city
buildings re-radiate sunlight (increasing the ambient temperature) and
block the wind (decreasing evaporative cooling). Those who live in
residences lacking air conditioning have the greatest exposure when they
live in high crime areas where they might even be afraid to open the
windows for fans.

**Table 5-2.** Heat Index.

+------+-------+-------+-------+-------+-------+-------+-------+------+
|      |       | T     |       |       |       |       |       |      |
|      |       | emper |       |       |       |       |       |      |
|      |       | ature |       |       |       |       |       |      |
|      |       | (° F) |       |       |       |       |       |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      |       | 80    | 85    | 90    | 95    | 100   | 105   | 110  |
+------+-------+-------+-------+-------+-------+-------+-------+------+
| >    | 40%   | 79    | 84    | 90    | 98    | 109   | 121   | 135  |
| Rela |       |       |       |       |       |       |       |      |
| tive |       |       |       |       |       |       |       |      |
| >    |       |       |       |       |       |       |       |      |
| >    |       |       |       |       |       |       |       |      |
| humi |       |       |       |       |       |       |       |      |
| dity |       |       |       |       |       |       |       |      |
| >    |       |       |       |       |       |       |       |      |
|  (%) |       |       |       |       |       |       |       |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      | 50%   | 80    | 86    | 94    | 105   | 118   | 133   |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      | 60%   | 81    | 90    | 99    | 113   | 129   | 148   |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      | 70%   | 82    | 92    | 105   | 122   | 142   |       |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      | 80%   | 84    | 96    | 113   | 133   |       |       |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+
|      | 90%   | 85    | 101   | 121   |       |       |       |      |
+------+-------+-------+-------+-------+-------+-------+-------+------+

Source: National Weather Service \< www.noaa.nws.gov\>

Note: This chart is based upon shady, light wind conditions. Exposure to
direct sunlight can increase the Heat Index as much as 15°F.)

  --------------- -------------------------------------------------------
  Heat Index      Possible Heat Disorder

  80°F - 90°F     Caution: Fatigue possible with prolonged exposure and
                  physical activity.

  90°F - 105°F    Extreme caution: Sunstroke, heat cramps and heat
                  exhaustion possible.

  105°F - 130°F   Danger: Sunstroke, heat cramps, heat exhaustion likely;
                  heat stroke possible.

  Greater than    Extreme danger: Heat stroke highly likely with
  130°F           continued exposure.
  --------------- -------------------------------------------------------

# Tornadoes

Tornadoes form when cold air from the north overrides a warmer air mass
and the cold air descends because of its greater weight. The descending
cold air is replaced by rising warm air, a process that initiates
rotational flow inside the air mass. As the tornado forms, pressure
drops inside the vortex and the wind speed increases. The resulting high
wind speed can destroy buildings, vehicles, and large trees. The
resulting debris becomes entrained in the wind field, which adds to the
tornado's destructive power.

There are approximately 900 tornadoes each year in the United States,
most of which strike Texas, Oklahoma, Arkansas, Missouri, and Kansas.
However, there is also significant vulnerability in the North Central
states and the Southeast from Louisiana to Florida. Tornadoes are most
common during the spring, with the months of April-June accounting for
50% of all tornadoes. There also is predictable diurnal variation, with
the hours from 4:00-8:00 pm being the most frequent time of impact.
Tornadoes have distinct directional tendencies as well, most frequently
traveling toward the northeast (54%), east (22%), and southeast (11%).
Only 8% travel north, 2% travel northwest, and 1% travel west,
southwest, or south, respectively. There also is a tendency for
tornadoes to follow low terrain (e.g., river valleys and to move in a
steady path, although they sometimes times skip about---missing some
structures and striking others. A tornado's forward movement speed
(i.e., the speed at which the funnel moves forward over the ground) can
range 0-60 mph but usually is about 30 mph. Tornadoes can vary
substantially in physical intensity and this attribute is characterized
by the *Fujita scale*, which has a low end of F0 (maximum wind speed of
40 mph) and a high end of F5 (maximum wind speed of 315 mph). The Fujita
scale has been criticized for neglecting the effects of construction
quality, thus overestimating windspeeds for tornadoes that are F3 and
higher. Discussion is underway to replace the existing Fujita scale with
an Enhanced Fujita scale (for further information, see
meted.ucar.edu/resource/wcm/html/230.htm for a powerpoint presentation.
For discussion of this change, see
www.wind.ttu.edu/F_Scale/default.htm).

Only about one third of all tornadoes exceed F2 (111 mph). The impact
area of a typical tornado is 4 miles (mi) in length but has been as much
as 150 mi. The typical width is 300-400 yards (yd) but has been as much
as 1 mi. It is important to recognize that 90% of the impact area is
affected by a wind speed of less than 112 mph, so many structures in a
stricken community will receive only moderate or minor damage. Only
about 3% of tornadoes cause deaths and 50% of those deaths are residents
of mobile homes---structures that are built substantially less sturdily
than site-built homes.

There has been an increased number of tornadoes reported during recent
years, but this is due in part to improved radar and spotter networks.
However, tornadoes have been observed in locations where they have not
previously been seen, suggesting some long-term changes in climate are
also involved. Detection is usually achieved by trained meteorologists
observing characteristic clues on Doppler radar. Over the years, warning
speed has been improved by NOAA Weather Radio, which provides timely and
specific warnings. Those who do not receive a warning can assess their
danger from a tornado's distinct physical cues; dark, heavy cumulonimbus
(thunderstorm squall line) clouds with intense lightning, hail and
downpour of rain immediately to the left of the tornado path, and noise
like a train or jet engine. The most appropriate protective action is to
shelter in-place, which is universally recommended to be a specially
constructed safe room (Federal Emergency Management Agency, no date, b).
If a safe room is not available, building occupants should shelter in an
interior room on the lowest floor. Mobile home residents should evacuate
to community shelter and those who are outside should seek refuge in a
low spot (e.g., a small ditch or depression) if in-place shelter is
unavailable.

*Hurricanes*

A hurricane is the most severe type of tropical storm. The earliest
stages of hurricane development are marked by thunderstorms that
intensify through a series of stages (tropical wave, tropical
disturbance, tropical depression, and tropical storm) that result in a
sustained surface wind speed exceeding 74 mph. At this point, the storm
becomes a hurricane that can intensify to any one of the five
Saffir-Simpson categories (see Table 5-3).

The nature of atmospheric processes is such that few of the minor storms
escalate to a major hurricane. In the average year there are 100
tropical disturbances, 10 tropical storms, 6 hurricanes, and only 2 of
these hurricanes strike the US coast. Hurricanes in Categories 3--5
account for 20% of landfalls, but over 80% of damage. Category 5
hurricanes are rare in the Atlantic (three during the 20^th^ Century),
but are more common in the Pacific. Tropical storms draw their energy
from warm sea water, so they form only when there is an increase in sea
surface temperature that exceeds 80°F. For most Atlantic hurricanes,
this takes place in tropical water off the West African coast. These
storms are generated when the surface water absorbs heat and evaporates,
and the resulting water vapor rises to higher altitudes. When it
condenses there, it releases rain and latent heat of evaporation. An
easterly steering wind (which is named for its direction of origin, so
an easterly wind blows from east to west) pushes these storms westward
across the Atlantic. The hurricane season begins the first of June,
reaches its peak during the month September, and then decreases through
the end of November.

**Table 5-3.** Saffir-Simpson Hurricane Categories.

+-------+------+--------+----------------------------------------------+
| Sa    | Wind | Ve     | Wind Effects                                 |
| ffir/ | S    | locity |                                              |
| Si    | peed | Pr     |                                              |
| mpson | (    | essure |                                              |
| Cat   | mph) | (psf)  |                                              |
| egory |      |        |                                              |
+-------+------+--------+----------------------------------------------+
| 1     | 74   | 19.0   | • Vegetation: some damage to foliage.        |
|       |      |        |                                              |
|       | -95  |        | • Street signs: minimal damage.              |
|       |      |        |                                              |
|       |      |        | • Mobile homes: some damage to unanchored    |
|       |      |        | structures.                                  |
|       |      |        |                                              |
|       |      |        | • Other buildings: little or no damage.      |
+-------+------+--------+----------------------------------------------+
| 2     | 96   | 30.6   | • Vegetation: much damage to foliage; some   |
|       |      |        | trees blown down.                            |
|       | -110 |        |                                              |
|       |      |        | • Street signs: extensive damage to poorly   |
|       |      |        | constructed signs.                           |
|       |      |        |                                              |
|       |      |        | • Mobile homes: major damage to unanchored   |
|       |      |        | structures.                                  |
|       |      |        |                                              |
|       |      |        | • Other buildings: some damage to roof       |
|       |      |        | materials, doors, and windows.               |
+-------+------+--------+----------------------------------------------+
| 3     | 111  | 41.0   | • Vegetation: major damage to foliage; large |
|       |      |        | trees blown down.                            |
|       | -130 |        |                                              |
|       |      |        | • Street signs: almost all poorly            |
|       |      |        | constructed signs blown away.                |
|       |      |        |                                              |
|       |      |        | • Mobile homes: destroyed.                   |
|       |      |        |                                              |
|       |      |        | • Other buildings: some structural damage to |
|       |      |        | small buildings.                             |
+-------+------+--------+----------------------------------------------+
| 4     | 131  | 57.2   | • Street signs: all down.                    |
|       |      |        |                                              |
|       | -155 |        | • Other buildings: extensive damage to roof  |
|       |      |        | materials, doors, and windows; many          |
|       |      |        | residential roof failures.                   |
+-------+------+--------+----------------------------------------------+
| 5     | \    | 81.3   | • Other buildings: some complete building    |
|       | >155 |        | failures.                                    |
+-------+------+--------+----------------------------------------------+

Source: Adapted from National Hurricane Center \<
www.nhc.noaa.gov/aboutsshs.shtml\>

Hurricanes have a definite structure that is very important to
understanding their effects. The hurricane *eye* is an area of calm
10-20 miles in radius that is surrounded by bands of high wind and rain
that spiral inward to a ring around the eye, called the *eyewall*. The
entire hurricane, which can be as much as 600 miles in diameter, rotates
counterclockwise in the Northern Hemisphere. This produces a storm surge
that is located in the *right front quadrant* relative to the storm
track. Hurricanes have a forward movement speed that averages about 12
mph, but any given hurricane can be faster or slower than this. Indeed,
each hurricane's speed can vary over time and the storm can even stall
at a given point for an extended period of time. Atlantic hurricanes
tend to track toward the west and north, but can loop and change
direction. Storm intensity weakens as it reaches the North Atlantic
(because it derives less energy from the cooler water at high latitudes)
or makes landfall (which cuts the storm off from its source of energy
and adds the friction of interaction with the rough land surface).

Hurricanes produce four specific threats---high wind, tornadoes, inland
flooding (from intense rainfall), and storm surge. The strength of the
wind can be seen in the third column of Table 5-3, which shows that the
pressure of the wind on vegetation and structures is proportional to the
square of the wind speed. That is, as the wind speed *doubles* from 80
mph in a Category 1 hurricane to 160 mph in a Category 5 hurricane, the
velocity pressure *quadruples* from less than 20 pounds per square foot
(psf) to over 80 psf. Damage from high wind (and the debris that is
entrained in the wind field) is a function of a structure's exposure.
Wind exposure is highest in areas directly downwind from open water or
fields. Upwind hills, woodlands, and tall buildings decrease exposure to
the direct force of the wind but increase exposure to flying debris such
as tree branches and building materials that have been torn from their
sources.

Storm clouds in the outer bands of a hurricane can sometimes produce
tornadoes that are mostly small and short-lived. Hurricanes can also
produce torrential rain at rates up to four inches/hour for short
periods of time and one US hurricane produced 23 inches over 24 hours.
Such downpours cause severe local ponding (water that fell and did not
move) and inland flooding (water that fell elsewhere and flowed in).
Both inland flooding and storm surge are discussed below under
hydrological hazards.

Hurricane disasters resulted in relatively few casualties in the US
during the 20^th^ Century. The worst hurricane disaster occurred in
Galveston, Texas, in 1900 when over 6000 lives were lost in a community
of about 18,000. However, coastal counties have experienced explosive
population growth in recent decades, which creates the potential for
another catastrophic loss of life---Hurricane Katrina being a notable
example. Moreover, economic losses are increasing substantially over
time. Inflation makes only a small contribution to the increase; most of
the increase is due to increased population in vulnerable areas and
increased wealth (per person) in those areas (Pielke & Landsea, 1998).
There is extreme variation in losses by decade due to variability in the
number of storms. For example, the two decades from 1950-1969
experienced 33 hurricanes whereas the equivalent period from 1970-1988
experienced only six hurricanes.

Hurricanes are rapidly detected by satellite and continually monitored
by specially equipped aircraft. Storm forecast models have been
developed that have provided increasing accuracy in the prediction of
the storm track. Nonetheless, there are forecast uncertainties about the
eventual location of landfall, as well as the storm's size, intensity,
forward movement speed, and rainfall. One of the biggest problems is
that the long time required to evacuate some urbanized areas (30 hours
or more, see Lindell, Prater, Perry & Wu, 2002) requires warnings to be
issued at a time when storm behavior remains uncertain. The strike
probability data in Table 5-4 indicate many coastal jurisdictions must
be warned to evacuate even though the storm will eventually miss them.
Moreover, there is significant uncertainty about the wind speed and,
thus, the inland distance that must be evacuated.

Appropriate protective actions for hurricanes are well understood;
within the storm surge/high wind field risk areas, shelter in-place is
recommended only for elevated portions (i.e., above the wave crests) of
reinforced concrete buildings having foundations anchored well below the
scour line (the depth to which wave action erodes the soil on which the
building rests). Authorities generally recommend that evacuation be
completed before evacuation routes are flooded or high wind can overturn
motor homes and other high profile vehicles. Outside storm surge risk
areas, shelter in-place is suitable for most permanent buildings with
solid construction, but debris sources should be controlled and
permanent shutters should be installed on windows (or temporary shutters
stored for quick installation). Evacuation is advisable for residents of
mobile homes in high wind zones.

**Table 5-4.** Uncertainties about Hurricane Conditions as a Function of
Time before Landfall.

  ---------------- ----------------- ------------- ---------- ---------------
  Forecast period  Absolute landfall Maximum       Miss/Hit   Average wind
  (hours)          error (nautical   probability   Ratio      speed error
                   miles)                                     (mph)

  72               \>200             10%           9 to 1     23

  48               150               13-18%        7 to 1     18

  36               100               20-25%        4 to 1     15

  24               75                35-50%        2 to 1     12

  12               50                60-80%        2/3 to 1   9
  ---------------- ----------------- ------------- ---------- ---------------

Source: Adapted from Emergency Management Institute/National Hurricane
Center (no date)

##### *Wildfires*

All fires require the three elements of the fire triangle: *fuel*, which
is any substance that will burn; *oxygen* that will combine with the
fuel; and enough *heat* to ignite fuel (and sustain combustion if an
external source is absent). The resulting combustion yields heat
(sustaining the reaction) and combustion products such as toxic gases
and unburned particles of fuel that are visible as smoke. Wildfires are
distinguished mostly by their fuel. *Wildland* fires burn areas with
nothing but natural vegetation for fuel, whereas *interface* fires burn
into areas containing a mixture of natural vegetation and built
structures. *Firestorms* are distinguished from other wildfires because
they burn so intensely that they warrant a special category---in this
case, they create their own local weather and are virtually impossible
to extinguish. Wildfires can occur almost anywhere in the United States
but are most common in the arid West where there are extensive stands of
conifer trees and brush that serve as ready fuels. Once a fire starts,
the three principal variables determining its severity are fuel,
weather, and topography. Fuels differ in a number of characteristics
that collectively define fuel type. These include the fuel's ignition
temperature (low is more dangerous), amount of moisture (dry is more
dangerous), and the amount of energy (resinous wood is more dangerous).
A given geographical area can be defined by its fuel loading, which is
the quantity of vegetation in tons per acre, and fuel continuity, which
refers to the proximity of individual elements of fuel. Horizontal
proximity can be defined, for example, in terms of the distance between
trees. Vertical proximity can be defined in terms of the distance
between different levels of vegetation (e.g., grasses, brush, and tree
branches). Weather affects fire behavior by wind speed and direction as
well as temperature and humidity. Wind speed and direction have the most
obvious effects on fire behavior, with strong wind pushing the fire
front forward and carrying burning embers far in advance of the main
front. High temperature and low humidity promote fires by decreasing
fuel moisture, but these can vary during the day (cooling and
humidifying at sunset) as well as over longer periods of time.
Topography affects fire behavior by directing prevailing wind currents
and the hot air produced by the fire. Canyons can accelerate the wind by
funneling it through narrow openings. Steep slopes (greater than 10)
take advantage of a fuel's location in the fire's heated updraft, which
allows the advancing fire front to dry nearby fuels through radiant
heating and also provide a ready path for igniting these fuels. A fire's
forward movement speed doubles on a 10 slope and quadruples on a 20
slope.

Wildland fires are a major problem in the US because an average of about
73,000 such fires per year burn over three million acres. Approximately
13% of these wildfires are caused by lightning, but people cause 24% of
them accidentally and 26% of them deliberately. The greatest loss of
life from a US wildfire occurred in the 1871 Peshtigo, Wisconsin
wildfire that killed 2200 people (Gess & Lutz, 2002). More recently, the
1991 Oakland Hills California wildfire killed 25 people, injured 150,
and damaged or destroyed over 3,000 homes. Major contributors to the
severity of this wildland urban interface fire were the housing
construction materials (predominantly wood siding and wood shingle
roofs), vegetation planted immediately adjacent to the houses, and
narrow winding roads that impeded access by fire fighting equipment.

The US Forest Service maintains a Fire Danger Rating System that
monitors changing weather and fuel conditions (e.g., fuel moisture
content) throughout the summer fire season. Some of the fuel data are
derived from satellite observations and the weather data come from
hundreds of weather stations. Appropriate protective actions include
evacuation out of the risk area, evacuating to a safe location (e.g., an
open space such as a park or baseball field having well-watered grass
that will not burn), and sheltering in-place within a fire-resistant
structure (e.g., a concrete building with no nearby vegetation).

##### Hydrological Hazards

####### The principal hydrological hazards of concern to environmental hazard managers are floods, storm surges, and tsunamis. 

*Floods*

Flooding is a widespread problem in the United States that accounts for
three-quarters of all Presidential Disaster Declarations. A flood is an
event in which an abnormally large amount of water accumulates in areas
where it is usually not found. Flooding is determined by a hydrological
cycle in which precipitation falls from clouds in the form of rain and
snow (see Figure 5-1). When it reaches the ground, the precipitation
either infiltrates the soil or travels downhill in the form of surface
runoff. Some of the water that infiltrates the soil is taken up by plant
roots and transported to the leaves where it is transpired into the
atmosphere. Another portion of the ground water gradually moves down to
the water table and flows underground until reaching water bodies such
as wetlands, rivers, lakes, or oceans. Surface runoff moves directly to
surface storage in these water bodies. At that point, water evaporates
from surface storage, returning to clouds in the atmosphere.

# [**Figure 5-1.** The Hydrological Cycle.]{.underline} 

There are seven different types of flooding that are widely recognized.
*Riverine (main stem) flooding* occurs when surface runoff gradually
rises to flood stage and overflows its banks. *Flash flooding* is
defined by runoff reaching its peak in less than six hours. This usually
occurs in hilly areas with steep slopes and sparse vegetation, but also
occurs in urbanized areas with rapid runoff from impermeable surfaces
such as streets, parking lots, and building roofs. *Alluvial fan
flooding* occurs in deposits of soil and rock found at the foot of steep
valley walls in arid Western regions. *Ice/debris dam failures* result
when an accumulation of downstream material raises the water surface
above the stream bank. *Surface ponding/local drainage* occurs when
water accumulates in areas so flat that runoff cannot carry away the
precipitation fast enough. *Fluctuating lake levels* can occur over
short-term, seasonal, or multiyear periods, especially in lakes that
have limited outlets or are entirely landlocked. *Control structure (dam
or levee) failure* has many characteristics in common with flash
flooding.

Floods are measured either by discharge or stage. Discharge, which is
defined as the volume of water per unit of time, is the unit used by
hydrologists. Stage, which is the height of water above a defined level,
is the unit needed by emergency managers because flood stage determines
the level of casualties and damage. Discharge is converted to stage by
means of a rating curve (see Figure 5-2). The horizontal axis shows
discharge in cubic feet per second and the vertical axis shows stage in
feet above flood stage. Note that high rates of discharge produce much
higher stages in a valley than on a plain because the valley walls
confine the water.

# [**Figure 5-2.** Stage Rating Curve.]{.underline} 

> ![](media/image1.wmf)

Flooding is affected by a number of factors. The first of these,
precipitation, must be considered at a given point and also across the
entire watershed (basin). The total precipitation at a point is equal to
the duration of precipitation times its intensity (frequently measured
in inches per hour). Total precipitation over a basin is equal to
precipitation summed over all points in the surface area of the basin.
The precipitation's contribution to flooding is a function of
temperature because rain (a liquid) is immediately available whereas
snow (a solid) must first be melted by warm air or rain. Moreover, as
indicated by Figure 5-3, the precipitation from a single storm might be
deposited over two or more basins and the amount of rainfall in one
basin might be quite different from that in the other basin.
Consequently, there might be severe flooding in a town on one river
(City A) and none at all in a town on another river (City B) even if the
two towns received the same amount of rainfall from a storm.

As the hydrological cycle makes clear, flooding is also affected by
surface runoff, which is determined by terrain and soil cover. One
important aspect of terrain is its slope, with runoff increasing as
slope increases. In addition to slope steepness, slope length and
orientation to prevailing wind (and, thus, the accumulation of rainfall
and snowfall) and sun (and, thus, the accumulation of snow) are also
important determinants of flooding.

# [**Figure 5-3.** Map of the Distribution of Precipitation from a Storm.]{.underline} 

Slope geometry is also an important consideration. Divergent slopes
(e.g., hills and ridges) provide rapid runoff dispersion. By contrast,
convergent slopes (e.g., valleys) provide runoff storage in puddles,
potholes, and ponds. Mixed slopes have combinations of these, so slope
mean (the average slope angle) and variance (the variability of slope
angles) determine the amount of storage. A slope with a zero mean and
high variance (a plain with many potholes) will provide a larger amount
of storage than a slope with a zero mean and low variance (a featureless
plain). Similarly, a slope with a positive mean and high variance (a
slope with many potholes) will provide a larger amount of storage than a
slope with a positive mean and low variance.

Soil cover also affects flooding because dense low plant growth slows
runoff and promotes infiltration. In areas with limited vegetation,
surface permeability is a major determinant of flooding. Surface
permeability increases with the proportion of organic matter content
because this material absorbs water like a sponge. Permeability also is
affected by surface texture (particle size and shape). Clay, stone, and
concrete are very impermeable because particles are small and smooth,
whereas gravel and sand are very permeable---especially when the
particles are large and have irregular shapes that prevent them from
compacting. Finally, surface permeability is affected by soil saturation
because even permeable surfaces resist infiltration when soil pores (the
spaces between soil particles that ordinarily are filled with air)
become filled with water. Groundwater flows via local transport to
streams at the foot of hill slopes and via remote transport through
aquifers. Rapid in- and outflow through valley fill increases peak flows
whereas very slow in- and outflow through upland areas maintains flows
between rains.

Evapotranspiration takes place via two mechanisms. First, there is
direct evaporation to atmosphere from surface storage in rivers and
lakes. Second, there is uptake from soil and subsequent transpiration by
plants. Transpiration draws moisture from the soil into plants' roots,
up through the stem, and out through the leaves' pores (similar to
people sweating). The latter mechanism is generally much higher in
summer than in winter due to increased heat and plant growth, but
transpiration is negligible during periods of high precipitation.

Stream channel flow is affected by channel wetting which infiltrates the
stream banks (horizontally) until they are saturated as the water rises.
In addition, there is seepage because porous channel bottoms allow water
to infiltrate (vertically) into groundwater. Channel geometry also
influences flow because a greater channel cross-section distributes the
water over a greater area, as does the length of a *reach* (distinct
section of river) because longer reaches provide greater water storage.
High levels of discharge to downstream reaches can also affect flooding
on upstream reaches because flooded downstream reaches slow flood
transit by decreasing the river's elevation drop.

Flooding increases when upstream areas experience deforestation and
overgrazing, which increase surface runoff to a moderate degree on
shallow slopes and to a major degree on steep slopes as the soil erodes.
The sediment is washed downstream where it can silt the channel and
raise the elevation of the river bottom. These problems of agricultural
development are aggravated by flood plain urbanization. Like other
cities throughout the world, US cities have been located in flood plains
because water was the most efficient means of transportation until the
mid-1800s. Consequently, many cities were located at the head of
navigation or at transshipment points between rivers. In addition,
cities have been located in flood plains because level alluvial soil is
very easy to excavate for building foundations. Finally, urban
development takes place in flood plains because of the aesthetic
attraction of water. People enjoy seeing lakes and rivers, and pay a
premium for real estate that is located there.

One consequence of urban development for flooding is that cities involve
the replacement of vegetation with *hardscape*---impermeable surfaces
such as building roofs, streets, and parking lots. This hardscape
decreases soil infiltration, thus increasing the speed at which flood
crests rise and fall. Another factor increasing flooding is intrusion
into the flood plain by developers who fill intermittently flooded areas
with soil to raise the elevation of the land. This decreases the channel
cross-section, forcing the river to rise in other areas to compensate
for the lost space.

Flood risk areas in the US are generally defined by the 100-year
flood---an event that is expected to have a 100-year recurrence interval
and, thus, a 1% chance of occurrence in any given year. It is important
to understand that these extreme events are essentially independent, so
it is possible for a community to experience two 100-year floods in the
same century. Indeed, it is *possible* to have them in the same year
even though that would be a very *improbable* event. This statistical
principle is misunderstood by many people who believe there can be only
one 100-year flood per century. The belief that a 100-year flood
occurring this year cannot be repeated for another 100 years (or at
least nearly 100 years) is a very dangerous fallacy. Moreover, a
100-year flood is an arbitrary standard of safety that reflects a
compromise between the goals of providing long-term safety and
developing economically valuable land. A 50-, 200-, or even a 500-year
standard could be used instead. Community adoption of a 50-year flood
standard would provide more area for residential, commercial, and
industrial development. However, the resulting encroachment into the
flood plain would lead to more frequent damaging floods than would a
100-year flood standard. Alternatively, a community might use different
standards for different types of structures. For example, it might
restrict the 100-year floodplain to low intensity uses (e.g., parks),
allow residential housing to be constructed within the 500-year
floodplain, and restrict nursing homes, hospitals, and schools to areas
outside the 500-year floodplain.

Emergency response to floods is supported by prompt detection, which is
local or regional in scope. This includes automated devices such as
radar for assessing rainfall amounts at variable points in a watershed,
rain gages for detecting rainfall amounts at predetermined points in a
watershed, and stream gages for detecting water depth at predetermined
points along a river. Detection also can be achieved by manual devices
such as spotters for assessing rainfall amounts, water depth, or levee
integrity at specific locations (planned before a flood or improvised
during one). Once data on the quantity and distribution of precipitation
have been collected, they are used to estimate discharge volumes over
time from the runoff characteristics of a given watershed (e.g., soil
permeability and surface steepness) at a given time (e.g., current soil
saturation). Once discharge volume is estimated, it can be used together
with downstream topography (e.g., mountain valley vs. plain) to predict
downstream flood heights.

Timely and specific warnings of floods are provided by commercial news
media as well as NOAA Weather Radio. The most appropriate protective
action for persons is to evacuate in a direction perpendicular to the
river channel. Because flash floods in mountain canyons can travel
faster than a motor vehicle, it is safest to climb the canyon wall
rather than try to drive out. It also is important to avoid crossing
running water. Just two feet of fast moving water can float a car and
push it downstream with 1000 pounds of force.

##### *Storm Surge*

Storm surge is an elevated water level that exceeds the height of normal
astronomical tides. It is most commonly associated with hurricanes, but
also can be caused by extratropical cyclones (nor'easters). The height
of a storm surge increases as atmospheric pressure decreases and a
storm's maximum wind speed increases. Storm surge is especially
significant where coastal topography and bathymetry (submarine
topography) have shallow slopes and the coast has a narrowing shoreline
that funnels the rising water. These factors are magnified when the
storm remains stationary through several tide cycles and the affected
coast is defined by low-lying barrier islands whose beaches and dunes
have been eroded either by human development or by recent storms. Storm
surge---together with astronomical high tide, rainfall, river flow, and
storm surf---floods and batters structures and scours areas beneath
foundations as much as 4-6 feet below the normal grade level. At one
time, storm surge was the primary source of casualties in all countries,
but inland flooding is now the primary cause of hurricanes deaths in the
US. However, surge is still is the primary source of casualties in
developing countries such as Bangladesh. In these countries, population
pressure pushes the poor to farm highly vulnerable areas and poverty
limits the development of dikes and seawalls, warning systems,
evacuation transportation systems, and vertical shelters (wind resistant
structures that are elevated above flood level).

##### *Tsunamis*

Tsunamis are commonly referred to as "tidal" waves but they are, in
fact, sea waves that are usually generated by earthquakes. In addition,
tsunamis can be caused by volcanic eruptions or landslides that usually,
but not always, occur undersea. Tsunamis are rare events because 15,000
earthquakes over the course of a century have generated only 124
tsunamis, a rate of less than 1% of all earthquakes and only 0.7
tsunamis per year. This low rate of tsunami generation is attributable
to earthquake intensity; two-thirds of all Pacific tsunamis are
generated by shallow earthquakes exceeding 7.5 in magnitude.

Tsunamis can travel across thousands of miles of open ocean (e.g., from
the Aleutians to Hawaii or from Chile to Japan) at speeds up to 400 mph
in the open ocean, but they slow to 25 mph as they begin to break in
shallow water and run up onto the land. Tsunamis are largely invisible
in the open ocean because they are only 1-2 feet high. However, they
have wave lengths up to 60 miles and periods as great as one hour. This
contrasts significantly with ordinary ocean waves having wave heights up
to 30 feet, wave lengths of about 500 feet, and periods of about 10
seconds. Tsunamis can have devastating effects in some of the places
where they make landfall because the waves encounter bottom friction
when the water depth is less than 1/20 of their wavelength. At this
point, the bottom of the wave front slows and is overtaken by the rest
of the wave, which must rise over it. For example, when a wave reaches a
depth of 330 feet, its speed is reduced from 400 mph to 60 mph. Later,
reaching a depth of 154 feet reduces its speed to 44 mph. This causes
the next 650 feet of the wave to overtake the wave front in a single
second. As the wave continues shoreward, each succeeding segment of the
wave must rise above the previous segment because it can't go down
(water is not compressible) or back (the rest of the wave is pressing it
forward). Because the wavelength is so long and wave speed is so fast, a
large volume of water can pile up to a very great height---especially
where the continental shelf is very narrow. It is important to note that
the initial cue to tsunami arrival might be that the water level drops,
rather than rises. Indeed, this was the case in the 2004 Indian Ocean
tsunami. People's failure to understand the significance of the receding
water contributed to a death toll exceeding 200,000. An initial *wave*
is created only if the seafloor rises suddenly, whereas an initial
*trough* is created if the seafloor drops. In either case, the initial
phase will be followed by the alternate phase (i.e., a wave is followed
by a trough or vice-versa).

Tsunamis threaten shorelines worldwide and have no known temporal (i.e.,
diurnal or seasonal) variation. If a tsunami is initiated locally (i.e.,
within a hundred miles), the potential for a tsunami can be detected by
severe earthquake shaking. However, coastal residents' only physical cue
to a remotely initiated tsunami is wave arrival at coast, although the
arrival of a trough (making it appear that the tide went out
unexpectedly) should be recognized as a danger sign. International
tsunami warning systems base their detection of remote tsunamis on
seismic monitoring to detect major earthquakes and tidal gauges located
throughout the Pacific basin to verify tsunami generation. Once tsunami
generation has been confirmed, alerts can be transmitted throughout the
Pacific basin. The need for prompt action can be inferred from tsunami's
forward movement speed; a tsunami generated 100 miles away from a coast
can arrive in about 15 minutes.

The physical magnitude of a tsunami is extremely impressive. Wave crests
can arrive at 10-45 minute intervals for up to six hours and the highest
wave, as much as 100 ft at the shoreline, can be anywhere in the wave
train. The area flooded by a tsunami is known as the inundation zone,
which equivalent to a 100-year floodplain or hurricane storm surge risk
area. Because of the complexities in accounting for wave behavior and
the characteristics of the offshore bathymetry and onshore topography,
tsunami inundation zones must be calculated by competent analysts using
sophisticated computer programs. The physical impacts include casualties
caused by deaths from drowning and traumatic injuries from wave impact.
Property damage is caused by the same mechanisms.

Regarding protective measures, sheltering in-place in elevated
structures can protect against surge. However, steel reinforced concrete
structures on deep pilings are required to withstand wave battering and
foundation scour. Consequently, evacuation to higher ground is the most
effective method of population protection. Evacuation to a safe distance
out of the runup zone is obviously difficult on low-lying coasts, but it
also can be difficult where there are nearby hills if the primary
evacuation route runs parallel to the coastline.

##### Geophysical Hazards

To properly understand geophysical hazards, it is important to recognize
the earth's three distinct geological components. The *core* consists of
molten rock at the center of the earth, the *crust* is solid rock and
other materials at the earth's surface that vary in depth from four
miles under the oceans to 40 miles in the Himalayas, and the *mantle* is
an 1800 mile thick layer between the core and the crust. According to
tectonic theory, the earth's crust is defined by large plates that float
on the mantle and move gradually in different directions over time.

Tectonic plates can diverge, converge, or move laterally past each
other. When they diverge, new material is generated from below the
earth's mantle, usually at mid-ocean ridges, that flows very slowly (at
a rate of a few inches per year) away from the source. This process
produces a gradual expansion of the plate toward an adjoining plate.
Thus, one plate converges with another plate and the heavier material (a
seafloor) is *subducted* under lighter material (a continent). In the
US, this process is taking place in the Cascadian Zone along the Pacific
coast of Washington, Oregon, and Northern California. Tectonic activity
produces intermittent movement, which causes earthquakes and sometimes
tsunamis. In addition, the subducted material travels to great depth
within the earth where it is liquefied under intense heat and pressure.
The resulting magma causes volcanic activity.

Crustal plates also move laterally past each other as, for example, the
North American plate is moving northward past the Pacific Plate along
the San Andreas fault. Friction can lock the fault and increases strain
until it is released suddenly in an earthquake; the longer the fault is
locked, the more energy is stored until it is released. Finally, there
is some intraplate activity such as the mid-ocean "hot spots" that have
formed the Hawaiian Islands and mid-continental fault zones. One US
example is the New Madrid Seismic Zone affecting Missouri, Illinois,
Indiana, Kentucky, Tennessee, Mississippi, and Arkansas.

####### These tectonic processes give rise to the most important geophysical hazards in the US---volcanic eruptions and earthquakes. However, landslides are another geophysical hazard that will also be addressed in this section.

# Volcanic Eruptions

Volcanoes are formed when a column of magma (molten rock) rises from the
earth's mantle into a magma chamber and later erupts at the surface,
where it is called lava. Successive eruptions, deposited in layers of
lava or ash, build a mountain. Major eruptions create craters that are
gradually replaced in dome-building eruptions. Cataclysmic eruptions
create calderas that leave only a depression where the mountain once
stood. US volcanoes (recently erupted) are located principally in Alaska
(92) and Hawaii (21), as well as along the west coast of the 48
contiguous states (73): Oregon has 22, California has 20, and Washington
has 8. Vulcanologists distinguish among 20 different types of volcanoes
that vary in the type of ejected material, size, shape, and other
characteristics, but the two most important types of volcanoes are
shield volcanoes and stratovolcanoes. Shield volcanoes produce
relatively gentle *effusive* eruptions of low-viscosity lava, resulting
in shallow slopes and broad bases (e.g., Kilauea, Hawaii).
Stratovolcanoes produce *explosive* eruptions of highly acidic lava,
gas, and ash, resulting in steep slopes and narrow bases. One well known
stratovolcano is Mt. St. Helens, Washington, which erupted spectacularly
in 1980 (see Perry & Greene, 1983; Perry & Lindell, 1990).

The principal threats from volcanoes include gases and tephra that are
blasted into the air, pyroclastic flows that blast laterally from
volcano flanks, and the heavier lava and lahars that generally travel
downslope. Many gases are dangerous because they are heavier than air,
so they accumulate in low-lying areas. Other than harmless water vapor
(H~2~O), some gases are simple asphyxiants that are dangerous because
they displace atmospheric oxygen (carbon dioxide, CO~2~; methane,
CH~4~). There are also chemical asphyxiants (carbon monoxide, CO) that
are dangerous because they prevent the oxygen that is breathed in from
reaching the body's tissues. In addition, there are corrosives (sulfur
dioxide, SO~2~; hydrogen sulfide, H~2~S; hydrogen chloride, HCl;
hydrogen fluoride, HF; and sulfuric acid, H~2~SO~4~) and radioactive
gases (such as radon, Ra). Tephra consists of solid particles of rock
ranging in size from talcum powder ("ash") to boulders ("bombs").
Pyroclastic flows are hot gas and ash mixtures (up to 1600F) discharged
from the crater vent. Lahars are mudflows and floods, usually from
glacier snowmelt, with varying concentrations of ash. The impacts of
volcanic eruption tend to be strongly directional because ashfall and
gases disperse downwind; pyroclastic flows follow blast direction and
lava and lahars travel downslope through drainage basins. The forward
movement speed of the hazard varies. Gas and tephra movements are
determined by wind speed, usually less than 25 mph. Pyroclastic flows
can move at over 100 mph. Lava typically moves at walking speed (5 mph)
but can travel faster (35 mph) on steep slopes. Lahars move at the speed
of water flow, usually less than 25 mph, but can exceed 50 mph in some
instances.

The physical magnitude of the hazard also differs for each specific
threat. Inundation depths for ashfall and lahars can range up to tens of
meters in depth. Lava flows and pyroclastic flows are so hot that any
impact is considered to be unsurvivable. Similarly, the impact area also
varies by threat. Tephra deposition depends on eruption magnitude, wind
speed, and particle size, with traces of ash circling the globe. Lava
flows, lahars, and pyroclastic flows follow localized drainage patterns,
so safe locations can be found only a short distance from areas that are
totally devastated. These considerations indicate volcano risk areas can
be defined as listed in Table 5-5.

**Table 5-5.** Volcano Risk Areas.

+--------+--------+------------+-------------------------------------+
| Ca     | Name   | ##         | ## Threats                          |
| tegory |        | Distance\* |                                     |
+--------+--------+------------+-------------------------------------+
| 1      | E      | 0-100 m    | High risk of heat, ash, lava,       |
|        | xtreme |            | gases, rock falls, and projectiles  |
+--------+--------+------------+-------------------------------------+
| 2      | High   | 100-300 m  | # High risk of projectiles          |
+--------+--------+------------+-------------------------------------+
| 3      | Medium | 300-3000 m | Medium risk of projectiles          |
+--------+--------+------------+-------------------------------------+
| 4      | Low    | 3 km -- 10 | Low risk of projectiles             |
|        |        | km         |                                     |
+--------+--------+------------+-------------------------------------+
| 5      | Safe   | \> 10 km   | Minimal risk of projectiles         |
+--------+--------+------------+-------------------------------------+

\* (In meters and kilometers; these distances do not include mudflows
and floods that can travel up to 100 km or tsunamis that can travel
thousands of km.

Source: Adapted from \< www.volcanodiscovery.com\>

The physical impacts of a volcanic eruption vary with the type of
threat. Gases can cause deaths and injuries from inhalation, but
pyroclastic flows are more dangerous because they can cause deaths and
injuries from blast, thermal exposure, and inhalation of gas and ash. In
addition, they also can cause property damage from blast, heat, and
coverage by ash (even after it has cooled). Tephra causes property
damage from excess roof loading, shorting of electric circuits, clogged
air filters in vehicles, and abrasion of machinery. Deaths and injuries
can be caused by bomb impact trauma, and health effects can result from
ash inhalation (including fluoride poisoning of grazing animals). Lava
causes property damage from excess heat and coverage by rock (when
cooled). Deaths and injuries from thermal exposure to lava can occur,
but are rare because it moves so slowly. Lahars can cause property
damage from flooding and coverage by ash (when water drains off) and
deaths from drowning. Tsunamis cause property damage from wave impact
and water saturation, as well as deaths from drowning and traumatic
injuries. In addition, volcanic eruptions can cause tsunamis and
wildfires as secondary hazards.

The threat of volcanic eruption can be detected by physical cues
indicating rising magma. These include earthquake swarms, outgassing,
ash and steam eruptions, and topological deformation (changes in slope,
flank swelling). Appropriate protective measures include sweeping ash
from building roofs and evacuating an area at least six miles in radius
for a crater eruption and 12-18 miles in the direction of a
flank/lateral eruption. People also should be evacuated from floodplains
threatened by lahars. The principal problem in implementing evacuations
is that there are substantial uncertainties in the timing (onset and
duration) of eruptions, so people have sometimes been forced to stay
away from their homes and businesses for months at a time. In some
cases, the expected eruption never did materialize, causing severe
conflict among physical scientists, local civil officials, and disrupted
residents.

####### *Earthquakes*

When an earthquake occurs, energy is released at the *hypocenter,* which
is a point deep within the earth. However, the location of an earthquake
is usually identified by a point on the earth's surface directly above
the hypocenter known as the *epicenter*. Earthquake energy is carried by
three different types of waves, P-waves, S-waves, and surface waves.
P-waves, typically called primary waves but are more properly known as
pressure waves, travel rapidly. By contrast, S-waves, typically called
secondary waves but technically known as shear waves, travel more slowly
but cause more damage. The third type, surface waves, includes Love
waves and Rayleigh waves. These have very low frequency and are
especially damaging to tall buildings.

The physical magnitude of an earthquake is different from its intensity.
*Magnitude* is measured on a logarithmic scale where a one-unit increase
represents a 10-fold increase in seismic wave amplitude and a 30-fold
increase in energy release from the source. Thus, a M8.0 earthquake
releases 900 (30 x 30) times as much energy as a M6.0 earthquake. By
contrast, *intensity* measures the impact at a given location and can be
assessed either by behavioral effects or physical measurements. The
behavioral effects of earthquakes are classified by the Modified
Mercalli Intensity Scale, which defines each category (see Table 5-6,
column 1) in terms of its behavioral effects of earthquake motion on
people, buildings, and objects in the physical environment (column 3).
Physical measurements can be assessed in terms of average peak
acceleration (column 4), which describe seismic forces in horizontal and
vertical directions. This acceleration is measured either as the number
of millimeters per second squared (mm/sec^2^) or as a multiple of the
force of gravity (*g* = 9.8 meters/sec^2^)

The impact of an earthquake at a given point is determined by a number
of factors. First, intensity decreases with distance from the epicenter,
with slow attenuation along the fault line and more rapid attenuation
perpendicular to the fault line. In addition, soft soil transmits energy
waves much more readily than bedrock, and basins (loose fill surrounded
by rock) focus energy waves. Thus, isoseismal contours (lines of
constant seismic energy) can be extremely irregular, depending on fault
direction and soil characteristics. The complex interplay of these
factors can be seen in Figure 5-4, which displays the isoseismal
contours (lines of equal seismic intensity) for the 1994 Northridge
earthquake.

Within the impact area, the primary earthquake threats (mostly
associated with plate boundaries) are ground shaking, surface faulting,
and ground failure. Ground shaking creates lateral and upward motion in
structures designed only for (downward) gravity loads. In addition,
unreinforced structures respond poorly to tensile (upward stretching)
and shear (lateral) forces, as do "soft-story" (e.g., buildings with
pillars rather than walls on the ground floor) and asymmetric (e.g.,
L-shaped) structures. Moreover, high-rise buildings can demonstrate
resonance, which is a tendency to sway in synchrony with the seismic
waves, thus amplifying their effects.

Surface faulting---cracks in the earth's surface---is a widespread fear
about earthquakes that actually is far less of a problem than popularly
imagined. The vulnerability of buildings to surface faulting is easily
avoided by zoning regulations that prevent building construction within
50 feet of a fault line. Unfortunately, zoning restrictions are
infeasible for utility networks (water, wastewater, and fuel pipelines,
electric power and communications lines, roads and railroads) that must
cross the fault lines.

**Table 5-6.** Modified Mercalli Intensity (MMI) Scale for Earthquakes.

+-------+---------+------------------------------------------+---------+
| Cat   | ## In   | # Type of Damage                         | Max.    |
| egory | tensity |                                          | accel   |
|       |         |                                          | eration |
|       |         |                                          |         |
|       |         |                                          | (mm/s   |
|       |         |                                          | ec^-2^) |
+-------+---------+------------------------------------------+---------+
| I     | Instr   | Detected only on seismographs            | \< 10   |
|       | umental |                                          |         |
+-------+---------+------------------------------------------+---------+
| II    | Feeble  | Some people feel it                      | \< 25   |
+-------+---------+------------------------------------------+---------+
| III   | Slight  | Felt by people resting; like a large     | \< 50   |
|       |         | truck rumbling by                        |         |
+-------+---------+------------------------------------------+---------+
| ## IV | ## M    | Felt by people walking; loose objects    | \< 100  |
|       | oderate | rattle on shelves                        |         |
+-------+---------+------------------------------------------+---------+
| V     | S       | #### Sleepers awake; church bells ring   | \< 250  |
|       | lightly |                                          |         |
|       | strong  |                                          |         |
+-------+---------+------------------------------------------+---------+
| VI    | Strong  | Trees sway; suspended objects swing;     | \< 500  |
|       |         | objects fall off shelves                 |         |
+-------+---------+------------------------------------------+---------+
| VII   | Very    | Mild alarm; walls crack; plaster falls   | \< 1000 |
|       | strong  |                                          |         |
+-------+---------+------------------------------------------+---------+
| VIII  | Dest    | Moving cars uncontrollable; chimneys     | \< 2500 |
|       | ructive | fall and masonry fractures; poorly       |         |
|       |         | constructed buildings damaged            |         |
+-------+---------+------------------------------------------+---------+
| IX    | Ruinous | Some houses collapse; ground cracks;     | \< 5000 |
|       |         | pipes break open                         |         |
+-------+---------+------------------------------------------+---------+
| X     | Dis     | Ground cracks profusely; many buildings  | \< 7500 |
|       | astrous | destroyed; liquefaction and landslides   |         |
|       |         | widespread                               |         |
+-------+---------+------------------------------------------+---------+
| XI    | Very    | Most buildings and bridges collapse;     | \< 9800 |
|       | Dis     | roads, railways, pipes and cables        |         |
|       | astrous | destroyed; general triggering of other   |         |
|       |         | hazards                                  |         |
+-------+---------+------------------------------------------+---------+
| XII   | Catas   | Total destruction; trees driven from     | \> 9800 |
|       | trophic | ground; ground rises and falls in waves  |         |
+-------+---------+------------------------------------------+---------+

Source: Adapted from Bryant (1991).

Ground failure is defined by a loss of soil bearing strength and takes
three different forms. Landsliding occurs when a marginally stable soil
assumes a more natural angle of repose (a more detailed discussion is
presented in the next section). Fissuring or differential settlement
occurs when loose fill, which is very prone to compaction and
consolidation, is located next to other soils that are less prone to
this behavior. Finally, soil liquefaction is caused by loss of
grain-to-grain support in saturated soils (e.g., where there is a high
water table). Ground failure is a threat because building foundations
need stable soil to support the rest of the structure. Even partial
failure of the soil under the foundation can destroy a building by
causing it to tilt at a dangerous angle.

Earthquakes also can cause major secondary threats such as tsunamis, dam
failures, hazardous materials releases, and building fires. Tsunamis
were addressed earlier, but dam failures can occur if ground shaking
causes earth or rock dams to rupture or the valley walls abutting the
dam to fail. Hazardous materials releases can occur if ground shaking
causes containment tanks or pipes to break. Fires are caused by broken
fuel and electric power lines that provide the necessary fuel and
ignition sources. In addition, fire spread is promoted when broken water
lines prevent fire departments from extinguishing the initial blazes.

As yet, there is no definitive evidence of physical cues that provide
reliable forewarning of an imminent earthquake. Unusual animal behavior
has been observed, but this has not proved to be a reliable indicator of
an imminent earthquake. The Chinese successfully predicted an earthquake
at Haicheng in 1975 and saved thousands of lives by evacuating the city.
However, there was no forewarning of the 1976 earthquake at Tangchan.
Currently, earth scientists are examining many potential predictors such
as increased radon gas in wells, increased electrical conductivity and
magnetic anomalies in soil, and topographic perturbations such as
changes in ground elevation, slope, and location ("creep"). There were
great expectations for short-term earthquake predictions 30 years ago,
but seismologists currently give only probabilities of occurrence within
long periods (5, 10, or 20 years).

[**Figure 5-4.** Isoseismal Contours for the Northridge
Earthquake.]{.underline}

![](media/image3.jpeg){width="3.75in" height="3.3020833333333335in"}

Source: Adapted from Dewey, et al. (1994).

Very short term forewarning of earthquakes can sometimes be initiated by
detection of the relatively harmless P-waves that arrive from distant
earthquakes a few seconds before the arrival of the damaging S-waves and
surface waves. Currently, however, there is no method of advance
detection and warning for local earthquakes because these are so close
to the impact area that P-waves and S-waves arrive almost
simultaneously. Protective measures can be best understood by the common
observation that "earthquakes don't kill people, falling buildings
(especially unreinforced masonry buildings) kill people". Thus, building
occupants are advised to shelter in-place under sturdy furniture while
the ground is shaking. Those who survive the collapse of their buildings
typically attempt to rescue those survivors who remain trapped, but the
success of this improvised response depends upon the type of building.
Unreinforced masonry buildings are much more likely to collapse, but
search and rescue from these structures can be relatively easy. By
contrast, steel-reinforced concrete buildings are much less likely to
collapse, but search and rescue is extremely difficult unless
sophisticated equipment is available to well-trained urban search and
rescue teams. Unfortunately, almost all victims will die by the time
remote urban search and rescue (USAR) teams arrive because crush
injuries usually kill within 24 hours. However, USAR teams take longer
than this to mobilize and travel to the incident site. Another problem
with earthquakes is that destruction of infrastructure (electric power,
fuel, water, wastewater, and telecommunications) impairs emergency
response. Consequently, households, businesses, and local governments
must be self-sufficient for at least 72 hours until outside assistance
can arrive.

####### *Landslides*

The term *landslide* is often used to refer generically to a number of
different physical phenomena involving the downward displacement of rock
or soil that moves because of gravitational forces. *Slides* occur
because a failure surface is created by two distinct soil strata and the
upper stratum is displaced downslope. Some slides are triggered by
earthquakes or volcanic eruptions. However, many are caused by heavy
rainfall that saturates soil, increasing the weight of the upper surface
and lubricating the failure surface. *Debris flows* have such a high
water content distributed throughout the soil mass that they act like a
viscous (thick) fluid. *Lateral spreads* involve the outward movement of
material on the sides and downward movement of material on the top of a
soil mass. *Topples* and *falls* involve rock masses that detach from
steep slopes and either tilt or fall free to a lower surface.

Slopes remain stable when shear stress is less than shear strength.
Shear stress increases with the steepness of the slope and the weight
placed on that slope. Shear strength depends upon the internal cohesion
(interlocking or sticking) of soil particles and the internal friction
of particles within a soil mass, which is reduced by soil saturation.
Thus, landslides are most common in areas having steep slopes composed
of susceptible soils types (i.e., ones with low internal cohesion) that
are stratified (creating failure surfaces) and saturated with water.
Slide probability is commonly increased by four different conditions.
The first occurs when slopes have been cleared of vegetation, whereas
the second occurs when excavations for houses and roads use the "cut and
fill" method on unstable steep slopes. (This technique is used to create
a level surface on a slope by cutting soil out of a section of hillside
and using it to fill the area below this cut.) The third condition
creating landslides occurs when the construction of many buildings and
roads significantly increases the weight placed on the slope and the
fourth condition occurs when construction of access roads removes
support from the foot of the slope.

Landslide risk areas can be mapped by conducting geological surveys to
identify areas having slopes with distinct soil strata that are likely
to separate when saturated or shaken. Visible cues of imminent slides
can also be seen at the head and toe of a potential slide area, which
can be monitored to determine whether to take protective actions. These
include installing slope drainage systems or retaining walls, and
temporarily evacuating or permanently relocating the population at risk.

**Technological Hazards**

Hazardous materials (also known as *hazmat*) are regulated by a number
of federal agencies including the US Department of Transportation, US
Environmental Protection Agency, US Nuclear Regulatory Commission, and
the Occupational Safety and Health Administration of the US Department
of Labor. In addition, the US Coast Guard and Federal Emergency
Management Agency of the US Department of Homeland Security have
responsibilities for emergency response to hazmat incidents. Because
these agencies have different responsibilities, they have
correspondingly different definitions of hazmat. According to the
Department of Transportation, hazmat is defined as substances that are
"capable of posing unreasonable risk to health, safety, and property"
(49CFR 171.8).

Until the late 1980s, the location, identity, and quantity of hazmat
throughout the United States was generally undocumented. However, Title
III of the Superfund Amendments and Reauthorization Act---SARA Title III
(also known as the Emergency Planning and Community Right to Know
Act---EPCRA) of 1986 required those who produce, handle, or store
amounts exceeding statutory threshold planning quantities of
approximately 400 Extremely Hazardous Substances (EHSs) to notify local
agencies, their State Emergency Response Commission (SERC), and the US
EPA. Nonetheless, the Chemical Abstract Service (CAS) lists 1.5 million
chemical formulations with 63,000 of them hazardous. There are over
600,000 shipments of hazmat per day (100,000 of which are shipments of
petroleum products). Fortunately, only a small proportion of these
chemicals account for most of the number of shipments and the volume of
materials shipped (see Table 5-7, adapted from Lindell & Perry, 1997a).
These hazmat shipments result in an average of 280 liquid spills or
gaseous releases per year, the vast majority of which occur in
transport. Of these spills and releases, 81% take place on the highway
and 15% are in rail transportation. These incidents cause approximately
11 deaths and 311 injuries per year.

**Table 5-7.** Volume of production for top 12 EHSs, 1970--1994.

+-----+------------+---------+---------+---------+---------+--------+
| R   | Chemical   | Year    |         |         |         | \%     |
| ank |            |         |         |         |         | in     |
| in  | name       |         |         |         |         | crease |
|     |            |         |         |         |         |        |
| top |            |         |         |         |         | 197    |
| 50  |            |         |         |         |         | 0-1994 |
+-----+------------+---------+---------+---------+---------+--------+
|     |            | 1970    | 1980    | 1990    | 1994    |        |
+-----+------------+---------+---------+---------+---------+--------+
| 1   | Sulfuric   | 29,525  | 44,157  | 44,337  | 44,599  | 51     |
|     | acid       |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 8   | Ammonia    | 13,824  | 19,653  | 17,003  | 17,965  | 30     |
+-----+------------+---------+---------+---------+---------+--------+
| 10  | Chlorine   | 9,764   | 11,421  | 11,809  | 12,098  | 24     |
+-----+------------+---------+---------+---------+---------+--------+
| 13  | Nitric     | 7,603   | 9,232   | 7,931   | 8,824   | 16     |
|     | acid       |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 23  | Fo         | 2,214   | 2,778   | 3,360   | 4,277   | 93     |
|     | rmaldehyde |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 25  | Ethylene   | 1,933   | 2,810   | 2,678   | 3,391   | 75     |
|     | oxide      |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 31  | Phenol     | 854     | 1,284   | 1,769   | 2,026   | 137    |
+-----+------------+---------+---------+---------+---------+--------+
| 33  | Butadiene  | 1,551   | 1,400   | 1,544   | 1,713   | 10     |
+-----+------------+---------+---------+---------+---------+--------+
| 34  | Propylene  | 590     | 884     | 1,483   | 1,888   | 220    |
|     | oxide      |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 36  | Acr        | 520     | 915     | 1,338   | 1,543   | 197    |
|     | ylonitrile |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 37  | Vinyl      | 402     | 961     | 1,330   | 1,509   | 275    |
|     | acetate    |         |         |         |         |        |
+-----+------------+---------+---------+---------+---------+--------+
| 47  | Aniline    | 199     | 330     | 495     | 632     | 218    |
+-----+------------+---------+---------+---------+---------+--------+

Source: Adapted from Lindell and Perry (1997a).

Emergency managers typically expect to find hazmat produced, stored, or
used at fixed-site facilities such as petrochemical and manufacturing
plants. However, such materials are also found in facilities as diverse
as warehouses (e.g., agricultural fertilizers and pesticides), water
treatment plants (chlorine is used to purify the water), and breweries
(ammonia is used as a refrigerant). Hazmat is transported by a variety
of modes---ship, barge, pipeline, rail, truck, and air. In general, the
quantities of hazmat on ships, barges, and pipelines can be as large as
those at many fixed site facilities, but quantities usually are smaller
when transported by rail, smaller still when transported by truck, and
smallest when transported by air. Small to moderate size releases of
less hazardous materials at fixed site facilities are occupational
hazards but often pose little risk to public health and safety because
the risk area lies within the facility boundary lines. However, releases
of this size during hazmat transportation are frequently a public hazard
because passers-by can easily enter the risk area and become exposed.
The amount that is actually released is often much smaller than the
total quantity that is available in the container but prudence dictates
that the planning process assume the plausible worst case of complete
release within a short period of time (e.g., 10 minutes in the case of
toxic gases, see US Environmental Protection Agency, 1987). In addition
to the quantity of the hazmat released, the size of the risk area
depends upon its chemical and physical properties.

The US DOT groups hazmat into nine different classes---explosives,
gases, flammable liquids, flammable solids, oxidizers and organic
peroxides, toxic (poisonous) materials and infectious substances,
radioactive materials, corrosive materials, and miscellaneous dangerous
goods. Each of these hazmat classes is described in the remainder of
this section. It is important to be aware that classification of a
substance into one of these categories does not mean it cannot be a
member of another class. For example, hydrogen sulfide is transported as
a compressed gas that is both toxic and flammable.

*Explosives* are chemical compounds or mixtures that undergo a very
rapid chemical transformation (faster than the speed of sound)
generating a release of large quantities of heat and gas. For example,
one volume of nitroglycerin expands to 10,000 volumes when it explodes;
it is this rapid increase in volume that creates the surge in pressure
characteristic of a blast wave. Explosives vary in their sensitivity to
heat and impact. Class A consists of high explosives that *detonate* (up
to 4 mi/sec), producing overpressure, fire, and missile hazards. Class B
consists of low explosives that *deflagrate* (approximately .17
mi/sec---about 4% as fast as a detonation) and cause fires and flying
debris (usually referred to as missile hazards). Class C consists of low
explosives that are fire hazards only. Explosives can cause casualties
and property damage due to overpressure from atmospheric blast waves or
missile hazards. Destructive effects from the quantities of explosives
found in transportation can be felt as much as a mile or more away from
the incident site.

*Compressed gases* are divided into flammable and nonflammable gases.
Nonflammable gases---such as carbon dioxide, helium, and nitrogen---are
usually transported in small quantities. These are a significant hazard
only if the cylinder valve is broken, causing the contents to escape
rapidly through the opening and the container to become a missile
hazard. Flammable gases (acetylene, hydrogen, methane) are missile and
fire hazards. Rupture of gas containers can launch missiles up to a
mile, so evacuation out to this distance is advised if there is a fire.
Large quantities of flammable gases, such as railcars of liquefied
petroleum gas (LPG), are of significant concern because the released gas
will travel downwind after release until it reaches an ignition source
such as the pilot light in a water heater or the ignition system in a
car. At distances of one-half mile or more, the gas cloud can erupt in a
fireball that flashes back toward the release point. Emergency managers
need to understand the community-wide hazards associated with fires
arising from flammable gases. Consequently, this topic is discussed in
greater detail later in this chapter.

*Flammable liquids*, which evolve flammable vapors at 80F or less, pose
a threat similar to flammable gases. A volatile liquid such as gasoline
rapidly produces large quantities of vapor that can travel toward an
ignition source and erupt in flame when it is reached. When a flammable
liquid is spilled on land, there should be a downwind evacuation of at
least 300 yards. A flammable liquid that floats downstream on water
could be dangerous at even greater distances and one that is toxic
requires special consideration (see the section on toxic chemicals,
below). A fire involving a flammable liquid should stimulate
consideration of an evacuation of 800 yards in all directions.

*Flammable solids* self-ignite through friction, absorption of moisture,
or spontaneous chemical changes such as residual heat from
manufacturing. Flammable solids are somewhat less dangerous than
flammable gases or liquids, because they do not disperse over wide areas
as gases and liquids do. A large spill requires a downwind evacuation of
100 yards, but a fire should stimulate consideration of an evacuation of
800 yards in all directions.

*Oxidizers and organic peroxides* include halogens (e.g., chlorine and
fluorine), peroxides (e.g., hydrogen peroxide and benzoyl peroxide), and
hypochlorites. These chemicals destroy metals and organic substances and
also enhance the ignition of combustibles (a spill of liquid oxygen can
cause the ignition of asphalt roads on a hot summer day). Oxidizers and
organic peroxides do not burn, but are hazardous because they promote
combustion and some are shock sensitive. A large spill should prompt a
downwind evacuation of 500 yards and a fire should initiate an
evacuation of 800 yards in all directions.

*Toxic chemicals*, which can have large impact areas, are classified in
a number of ways. DOT Class 2 consists of nonflammable gases and Class 6
is defined as poisons. Class A includes gases and vapors, a small amount
of which is an inhalation hazard, whereas Class B consists of liquids or
solids that are ingestion or absorption hazards. Many of these chemicals
are defined by SARA Title III/EPCRA as EHSs. Toxic materials are a major
hazard because of the effects they can produce when inhaled into the
lungs, ingested into the stomach by means of contaminated water or food,
or absorbed through the skin by direct contact. Of these exposure
pathways, inhalation hazard is typically the greatest concern because
high concentrations achieved during acute exposure can kill in a matter
of seconds. Nonetheless, prolonged ingestion can cause cancers in those
who are exposed and also can cause genetic defects in their offspring.
Moreover, chemical contamination of victims poses problems for
volunteers and professionals providing first aid and transporting
victims to hospitals. These chemicals vary substantially in their
volatility and toxicity, so evacuation distances following a spill or
fire must be determined from the Table of Protective Action Distances in
the *Emergency Response Guidebook* (US Department of Transportation,
2000, see www.dot.gov). Emergency managers need to understand the
community-wide hazards that could result from a toxic chemical release.
Consequently, this topic is discussed in greater detail later in this
chapter.

*Infectious substances* have rarely been a significant threat to date
because there are relatively few shipments of these substances, they
usually are transported in small quantities, and they have restrictive
requirements for packaging and marking. However, infectious substances
have the potential to be used in terrorist attacks, so emergency
managers should knowledgeable about them. This topic is discussed in
greater detail in the section on biological hazards.

*Radioactive materials* are substances that undergo spontaneous decay,
emitting ionizing radiation in the process. The types and quantities of
materials transported in the US generally have very small impact areas.
With the exception of nuclear power plants, for which planning is
supported by state and federal agencies and electric utilities, releases
of radioactive materials are likely to involve small quantities.
Nonetheless, even a few grams of a lost radiographic source for
industrial or medical X-rays can generate a high level of public
concern. Here also, the recently recognized threat of terrorist attack
from a "dirty bomb" that uses a conventional explosive to scatter
radioactive material over a wide area deserves emergency managers'
attention because of the potential for long-term contamination of
central business districts. A large spill should prompt a downwind
evacuation of 100 yards and a fire should initiate an evacuation of 300
yards in all directions. Emergency managers need to understand the
community-wide hazards that could arise from a release of radioactive
materials. Consequently, this topic is discussed in greater detail later
in this chapter.

*Corrosives,* which are substances that destroy living tissue at the
point of contact, can be either acidic or alkaline. Examples of acids
include hydrochloric acid (HCl) and sulfuric acid (H~2~SO~4~), whereas
examples of alkaline substances (caustics) include sodium hydroxide
(NaOH), potassium hydroxide (KOH), and ammonia (NH~4~). In addition to
producing chemical burns of human and animal tissues, corrosives also
degrade metals and plastics. The most frequently used and transported
substances in this class are not highly volatile, so the geographical
area affected by a spill is likely to be no greater than 100 yards
unless the container is involved in a fire or the hazmat enters a
waterway (e.g., via storm sewers). These chemicals vary substantially in
volatility and toxicity, so evacuation distances following a spill or
fire must be determined from the Table of Protective Action Distances in
the *Emergency Response Guidebook.*

*Miscellaneous dangerous goods,* as the name of this category suggests,
this class comprises a diverse set of materials such as air bags,
certain vegetable oils, polychlorinated biphenyls (PCBs), and white
asbestos. Materials in this category are low to moderate fire or health
hazards to people within 10-25 yards.

*Fires*

Flammable materials support rapid oxidization that produces heat and
affects biological systems by thermal radiation (burns). As noted
earlier in the section on wildfires, combustion requires the three
elements of the fire triangle: *fuel*, which is any substance that will
burn; *oxygen* that will combine with the fuel; and enough *heat* to
ignite the fuel. Combustion usually yields enough heat to sustain the
combustion reaction, but it also produces combustion products that might
be more dangerous than the heat. Combustion of simple hydrocarbons or
alcohols as fuels generally yields carbon dioxide, carbon monoxide,
water vapor, and unburned vapors of the fuel as combustion products.
More complex and heavier substances such as pesticides also yield carbon
dioxide, carbon monoxide, water vapor, and unburned vapors of the fuel.
However, they also produce highly toxic chemicals. It can be very
difficult to predict what will be the combustion products from a
building fire (e.g., an agricultural warehouse) because the temperature
of the fire is variable over time and from one location to another in
the fire, and the chemicals reacting with each other often are variable
over time and from one location to another in the fire.

In understanding combustion, it is important to recognize an important
distinction between gases and liquids. A gas is a substance that, at
normal temperatures and pressures, will expand to fill the available
volume in a space. By contrast, a liquid is a substance that, at normal
temperatures and pressures, will spread to cover the available area on a
surface. Any liquid contains some molecules that are in a gaseous state;
this is called vapor. All liquids generate increasing amounts of vapor
as the temperature increases and the pressure decreases. Conversely, at
a given temperature and pressure, the amount of vapor in a liquid varies
from one substance to another. There are three temperatures of each
flammable liquid that are important because they determine the
production of vapor. In turn, vapor generation is important because *it
is the vapor that burns, not the liquid.* The three important
temperatures of a liquid substance are its boiling point, flash point,
and ignition temperature. The boiling point is the temperature of a
liquid at which its vapor pressure is equal to atmospheric pressure.
Vapor production is negligible when a fuel is below its boiling point
but increases significantly once it exceeds this temperature. The flash
point of a liquid is the temperature at which it gives off enough vapor
to flash momentarily when ignited by a spark or flame. A liquid is
defined as combustible if it has a flash point above 100°F. (e.g.,
kerosene) and flammable if it has a flash point below 100° F. (e.g.,
gasoline). The final temperature to understand is the ignition
temperature, which is the minimum temperature at which a substance
becomes so hot that its vapor will ignite even in the absence of an
external spark or flame.

Gases and vapors have flammable limits that are defined by the
concentration (percent by volume in air) at which ignition can occur in
open air or an explosion can occur in a confined space. The lower
flammable/explosive limit (LFL/LEL) is the minimum concentration at
which ignition will occur. Below that limit the fuel/air mixture is "too
lean" to burn. The upper flammable/explosive limit (UFL/UEL) is the
maximum concentration at which ignition will occur. Above that limit the
fuel/air mixture is "too rich" to burn. When released from a source, a
flammable gas or vapor disperses in an approximately circular pattern if
there is no wind but in an approximately elliptical pattern in the
normal situation in which the wind is blowing (see Figure 5-5).

[**Figure 5-5.** Flammable Plume.]{.underline}

The most dangerous flammable substances have a low ignition temperature,
low LEL, and wide flammable range. Indeed, gasoline is widely used
precisely because of these characteristics. It has a low flash point
(--45 to --36F), a low LFL (1.4-1.5%), and a reasonably wide range (6%).
By contrast, peanut oil is useful in cooking because it has the opposite
characteristics---a high flash point (540F) and an undefined LFL because
it does not vaporize.

An important hazard of flammable liquids is a *Boiling Liquid Expanding
Vapor Explosion* (BLEVE), which occurs when a container fails at the
same time as the temperature of the contained liquid exceeds its boiling
point at normal atmospheric pressure. BLEVEs involve flammable or
combustible compressed gases that are not classified as "explosive
substances", but can produce fireballs as large as 1000 feet in diameter
and launch shrapnel to distances up to one half mile from the source.

*Toxic Industrial Chemical Releases*

Toxic industrial chemical releases are of special concern to emergency
managers because the airborne dispersion of these chemicals can produce
lethal inhalation exposures at distances as great as 10 miles and
sometimes even more. The spread of a toxic chemical release can be
defined by a dispersion model that includes the hazmat's chemical and
physical characteristics, its release characteristics, the topographic
conditions in the release area, and the meteorological conditions at the
time of the release. The chemical and physical characteristics of the
hazmat include its quantity (measured by the total weight of the hazmat
released), volatility (as noted earlier, higher volatility means more
chemical becomes airborne per unit of time), buoyancy (whether it tends
to flow into low spots because it is heavier than air), and toxicity
(the biological effect due to cumulative dose or peak concentration). It
also includes the chemical's physical state---whether it is a solid,
liquid (remember, a substance above its boiling point is a vapor), or a
gas at ambient temperature and pressure. In general, vapors and gases
are major hazards because they are readily inhaled and this is the most
rapid path into the body.

Release characteristics are defined by the chemical's temperature and
pressure in relation to ambient conditions, its release rate (in pounds
per minute), and the size (surface area) of the spilled pool if the
substance is a liquid. Temperature and pressure are important because
the rate at which the chemical disperses in the atmosphere increases
when these parameters exceed ambient conditions. The release rate is
important because it determines the concentration of the chemical in the
atmosphere. Specifically, a higher release rate puts a larger volume of
chemical into a given volume of air, thus increasing its concentration
(where the latter is defined as the volume of chemical divided by the
volume of air in which it is located).

Topographical conditions relevant to liquid spills include the slope of
the ground and the presence of depressions. As is the case with
flooding, steep slopes allow a liquid to rapidly move away from the
location of the spill. Both flat slopes and depressions decrease the
size of a liquid pool which, in turn, affects the size of the pool's
surface area and reduces the rate at which vapor is generated from it.
Thus, dikes are erected around chemical tanks to confine spills in case
the tanks leak and hazmat responders build temporary dikes around spills
for the same reason. Topographical characteristics also affect the
dispersion of a chemical release in the atmosphere. Hills and valleys
are land features that channel the wind direction and can increase wind
speed at constriction points---for example, where a valley narrows and
causes wind speed to increase due to a "funnel" effect. Forests and
buildings are rough surfaces that increase turbulence in the wind field,
causing greater vertical mixing. By contrast, large water bodies have
very smooth surfaces that do not constrain wind direction and, because
they provide no wind turbulence, allow a chemical release to maintain a
high concentration at ground level where it is most dangerous to people
nearby.

The immediate meteorological conditions of concern during a hazmat
release are wind speed, wind direction, and atmospheric stability class.
The effect of wind speed on atmospheric dispersion can be seen in Figure
5-6, which shows a release dispersing uniformly in all directions when
there is no wind (Panel A). Thus, the plume isopleth (contour of
constant chemical concentration) corresponding to the Level of Concern
(LOC) for this chemical is a circle. The nearby town lies outside the
vulnerable zone so its inhabitants would not need to take protective
action. However, Panel B describes the situation in which there is a
strong wind, so the plume isopleth corresponding to the LOC for this
chemical takes the shape of an ellipse. In this case, the nearby town
lies inside the vulnerable zone and would need to take protective
action.

[**Figure 5-6.** Effects of Wind Speed on Plume Dispersion.]{.underline}

#  

#### As Table 5-8 indicates, the atmospheric stability class can vary from Class A through Class F. Class A, the most unstable condition, occurs during strong sunlight (e.g., midday) and light wind. This dilutes the released chemical by mixing it into a larger volume of air. Class F identifies the most stable atmospheric conditions, which take place during clear nighttime hours when there is a light wind. These conditions have very little vertical mixing, so the released chemical remains highly concentrated at ground level.

#### Table 5-8. Atmospheric Stability Classes.

  ------------- ----------- ----------- ----------- ------------ -----------
                Strength of                         Nighttime    
                sunlight                            conditions   

  Surface Wind  Strong      Moderate    Slight      Overcast 50% Overcast \<
  Speed (mph)                                                    50%

  \< 4.5        A           A-B         B           \-           \-

  4.5-6.7       A-B         B           C           E            F

  6.7-11.2      B           B-C         C           D            E

  11.2-13.4     C           C-D         D           D            D

  \>13.4        C           D           D           D            D
  ------------- ----------- ----------- ----------- ------------ -----------

A: Extremely Unstable Conditions

B: Moderately Unstable Conditions

C: Slightly Unstable Conditions

D: Neutral Conditions (heavy overcast day or night)

####### E: Slightly Stable Conditions

F: Moderately Stable Conditions

Source: Adapted from FEMA, DOT, EPA (no date, a).

It is important to recognize that meteorological characteristics can
sometimes remain stable for days at a time, but at other times can
change from one hour to the next. Figure 5-7, adapted from McKenna
(2000), displays the wind direction at each hour during the day of the
accident at the Three Mile Island (TMI) nuclear power plant in terms of
the orientation of an arrow. Wind speed is indicated by the length of
the arrow. The figure shows wind speed and direction changed repeatedly
during the course of the accident, so any recommendation to evacuate the
area downwind from the plant would have referred to different geographic
areas at different times during the day. This would have made evacuation
recommendations extremely problematic because the time required to
evacuate these areas would have taken many hours. Consequently, the
evacuation of one area would have still been in progress when the order
to initiate an evacuation in a very different direction was initiated.

[**Figure 5-7.** Wind Rose from 3:00 a.m. to 6:00 p.m. on the First Day
of the TMI Accident.]{.underline}

Source: Adapted from McKenna (2000).

The ultimate concern in emergency management is the protection of the
population at risk. The risk to this target population varies inversely
with distance from the source of the release. Specifically, the
concentration (*C*) of a hazardous material decreases with distance
(*d*) according to the inverse square law (i.e., *C* = *1/d^2^*).
However, distance is not the only factor that should be of concern. In
addition, the density of the population should be considered because a
greater number of persons per unit area increases risk area population.
Moreover, there might be differences in susceptibility within the risk
area population because individuals differ in their dose-response
relationships as a function of age (the youngest and oldest tend to be
the most susceptible) and physical condition (those with compromised
immune systems are the most susceptible).

Toxic chemicals differ in their exposure pathways---inhalation,
ingestion, and absorption. Inhalation is the means by which entry into
the lungs is achieved. This is generally a major concern because toxic
materials can pass rapidly through lungs to bloodstream and on to
specific organs within minutes of the time that exposure begins.
Ingestion is of less immediate concern because entry through the mouth
into the digestive system (stomach and intestines) is a slower route
into the bloodstream and on to specific organs. Depending on the
chemical's concentration and toxicity, ingestion exposures might be able
to be tolerated for days or months. Authorities might choose to prevent
ingestion exposures by withholding contaminated food from the market or
recommending that those in the risk area drink boiled or bottled water.
Absorption involves entry directly through the pores of the skin (or
through the eyes), so it is more likely to be a concern for first
responders than for local residents. Nonetheless, some chemicals can
affect local populations in this way, as was the case with the release
of methyl isocyanate during the accident in Bhopal, India, in 1984.

The harmful effects of toxic chemicals are caused by alteration of
cellular functions (cell damage or death), which can be either acute or
chronic in nature. Acute effects occur during the time period from 0--48
hours. Irritants cause chemical burns (dehydration and exothermic
reactions with cell tissue). Asphyxiants are of two types; simple
asphyxiants such as carbon dioxide (CO~2~) displace oxygen (O~2~) within
a confined space or are heavier than oxygen so they displace it in
low-lying areas such as ditches. By contrast, chemical asphyxiants
prevent the body from using the oxygen even if it is available in the
atmosphere. For example, carbon monoxide (CO) combines with the
hemoglobin in red blood cells more readily than does O~2~ so the CO
prevents the body from obtaining the available O~2~ in the air.
Anesthetics/narcotics depress the central nervous system and, in extreme
cases, suppress autonomic responses such as breathing and heart
function.

Chronic, or long-term, effects can be general cell toxins, known as
cytotoxins, or have organ specific toxic effects. In the latter case,
the word *toxin* is preceded by a prefix referring to the specific
system affected. Consequently, toxins affecting the circulatory system
are called hemotoxins, those affecting the liver are hepatotoxins, those
affecting the kidneys are nephrotoxins, and those affecting the nervous
system referred to as neurotoxins. Other chronic effects of toxic
chemicals are to cause cancers, so these chemicals are referred to as
carcinogens. Mutagens cause mutations in those directly exposed, whereas
teratogens cause mutations to the genetic material of those directly
exposed and, thus, mutations in their offspring. The severity of any
toxic effect is generally due to a chemical's rate and extent of
absorption into the bloodstream, its rate and extent of transformation
into breakdown products, and its rate and extent of excretion of the
chemical and its breakdown products from the body (i.e., the substances
into which the chemical decomposes).

Research on toxic chemicals has led to the development of dose limits.
Some important concepts in defining dose limits are the LD-50, which is
the dose (usually of a liquid or solid) that is lethal to half of those
exposed, and the LC-50, which is the concentration (usually of a gas)
that is lethal to half of those exposed. Based upon these dose levels,
authoritative sources have devised dose limits that are administrative
quantities that should not be exceeded. LOCs are values provided by EPA
indicating the *Level of Concern* or "concentration of an EHS
\[Extremely Hazardous Substance\] above which there may be serious
irreversible health effects or death as a result of a single exposure
for a relatively short period of time" (US Environmental Protection
Agency, 1987, p. XX). IDLHs are values provided by NIOSH/OSHA indicating
the concentration of a gas that is *Immediately Dangerous to Life or
Health* for those exposed more than 30 minutes. TLVs are *Threshold
Limit Values*, which are the amounts that the American Conference of
Government Industrial Hygienists has determined that a healthy person
can be exposed to 8--10 hours/day, 5 days/week throughout the work life
without adverse effects.

*Weaponized Toxic Chemicals*

Although it seems plausible that a deliberate attack might use
explosives to cause release toxic chemicals from a domestic source such
as a chemical plant, rail car, or tank truck, it also is possible that a
weaponized toxic agent might be used. Such agents were originally used
by the military in battles dating back to World War I. Over the years,
attention turned to increasingly toxic chemicals that, by their very
nature, require smaller doses to achieve a significant effect (e.g.,
disability or death). One consequence of the more advanced toxic agents
is that they can affect victims through absorption in secondary
*contamination*. That is, chemical residues on a victim's skin or
clothing can affect those who handle that individual. Indeed, any object
on which the chemical is deposited becomes an avenue of secondary
contamination (World Health Organization, 2004). A list of the most
likely weaponized toxic agents is presented in Table 5-9. Some of these
agents are produced by biological processes (botulism, anthrax, and
encephalitis) that affect victims through the production of toxins and,
thus, are more properly considered to be chemical weapons (World Health
Organization, 2004).

**Table 5-9**. Weaponized Toxic Agents.

  ------------------------------ ----------------------------------------
  Agent                          Example

  Tear gases/other sensory       Oleoresin capsicum ("pepper spray")
  irritants                      

  Choking agents (lung           Phosgene
  irritants)                     

  Blood gases                    Hydrogen cyanide

  Vesicants (blister gases)      Mustard gas

  Nerve gases                    O-Isopropyl Methylphosphonofluoridate
                                 (Sarin gas)

  Toxins                         Clostrinium botulinum ("botulism")

  Bacteria and rickettsiae       Bacillus anthracis ("anthrax")

  Viruses                        Equine encephalitis
  ------------------------------ ----------------------------------------

Source: Adapted from World Health Organization (2004)

A terrorist attack involving a toxic chemical agent might be detected
initially by fire, police, or emergency medical services personnel
responding to a report of a mass casualty incident. Likely symptoms
include headache, nausea, breathing difficulty, convulsions, or sudden
death---especially when these symptoms are displayed by a large number
of people in the same place at the same time. In this case, the
appropriate response will be the same as in any other hazardous
materials incident. Specifically, there will be a need to control access
to the incident site, decontaminate the victims as needed, and transport
them for definitive medical care. In addition to the normal coordination
with emergency medical services and hospital personnel, it is
appropriate for emergency managers to be aware of the assistance that is
available from local poison control centers. Other than that, the
capabilities needed to respond effectively to an attack using toxic
chemicals will be much the same as those needed for an industrial
accident involving these materials (World Health Organization, 2004).
Unfortunately, few communities---even those with a significant number of
chemical facilities---have hospitals with the capability to handle mass
casualties from toxic chemical exposure caused by either an industrial
accident or a terrorist attack.

In the event of a terrorist attack, emergency managers will need to deal
with a consideration that is not encountered in most other incidents to
which they respond. Specifically, the incident site will be considered a
crime scene by law enforcement authorities. Consequently, emergency
mangers must learn about the basic procedures these personnel will
follow, including collecting evidence, maintaining a chain of custody
over that evidence, and controlling access to the incident scene. This
latter issue should be carefully coordinated to avoid a conflict between
emergency management procedures for victim rescue and law enforcement
procedures for crime scene security.

*Radiological Material Releases*

There are 123 nuclear power plants in the US, most of which are located
in Northeast, Southeast, and Midwest. To understand the radiological
hazards of these plants, it is necessary to understand the *atomic*
*fission reaction*. The atoms of chemical elements consist of positively
charged protons and neutrally charged neutrons in the atom's nucleus,
together with negatively charged electrons orbiting around the nucleus.
Some unstable chemical elements undergo a process of spontaneous decay
in which a single atom divides into two less massive atoms (known as
fission products) while emitting energy in the form of heat and ionizing
radiation. The ionizing radiation can take the form of alpha, beta, or
gamma radiation. Alpha radiation can travel only a very short distance
and is easily blocked by a sheet of paper but is dangerous when inhaled
(e.g., Pu-plutonium). Beta radiation can travel a moderate distance but
be blocked by a sheet of aluminum foil. Gamma radiation can travel a
long distance and can be blocked only by very dense substances such as
stone, concrete, and lead.

Radioactive materials are used for a variety of purposes. Small
quantities of some materials are used as sources of radiation for
medical and industrial diagnostic purposes (e.g., imaging fractured
bones and faulty welds). Large quantities of other radiological
materials are used as sources of heat to produce the steam needed to
drive electric generators at power plants. In these nuclear power
plants, enriched uranium fuel fissions when struck by a free neutron
(see Figure 5-8). The thermal energy released is used to heat water and,
thus, produce steam. The free neutrons are used to continue a sustained
chain reaction and the fission products are waste products that must
eventually be disposed in a permanent repository.

The fuel temperature is controlled by cooling water and the reaction
rate is controlled by neutron absorbing rods. The amount of fission
products increases with age, so the reactor is refueled by moving the
fuel in stages toward the center of the reactor vessel. Spent fuel,
which contains a significant amount of radioactive fission products and
some uranium, is stored onsite until transfer to a repository. The
nuclear fuel is located in the plant's reactor coolant system (RCS),
where it is contained in fuel pins that are welded shut and inserted
into long rods that are integrated into assemblies. Cooling water is
pumped into the reactor vessel where it circulates, picks up heat (and
small amounts of radioactive fission products) from the fuel, and flows
out of the reactor vessel.

[**Figure 5-8.** The Atomic Fission Reaction.]{.underline}

###### 

There are two types of RCSs, Pressurized Water Reactors (PWRs) and
Boiling Water Reactors (BWRs). In PWRs, the core coolant water is
pressurized (the pressurizer is a device used to control the pressure in
the reactor vessel) to prevent it from boiling. The hot water passes
through a heat exchanger (called the steam generator), gives up its
heat, and returns to the reactor vessel, completing the primary coolant
system. The water in the secondary coolant system is allowed to boil,
producing steam in the steam generator. In BWRs, the core coolant water
is allowed to boil, generating steam directly. The steam is delivered to
the turbine, spinning it to make electricity. The RCS is located in a
containment building (the turbine is in an adjacent building), which is
constructed with thick walls of steel-reinforced concrete to withstand
high internal pressures or external missile impact. However, it has many
penetrations for water pipes, steam pipes and instrumentation and
control cables. These penetrations are sealed during normal operations,
but the seals could be damaged during an accident that allows
radioactive material to escape from the containment building into the
environment.

During a severe accident involving irreversible loss of coolant, the
fuel will first melt through the steel cladding, then melt through the
RCS, and finally escape the containment building (probably through a
basemat melt-through or steam explosion). This process could produce a
release as soon as 45-90 minutes after core uncovery. If the core melts,
the danger to offsite locations depends upon containment integrity.
Early health effects are likely if there is early total containment
failure and are possible if there is early major containment leakage.
Otherwise, early health effects are unlikely. The problem is that
containment failure might not be predictable (McKenna, 2001).

A radioactive release would involve a mix of radionuclides (i.e., a
variety of radioactive substances that vary in their atomic weight) and
this mix is called the *source term*. The source term is defined by
three classes of radionuclides---particulates, radioiodine, and noble
gases. Particulates include uranium (U) and strontium (Sr), the latter
of which is dangerous because it is chemically similar to calcium (Ca)
and therefore tends to be deposited in bone marrow. Radioiodine (I-131)
is dangerous because it substitutes for nonradioactive iodine in the
thyroid and, thus, can cause thyroid cancer. Noble gases such as krypton
(Kr) do not react chemically with anything, but are easily inhaled to
produce radiation exposures while they remain in the lungs.

The source term is also characterized by its volatility. As noted in
connection with toxic chemicals, volatility is an important
characteristic of a substance because higher volatility means more of
the radionuclide becomes airborne per unit of time and stays airborne.
The quantity of radioactivity released can be measured in terms of the
number of pounds or kilograms, but this is not a very useful measure
because two different source terms with the same mass might emit very
different levels of radiation. Consequently, the amount of radioactivity
(ionizing radiation) released is measured by the number of
disintegrations per unit time (Curies). In fact, these disintegrations
are what a Geiger counter measures. The amount of radioactivity is
usually measured in curies of an individual radionuclide or class of
radionuclides.

Exposure pathways for radiological materials are similar to those of
toxic chemicals. Breathing air that is contaminated with radioactive
materials can cause inhalation exposure and eating food (e.g., unwashed
local produce) or drinking liquids (e.g., water or milk) that is
contaminated can cause ingestion exposure. Contamination also can enter
the body through an open wound such as a compound fracture, laceration,
or abrasion, but radiological materials do not cause absorption
exposures because they do not pass through the skin. However, because
radiological materials release energy, they can produce exposures via
direct radiation (also known as "shine") from a plume that is passing
overhead. If the plume has a significant component of particulates,
these might be deposited on the ground, vegetation, vehicles, or
buildings and the direct radiation from the deposited particulates would
produce a continuing exposure. In some cases, the small particles of
deposited material could become resuspended and inhaled or ingested. In
this connection, it is important to recognize the distinction between
irradiation and contamination. Irradiation involves the transmission of
energy to a target that absorbs it, whereas contamination occurs when
radioactive particles are deposited in a location within the body where
they provide continuing irradiation.

Measuring radiation dose is somewhat more complicated than measuring
doses of toxic chemicals. As noted earlier, a Curie is a measure of the
activity of a radioactive source in atomic disintegrations/second,
whereas a Roentgen is a measure of exposure to ionizing radiation. A rad
is a measure of absorbed dose, and a rem ("Roentgen equivalent man") is
a measure of committed dose equivalent. The term *committed* refers to
the fact that contamination by radioactive material on the skin or
absorbed into the body will continue to administer a dose until it
decays or is removed. The term *equivalent* refers to the fact that
there are differences in the biological effects of alpha, beta, and
gamma radiation. Weighting factors are used to make adjustments for the
biological effects of the different types of radiation. However, for
offsite emergency planning purposes, one rem is approximately equal to
one rad.

The health effects of exposure to ionizing radiation are defined as
early fatalities, prodromal effects, and delayed effects. Early
fatalities occur within a period of days or weeks and are readily
interpreted as effects of radiation exposure. Early fatalities begin to
appear at whole body absorbed doses of 140 rad (which is equal to 1.4
Gray, the new international scientific unit) but less than 5% of the
population would be expected to die from such exposures. Approximately
50% of an exposed population would be expected to die from a whole body
dose of 300 rad and 95% would be expected to die from a dose of 460 rad.

Prodromal effects are early symptoms of more serious health effects
(e.g., abnormal skin redness, loss of appetite, nausea, diarrhea,
nonmalignant skin damage), whereas delayed effects are cancers that
might take decades to manifest themselves and might only be associated
with a particular exposure on a statistical basis. Genetic disorders do
not reveal their effects until the next generation is born. Prodromal
effects would be expected to manifest themselves in less than 2% of the
population at a dose of 50 rad, whereas 50% would be expected to exhibit
prodromal symptoms at 150 rad and 98% would be expected to show these
symptoms at 250 rad.

The delayed effects of radiation exposure can be seen in Table 5-10,
which lists the number of fatal cancers, nonfatal cancers, and genetic
disorders that can be expected as a function of the number of person-rem
(that is, the number of persons exposed times the number of rems of
exposure per person). The small numbers involved are indicated by the
fact that the coefficients are presented in scientific notation (i.e.,
2.8 E-4 = .00028). That is, 2.8 fatal cancers, 2.4 nonfatal cancers, and
1 genetic effect are expected if 10,000 people are each exposed to 1 rem
of radiation to the whole body.

[**Table 5-10.** Average Risk of Delayed Effects (Per
Person-Rem)]{.underline}

## [Effect Whole body Thyroid Skin]{.underline} 

# Fatal cancers 2.8 E-4 3.6 E-5 3.0 E-6

##### Nonfatal cancers 2.4 E-4 3.2 E-4 3.0 E-4

##### [Genetic disorders 1.0 E-4]{.underline} 

Source: Adapted from McKenna (2000).

It is important to be aware of the differential biological affinity of
radionuclides for specific organs. Whole body radiation refers to the
response of the "typical" cell to irradiation, reflecting the common
components and structures all cells share. By contrast, the thyroid is
sensitive to I-131 and bone marrow is sensitive to Sr-90. Organ
differences in dose-response arise because rapidly dividing cells, found
in the gut (damage causes diarrhea and vomiting) and hair follicles
(damage causes hair loss), are especially susceptible. There also are
individual differences in dose-response. For example, fetuses are
extremely susceptible because all of their cells are dividing rapidly,
and the same is generally true of preschool children. Unfortunately,
recommendations for protective action by pregnant women are easily
misinterpreted. The concern is for the health of the highly susceptible
fetus, not that of the much less susceptible adult woman. Other
population segments include those at risk of any environmental insult:
the very old, the very young, and those with compromised immune systems.

Population protective actions for radiological emergencies are based
upon three fundamental attenuation factors---*time, distance,* and
*shielding*. Evacuation reduces the amount of time exposed and increases
distance from the source, whereas sheltering in-place can provide
shielding if this is done within dense materials that absorb energy and
are airtight. To determine when protective action should be initiated,
the EPA has developed Early Phase Protective Action Guides (PAGs), which
are specific criteria for initiating population protective action in
radiological emergencies (Conklin & Edwards, 2001). Note that the whole
body dose listed in Table 5-11 for initiating evacuation (1 rem) is only
a small fraction of the exposure level that would be expected to produce
prodromal effects in the most susceptible 2% of the general population.

**Table 5-11.** EPA Protective Action Guides

+-------------------------+---------------------+---------------------+
| ####### Organ           | EPA PAGs^a^         | Protective          |
|                         | (rem/Sv)            | Action^b^           |
+-------------------------+---------------------+---------------------+
| Whole body              | 1-5 (.01-.05)       | Evacuation          |
+-------------------------+---------------------+---------------------+
| Thyroid                 | 25 (.25)            | Stable Iodine (KI)  |
+-------------------------+---------------------+---------------------+

^a^ Dose inhalation from and external exposure from plume and ground
deposition.

^b^ Actions should be taken to avert PAG dose.

\* Evacuation is considered to be the most effective protective action
for nuclear power plant accidents at American sites.

**Biological Hazards**

According to the World Health Organization (2004, p. 5), biological
weapons are "those that achieve their intended target effects through
the infectivity of disease-causing micro-organisms and other such
entities including viruses, infectious nucleic acids, and prions". Some
biological agents produce toxins and, thus, are actually chemical
weapons whose "chemical action on life processes \[is\] capable of
causing death, temporary incapacitation or permanent harm" (World Health
Organization, 2004, p. 6).

Emergency managers should recognize that most biological agents likely
to be used in deliberate attacks on their communities also exist as
natural hazards. They also could be released accidentally from
fixed-site facilities (e.g., commercial or academic laboratories) or in
transportation among those facilities. These biological agents exist at
low levels of prevalence in human populations or, alternatively, in
animal populations from which they can spread to human populations.
Indeed, one quarter of the world's deaths in 1998 were caused by
infectious diseases. The major consequence of most biological agents is
the *magnification* of their effects by infection, unlike chemical
agents that generally experience *dissipation* over time and distance.
Biological agents magnify their effects by multiplying within the target
organisms, but chemical agents cannot do this.

Biological agents can be dispersed by contaminating food or water to
achieve exposure through ingestion. For example, a terrorist attack
might attempt to introduce a plant or animal infection that would affect
people through the food distribution system. However, this system is
routinely monitored by the US Department of Agriculture and state
departments of agriculture. In some cases, these agencies already
receive support from state emergency management agencies when natural
outbreaks occur. For example, collaborative relationships have been
demonstrated in recent cases of Bovine Spongiform Encephalopathy
(BSE---"mad cow" disease) and naturally occurring outbreaks of livestock
anthrax.

Alternatively, a biological agent can be used to create an aerosol cloud
of liquid droplets or solid particles to achieve an inhalation hazard.
The aerosol can be dispersed either in the open environment or through a
building's heating, ventilation, and air conditioning (HVAC) system, but
the latter is likely to produce more casualties because the
concentration of the biological agent will be greater. The effectiveness
of the dispersion will depend on the hazard agent's physical (particle
size and weight) characteristics. Micrometeorological variation can
produce corresponding variation in the dispersion of the hazard agent
and, under certain conditions, extreme dilution or loss of its
viability. Nonetheless, epidemic spread could compensate for poor
initial dispersion.

As is the case with some toxic chemical agents, biological agents can be
very difficult to detect when symptoms do not appear until long after
exposure occurs. The incubation period for biological agents is free of
symptoms, so tourists or business travelers might travel a long way from
the attack site before they become symptomatic. Consequently, infection
with a contagious agent could cause secondary outbreaks that are caused
by victims of the initial exposure transmitting the agent to people with
whom they come into contact during their travels. Thus, infection can
spread widely before local authorities are aware that an attack has even
occurred.

The dispersal of the victims at the time the symptoms are manifested and
the similarity of these symptoms to those of routinely encountered
diseases such as influenza could impede prompt recognition of an attack.
The major problem here is that the symptoms of biological agents are
frequently indistinguishable from common maladies such as colds and
influenza. Consequently, the occurrence of a covert biological agent
release is most likely to be identified by noting a significant increase
in the incidence of such symptoms. This would either be achieved by
health care providers in emergency rooms and clinics supplemented by the
health surveillance system operated by the public health department.

There is an emerging sensor technology for detecting many biological
agents. These sensors can identify the presence of agents at a very
early stage rather than awaiting the development of symptoms in human
populations. However, they can only detect these agents at specific
locations and, because of their expense, cannot currently be widely
distributed. For the foreseeable future, their deployment is likely to
be limited to the most critical facilities. Consequently, it is
important for emergency managers to establish a working relationship
with their local health departments. In turn, these will have
established contacts with regional laboratories and state and federal
public health agencies to provide assistance in identifying the agent,
treating the victims, and decontaminating the incident site.

Countermeasures for biological agents include isolation and quarantine.
*Isolation* is the action taken to prevent those who are known to be ill
with a contagious disease from infecting others. It typically is
associated with special treatment to remedy the disease. By contrast,
*quarantine* is used to prevent those who might have been exposed to a
biological agent but do not currently exhibit symptoms. Thus, they might
not become ill and, indeed, they might not even have the disease.
However, it is critical to prevent them from infecting others. Thus,
quarantine is somewhat similar to sheltering in-place from toxic
chemical hazards. The difference is that people being quarantined are
asked (or legally required) to remain indoors in order to protect others
from themselves (because *they* are the hazard) rather than to protect
themselves from an external hazard. Although there is extensive research
on household compliance with evacuation warnings, the same cannot be
said for isolation and quarantine. Nonetheless, it seems safe to say the
level of compliance will be less than perfect, so emergency managers
should try to assess local residents' perceptions of these protective
actions if the need to implement them arises.

In addition, biological agents can be combated by vaccines that provide
protection against specific agents and other therapeutic agents that
seek to block the body's reaction to the agent. Emergency managers will
be particularly interested in the latter type of therapy because a
generic therapeutic mechanism would be effective against a wide variety
of biological agents, just as a wide-spectrum antibiotic is effective
against a range of bacteria.