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Volume 31, Issue 1

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Emergency situations take many different forms. A house fire may affect a family; an automobile accident may affect several families; an earthquake may affect an entire country; and a hurricane may affect an entire coastline over a period of days. Many emergencies have common threads, which, if isolated, may serve as a basis for comparison and standard setting in planning activities. In emergency response operations, a decision must often be made to issue an order for a population to either evacuate or to shelter in place. This decision may have large-scale consequences that deserve thorough examination.

All emergency managers may someday face the decision to call for an evacuation or a shelter in place action. In the heat of an emergency there is often little time to consider the consequences of this crucial decision. Due to the need for quick decisions in emergency situations, researchers are spending considerable time and effort studying evacuation decisions.

This paper will briefly illustrate similarities and differences that can be used as decision making tools in four selected types of emergencies. This article compares (a) hurricanes; (b) floods; (c) chemical releases; and (d) radiation releases.

Situational comparisons

By definition, a situational comparison requires that there be common grounds for comparison in the selected situations.

All four of these situations require:

(a) A strategy to protect members of the public (relocation, evacuation, or shelter in place).
(b) A timely decision on the most desirable protection method for the situation (available time will vary).
(c) Time to react (warning time before impact).

By considering these variables, an appropriate decision on response can be reached.

Studies on emergency actions

There are vast differences in situations that require emergency managers to make quick decisions. These decisions must often be based on limited information and under conditions of extreme duress.

Information was found on hurricanes in several documents. Zelinski wrote in The Emergency Evacuation of Cities that 'a wide array of disasters can generate the temporary emptying of a city . . . within the natural realm, they include major storms, floods, earthquakes, and volcanic eruptions.' He also wrote that 'Among human disasters, the approach of hostile armies and actual or threatened aerial bombardment or artillery shelling may trigger urban flight, and many of a growing variety of industrial and freight traffic accidents can prompt an evacuation of densely settled areas.' Two of the categories described by Zelinski, (hurricanes and floods) fit into the natural disaster category. The other two categories of discussion (chemical and radiological releases) may be classified as human disasters originating in industrial or freight accidents.

President Richard Nixon signed the Disaster Relief Act of 1970 into law. To support this law, extensive research was put into managing and mitigating natural disasters. The result of this study was the Report to Congress, Disaster Preparedness. This report focused on natural disasters and was structured to offer specific information on disaster protection, disaster mitigation, the application of science and technology, and historical data. The last volume of this document contains selected physical studies of individual types of natural disasters. Extensive information is available in this report on river floods, hurricanes, and storm surges.

In 1974, J.M. Hans published a report for the Environmental Protection Agency titled Evacuation Risks-An Evaluation. This report focused on three risk factors that may result from evacuation decisions. These areas are (a) risk of death; (b) risk of injury; and (c) cost of evacuation. Hans described two types of evacuations. 'The first type of evacuation is to evacuate people from unprotected outdoor areas to indoor locations within the affected geographical area.' This form of sheltering in place, described by Hans as a method of protecting populations from the effects of radiation releases, is also very effective in reducing exposure in chemical releases. Hans wrote that, 'the second type of evacuation is to move persons outside the boundaries of the potentially affected geographical area.' This second type of evacuation is what we think of today as an evacuation.

Factors influencing decision-making

Given adequate warning time, the evacuation of a population is often the preferred response alternative. In cases involving chemical and radiological releases, the impact stage of the emergency may occur with little warning. When an emergency occurs rapidly and there is little time to safely evacuate the population, the emergency manager must consider the alternative decision of sheltering in place.

If the emergency manager chooses evacuation as the preferred alternative, associated logistical problems must be considered. Southworth wrote, 'The timing of evacuee response to a crisis can have a significant effect on the level of traffic congestion experienced during the flight to safety. In practice, we can expect that some people will leave an area when asked to do so, others will delay their decision until they gain more confidence in the official instructions, and others are likely to remain behind as either a residual at risk population, or until forced to move.'

Those who decide to evacuate in the late stages of an emergency are often forced to shelter in place as a last resort due to the fact that they have waited too long to safely leave the area. One of the most crucial problems encountered in an emergency situation is the time of onset variable. This variable often determines the severity of human suffering inflicted by a disaster.

Recommended actions

The recommendations for action in each situation will vary with the nature of the occurrence. Similarities and differences are illustrated in the following comparison of the four selected emergency situations.

a. Hurricanes
In recent years, hurricane planning has become much more important than in earlier times. Zelinski pointed out, 'In recent decades, . . . actual evacuation of large numbers of residents or visitors just before or during major storms has occurred frequently, or has been considered seriously as cities have flourished along the littoral of a boom region, and much of its shoreline has been developed for tourists and pleasure seekers.' He also pointed out that evacuation in these areas has become routine due to experience in the past.

Drabek wrote that hurricanes display a 'high degree of locational predictability.' The Office of Emergency Preparedness wrote that the words 'Predict and Warn . . . succinctly define the purpose of the hurricane warning service.' Hurricanes are characterized by early prediction and significant warning time, but due to the unpredictable nature of hurricanes, the warnings are not always directed to the correct populations. Zelinski wrote that 'Hurricanes have been known to exhibit the most bizarre sorts of locational behavior, sometimes following zigzag or circular paths, while changing direction suddenly, sometimes advancing hundreds of miles in a day and other times dawdling for days over the same locality.'

Thygerson wrote that 'the hurricane's worst killer is storm surges, wind driven waters that sweep over low lying coastal areas.' He also wrote that 'reduced atmospheric pressure in the storm center . . . may produce large swells - reaching heights of 50 feet' and that 'much of the densely populated Atlantic and Gulf coast area lies within 10 feet of sea level and is thus vulnerable to storm surges.' Depending on the situation and the amount of warning time available, evacuation is usually the clearly preferred emergency response option before a hurricane strikes a population center.

b. Floods

Floods have been known to take many forms. Thygerson distinguished between river floods and flash floods. Hurricane floods were considered separately under the category of storm surges. Flash floods are characterized by Thygerson as 'especially vicious: and the potential for frightful devastation is compounded by the fact that flash floods often occur with little or no warning.'

According to the President's Report to Congress on Disaster Preparedness 'warning time varies by the type and location of flooding. In mountainous areas or other areas where the runoff is rapid and subject to flash floods, forewarning may be only an hour or less. In areas where the terrain will retard the runoff or where flooding results from a melting snow-pack, the warning time can be days or even weeks.' The report goes on to say 'the amount of warning can have a direct relation to the emergency measures taken. With short forewarning, it may be possible only to evacuate people to high ground. With longer warning, property can be evacuated or protected, and even emergency engineering protective works can be constructed.' In both river floods and the more rapidly occurring flash floods, sheltering in place may be totally ineffective, and should only be taken as a last resort.

Thygerson described the difference between Flood Warnings and Flash Flood Warnings. He wrote that 'Flood warnings are forecasts of impending floods, and are distributed to the public by radio and through local government emergency forces. The warning message tells the expected severity of the flooding (minor, moderate, or major).' He wrote, 'Flash flood warnings are the most urgent type of flood warning issued.' He recommended 'before a flood, find out how many feet your property is above or below possible flood levels, so that when predicted flood levels are broadcast you can determine whether or not you may be flooded. Also locate the nearest safe area.' Thygerson recommended 'above all, move to a safe area before access is cut off by floodwaters. In these situations, shelter in place is not recommended or safe, evacuation is clearly the preferred option.'

c. Chemical Releases

The decision to evacuate or shelter in place during a chemical emergency is much more difficult than in the event of a hurricane or flood. The factors affecting such a decision are numerous and complex. The Michigan Department of State Police publication, Warning, Evacuation, and Shelter in Place Handbook, states 'we know that evacuation is often difficult and time consuming to accomplish and may even result in exposure to harmful intoxicants while evacuating . . . particularly when a toxic gas is released only for a short time, indoor sheltering may be the best choice.' The handbook goes on to identify the factors that affect protective action decisions. These factors are: 'the hazardous material involved; the population at risk; time factors involved; the effect of the present and predicted meteorological conditions; the capability to communicate with both the community at risk and the emergency response personnel; and the capabilities and the resources of the response organizations to implement, control, monitor, and terminate the protective action.' The handbook also states that 'In deciding on the most appropriate protective action, two questions need to be answered: (a) will in place protection provide adequate protection, and (b) is there time to evacuate?'

The Environmental Protection Agency (EPA) uses vulnerable zone estimates to determine worst-case accident scenarios. These zones are based on EPA methodology that takes into account (a) Whether the chemical is a solid, liquid, or gas, (b) the rate of release of the chemical and (c) the level of concern of the chemical. In 1995, Tickner wrote 'The methodology serves as a screening tool for facilities and local emergency management agencies to estimate the potential immediate human impacts of worst case accidents.'

The decision to either shelter-in-place or evacuate during a chemical emergency must be made using all available information on assessing the potential risk involved, estimating the time it will take to evacuate the population, and comparing the consequences of the two alternatives. The Michigan State Police Warning, Evacuation, and Shelter In Place Protection Handbook states that if evacuation is chosen, 'for an area that is only threatened by a hazardous release, it should be determined whether potential evacuees can be evacuated before hazards reach the area. To safely evacuate the area, a significant amount of lead time may be required.' If In-Place-Protection is chosen due to lack of time 'often the most prudent course of action for the protection of the nearby residents is to remain inside with the doors and windows closed and the heating and air conditioning systems shut off. An airborne cloud will frequently move past quickly. Vulnerable populations, such as the sick and elderly, may sustain more injury during an evacuation than they would sitting inside and putting simple countermeasures into effect.'

The decision to evacuate or shelter-in-place in this type of emergency must be made based on these factors. This decision is rarely clear-cut for a chemical release event.

d. Radiation Releases

In dealing with radiation releases, there are two primary considerations, (a) the public needs protection from exposure caused by inhalation or ingestion of the material (as in chemical release scenarios) and (b) the public needs protection from direct exposure to radiation emitted during a release. In a direct exposure scenario, the shielding capability of the chosen shelter becomes a key consideration.

The EPA report Protective Action Evaluation Part I, the Effectiveness of Sheltering as a Protective Action Against Nuclear Accidents Involving Gaseous Releases states that 'in the event of an airborne release of radioactive material from a nuclear power plant accident, sheltering of individuals is an important consideration in emergency protective action planning as it may be (a) an effective means of significantly reducing radiation dosages and (b) the only practical option in view of possible time and logistical constraints.'

The EPA report states that 'The basic shelter model characteristics considered are gamma-ray attenuation, source geometry, gaseous fission-product ingress and air change rate.' This report will focus on gamma ray attenuation and gaseous fission-product ingress. Source geometry and air change rates are important considerations, but will not be included in this discussion. In the event of a release from a nuclear power plant, radioiodine is one of the three most significant radionuclides emitted. Often, radioiodine is used as an indicator of the severity of the accident that led to the release.

When considering the amount of material (radioiodine) that will enter a structure, the EPA report explains that 'the extent to which radioiodine will penetrate a structural shielding facility is dependent on the gross tightness of the structure, the ventilation rate, filtration, the chemical and physical properties of the released material, and the interacting species.'
In the preliminary stages of a radiation accident, the decision to shelter in place or evacuate will be based on the initial threat posed. The severity of this threat will be based on cloud gamma attenuation. Attenuation is the process by which radiation is scattered and absorbed, thus reducing intensity. The second major attenuation factor (fallout attenuation) is not considered in this report. According to this EPA report, 'attenuation of cloud gamma radiation for large structures such as office buildings and multistory structures could be significantly more than for simple structures such as single family dwellings.'

The EPA report describes relative attenuation factors. A wood frame house with no basement should have an attenuation factor of 0.9, indicating little or no protection from direct gamma radiation penetrating through the building. If that wood frame house has a basement, however, an attenuation factor of 0.6 is indicated. This number represents a fairly significant dose reduction. The attenuation factor of large office or industrial building may be 0.2 or less, which indicates significant shielding and dose reduction.

In a radiological release emergency, the most significant variation affecting decision-making that distinguishes it from a chemical emergency is the radiation dose factor. Most other factors, such as tightness of the facility and air ingress remain similar in both types of events. This radiological difference, however, means that the dose received by the individual will depend largely on the type of structure in which that individual sought shelter, and is not just a function of air tightness.

The public can be instructed in how to respond to accidents through public education campaigns, public-warning systems, and emergency information systems. As in the case with chemical release emergencies, the decision to evacuate or shelter-in-place will based on limited information. In many cases, that decision will need to be made quickly.


In summary, evacuation is the safest option in hurricane and flood situations if time permits. When making a decision on whether to evacuate or shelter in place during a chemical or radiation release, a large amount of information must be processed in a very short period of time. The rapid time of onset that characterizes many radiation and chemical emergencies will make these decisions much more complex, and in certain situations, evacuation may not be the best available option.

Dr. Charles Richard Yard currently works for the State of Tennessee Department of Environment and Conservation specializing in the areas of radiation protection, emergency response planning, nuclear materials management, facility safety, and transportation. He received the degrees of Master of Public Health (Occupational Health and Safety) and Doctor of Philosophy (Health and Safety Sciences) from the University of Tennessee in Knoxville. email This email address is being protected from spambots. You need JavaScript enabled to view it..