A chemical accident that produces a toxic cloud can pose a life-threatening community emergency. Evacuation is one emergency response option, but having the public seek refuge indoors may sometimes be a better way to minimize the risk.
Scarcely a month passes that one does not hear or read about an accident involving the release of a toxic chemical. These accidents can create a toxic vapor cloud that can put a community in the midst of a life-threatening emergency. The tragedy that occurred in Bhopal, India, and other lesser events have spurred efforts within both industry and government to prevent such accidents from occurring. Yet, because we will never be able to prevent them all, efforts are also being made to improve our capabilities to respond to these emergencies when they do occur.
When an accident poses a threat to the general public, local authorities normally initiate emergency response measures in an attempt to minimize the harm. In general, the options they have are few: they can keep members of the public sheltered in homes or other buildings or they can evacuate the public from the affected area. In the United States, those with responsibility for protecting the public during an emergency have traditionally relied upon the use of evacuation during life-threatening emergencies. This strategy is not surprising given the types of incidents they typically have been called on to respond to. Indeed, evacuation probably is the best response for fires and many natural hazards such as floods and earthquakes. But there is increasing evidence that in certain circumstances sheltering members of the public in place may be the better option during a toxic chemical emergency.
Why shelter in place?
Sheltering in place means seeking refuge indoors when a toxic chemical forms a vapor cloud that threatens to engulf an area. The public, once inside, conducts several self-protective measures including closing all windows, doors, and vents, shutting off ventilation systems, and turning to the emergency broadcast system (EBS) on television or radio for further instructions. But the emergency response does not end here. When outdoor concentrations have fallen to safe levels, the public must evacuate and ventilate the building to diminish peoples' exposure. This final step is necessary because some toxic vapors may have infiltrated the structure; once inside, these vapors will take much longer to dissipate than those in the outdoor air. By remaining indoors too long the occupants could receive an unnecessarily high dose.
Sheltering in place provides protection during a toxic chemical emergency because closed structures supply a reservoir of clean air and serve to shield the occupants from direct exposure to the contaminated air outside. Because of infiltration effects, sheltering in place would probably not eliminate the threat entirely, but at least it would reduce the concentrations to which building occupants would be exposed were they to go outside.
There are other advantages to sheltering in place. Because it takes time for authorities to respond to an emergency, sheltering in place can be an effective interim measure while emergency personnel assess the situation, determine the best response actions, and, if necessary, mobilize the resources needed for an evacuation. Furthermore, if the public is indoors and tuned to the EBS, emergency personnel will be able to communicate with them directly. If an evacuation is necessary, they will be able to provide precise, up-to-the-minute instructions via the airwaves, which will greatly facilitate the evacuation.
Sheltering in place is by no means an all-purpose solution. A significant number of chemical emergencies involve a flammable or explosive substance, and the threat that the vapor cloud will ignite or explode may be the most immediate concern. With this type of situation, evacuation may be the only suitable option.
The degree to which sheltering in place provides protection from an airborne toxic chemical depends on a number of factors. The most important is the infiltration rate of the building. The infiltration rate refers to the volume and speed at which outdoor air is exchanged with the indoor air through cracks, vents, and other openings in a building. It is typically measured by the number of air changes per hour (ACH).
In the United States, the range of infiltration rates is large—from 0.2 ACH for highly energy-efficient buildings to 10.0 ACH for drafty buildings with windows and doors open. While infiltration rates can vary enormously from building to building, some generalizations can be made. Buildings in colder climates tend to have lower infiltration rates than those in warmer climates, and older buildings tend to be leakier than newer ones. Meteorological conditions such as wind speed and direction and the differences between indoor and outdoor air temperature will also affect the infiltration rate. Interior rooms may afford better protection than rooms with exterior walls, doors, or windows, and upper floors of a multistory building may provide greater protection than ground floors. The type of chemical is also a factor.
Closing windows and doors and turning off ventilation systems helps to reduce infiltration. Covering cracks around windows and doors with tape or wet rags will also slow infiltration. The degree to which these measures are effective, however, depends on the infiltration rate of the house; greater gains can probably be had in drafty structures with high infiltration rates than in well-weatherproofed ones.
The protectiveness of sheltering in place also depends on the duration and nature of the release. Releases can be long-term and continuous (a slow leak) or short-term and instantaneous (a "puff'). According to one expert, beginning with a typical infiltration rate of 1.0 ACH and a chemical release of 10 minutes' duration, those who shelter themselves in place will receive only about one-tenth of the dose of those who are outdoors and exposed directly to the chemical. Given a puff-type of release of only about one minute's duration, the indoor dose will be approximately one one-hundredth of the outdoor dose.
Automobiles, with windows shut and ventilations systems off, provide some degree of protection from airborne toxic chemicals. However, in general, infiltration rates in stationary autos are high, and the rate increases with the vehicle's speed.
Community preparedness
In a few places around the United States, sheltering in place has already been integrated into the community's emergency response plans. For example, in the area surrounding the Bridesburg chemical complex in Philadelphia, an information sheet prepared by Rohm & Haas and Allied Chemical instructs local residents to seek shelter indoors when they hear a long rotating tone from sirens installed in the area, and then to take self-protective measures. In the Kanawha Valley in West Virginia, which is home to a number of chemical manufacturing facilities, the community interest pages of the local phone book contain the statement that "sheltering in place is a proven, effective emergency protective action which is used when there is insufficient time to evacuate in the event of an airborne hazardous material release."
These examples of emergency planning, however, are exceptions rather than the rule. In both the Bridesburg area and the Kanawha Valley, public concerns prompted the industry and local government to develop plans much more sophisticated than the level of planning found in most areas in the United States. For the rest of the country, the impetus to begin preparing for chemical emergencies has come with the passage of the Emergency Planning and Community Right-to-Know Act in 1986 (see box).
Shelter in place or evacuate?
A community's emergency response plan and the information furnished to the local authorities during an emergency will provide the basis for determining the best response actions. As Title III of the 1986 act requires, emergency response decisions and actions are directed by local authorities. Most likely, the fire and police chiefs will be closely involved in response decisions, though the final decision to shelter in place or evacuate usually rests with an elected official—the mayor, the city manager, or a county executive.
In deciding whether to shelter in place or evacuate, emergency personnel face a difficult task. Important factors they must consider in making this decision include, first, whether there is time to complete an evacuation before the toxic chemical vapors reach populated areas. This determination will require estimates of how long it will take the vapors to arrive and how long it will take to evacuate a threatened area. Second, emergency personnel must determine whether the homes or buildings where the public is sheltered will be protective enough when the toxic vapors reach them. In many cases, emergency personnel are notified when the release has already occurred and there is no time even to begin an evacuation. Under these circumstances, sheltering in place is the only option.
Yet in a situation when there is no time to evacuate and where sheltering in place may not be protective enough, emergency personnel may have to direct the public through the EBS to conduct additional protective actions such as breathing through a cloth; or, under extreme circumstances, they may have to under-take rescue actions and provide sheltered occupants with protective breathing apparatus. In situations where the release has not yet occurred or is small, but where it has the potential to escalate into a large release, emergency personnel will usually opt for evacuation as a precautionary measure, if time permits.
During an actual emergency the best response may not be obvious. No doubt some uncertainties arise in a given situation because the information the emergency personnel receive may be incomplete or even incorrect. Furthermore, there may only be a few minutes to assess the situation and implement a response.
In preparing for a decision to shelter in place or evacuate, there are several steps a community can take. Evacuation times depend on many factors, such as the size and the population density of the area to be evacuated, the time of day, the weather conditions, and the road network—much of which can be assessed before an accident occurs. Planners may also be able to reduce the time needed to implement an evacuation through a good education program and practice drills. Emergency personnel are well aware that a total evacuation can easily take several hours. In comparison, the time it takes to shelter in place will generally be considerably less and, if the public has been educated to recognize warning signals, sheltering in place can begin within minutes.
Growing support
The United States Environmental Protection Agency (EPA), the Federal Emergency Management Agency (FEMA), and other federal agencies are responsible for providing guidance and training to the local emergency planning committees (LEPCs) to assist in developing emergency response plans. Unfortunately, the existing guidelines do not emphasize sheltering in place and as a result it has received little or no attention by LEPCs.
This situation is changing, however. A recent conference sponsored by EPA and FEMA and organized by RFF's Center for Risk Management brought together representatives from all levels of government (including emergency planners and response personnel), industry representatives, and a variety of technical experts on evacuation and sheltering in place. This diverse group reached the consensus that sheltering in place is a viable option for responding to chemical emergencies and should be considered as an alternative to evacuation during chemical emergencies.
The conference also pointed out some of the issues that need to be resolved if sheltering in place is to become a more widely accepted practice. For example, few guidelines exist to help emergency personnel make the important decision between sheltering in place and evacuation. In addition, there is a human factor in implementing emergency response actions: experts are concerned that because of the urge to flee from danger it might be difficult to convince some of the public to accept sheltering in place.
In many emergencies, there may be no time to begin an evacuation.
As one result of the conference, the federal government is considering an expansion of its guidance and training of local authorities to give sheltering in place a more prominent role. The intention is that the sheltering option will become more widely integrated into local emergency response actions.
In spite of the availability of protective response measures, the focus should remain—as always—on preventing accidents involving toxic chemicals from occurring in the first place. Even with the best preventative measures these accidents will continue to occur, but in giving greater attention to mitigating actions, options like sheltering in place may help reduce the harm from such potentially catastrophic events.
Material for this article was drawn from an RFF research project on sheltering in place during chemical emergencies, carried out for EPA's Emergency Preparedness Staff. The project was conducted by Theodore S. Glickman and Alyce M. Ujihara of RFF's Center for Risk Management.
A version of this article appeared in print in the January 1989 issue of Resources magazine.
