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Using Buildings as Shelters Against Fallout

Quickly going inside and staying inside the nearest and most protective building in order to minimize exposure to radioactive fallout is the most critical lifesaving action for the public after a nuclear detonation.1,2 This listing of frequently asked questions (FAQ) is a tool to aid building owners, operators, and occupants in judging a structure’s ability to minimize fallout exposure and in identifying the best areas within a building to shelter. This FAQ is useful to commercial building owners and operators, apartment complex managers, safety officers for businesses and schools, neighborhood associations, individual homeowners, and apartment dwellers. The principal goals of this FAQ are to raise people’s general awareness of the protective qualities of the spaces around them and to encourage individuals who oversee large properties to serve as key community resources and educators on fallout protection.

How does a building protect people from radiation? Radiation from dangerous fallout can be blocked by dense material such as earth, cement, and concrete, and it can be reduced by increasing a person’s distance from deposited fallout.2 The more dense the material that separates people from radioactive fallout, the more protected they are from radiation. Dense materials such as brick, cement, and earth provide better protection than wood, drywall, and thin sheet metal. Similarly, the further a person is away from where fallout has settled on and around buildings, the more protected s/he is. A large concrete and steel building is doubly protective, for example, because its dense materials block radiation well, and because people can move to the core of the building, creating greater distance from the radioactive fallout outside.

Materials that provide best (1) to least (5) protection from radioactive fallout:3

  1. Lead
  2. Steel
  3. Concrete
  4. Earth
  5. Wood

Is it more important to know where to shelter at home or at the workplace? A nuclear detonation could occur at any time. It is important for people to know where the best places are around them to take immediate shelter—both during the day and at night. People are encouraged to be aware of their surroundings, creating a mental map of sheltering spaces around them—whether they are at home, at work, in a mall, or on the road. Individuals who hold authority over large properties are encouraged to assist tenants in identifying areas to shelter (see below).

What buildings offer the best protection? What places within buildings offer the best protection? Both the density of building materials and the distance from fallout affect how much a building can protect occupants from radiation. The best areas to shelter are deep inside buildings made of dense material, far away from fallout that has settled outside. Areas within a building, such as restrooms and stairwell cores, which are distant from deposited radioactive fallout, provide better protection than those close to roofs, windows, and exterior walls. Multistory brick or concrete structures, cores of large office buildings, multistory shopping malls, basements, tunnels, subways, and other underground areas are examples of good shelters. Poor shelters have little material to block radiation and provide little distance from fallout that has settled on rooftops and the ground outside. Poor shelters include outdoor areas, cars and other vehicles, mobile homes, single-story wood-frame houses, strip malls, and other single-story light structures.4

What do scientists mean when they refer to “protection factor” (PF?) The protection factor for radiation is similar in concept to the sun protection factor (SPF) values used in sunscreens.4 That is, the higher a shelter’s protection factor, the greater the ability of that shelter to minimize exposure to radioactive fallout. Scientists assign PF values to buildings based on information about how radiation reduces in intensity as it moves through walls, floors, and over distance. This information was originally derived from mathematical and computer calculations on radiation penetration as well as small-scale models and full-scale experiments with radioactive material.5 Some of the research used to calculate PF was completed during the Cold War, but more advanced mathematical modeling has recently improved guidance on shelters, including information in the image below:

Sample protection factors for a variety of building types and locations:1  Shelters with a higher PF are made of more dense material and place people farther away from radiation sources, reducing the radiation exposure of those inside. To understand how PF corresponds to the reduction in radiation exposure, first assume that a person outside in the dangerous fallout zone for an hour immediately after the detonation would receive a potentially lethal dose of 500 rem (a “rem” is a unit of measurement for radiation). In contrast, a person in a nearby shelter with a protection factor of 10 would receive just 1/10 of the radiation, or 50 rem, in the same amount of time. The PF represents the fraction of radiation received by people sheltering, compared to full exposure outside the shelter.

Does a building have to be airtight to keep out fallout? No. Federal guidance has stated that buildings do not have to be airtight to protect against fallout and that broken windows will not greatly reduce the protection offered by a shelter.1 Nonetheless, building owners, managers, maintenance engineers, safety officers, building emergency coordinators, and others can take steps to “harden” buildings to prevent airborne hazards such as fallout from entering buildings. The county of Los Angeles has developed a checklist that may help prevent dangerous fallout from entering buildings.6 The prompt shutdown and isolation of air movement in large buildings could protect building occupants from fallout introduced through the ventilation system. Additionally, high-quality filters, properly maintained, could remove significant amounts of airborne fallout, especially the largest and most dangerous particles.4 Managers assessing the need to harden a building should have an understanding of critical systems, such as ventilation systems, and how they may move contamination through the building. Additionally, an engineering analysis of the ventilation system may need to be performed. In the case of individual householders, Los Angeles County has recommended that occupants close the doors and windows and turn off fans that bring in air from the outside.4 In-room fans that only recirculate air can still be used, as can heating or air-conditioning systems that do not bring in air from the outside.

What supplies does someone need to be able to shelter-in-place? Fallout preparedness hinges on a very familiar formula for disaster readiness: the ability to shelter-in-place. For effective sheltering, people should equip their emergency kit at home and at work with adequate food, water, and medications; a battery-powered or hand-crank radio; and other supplies. Emergency professionals recommend that if you have not yet set aside anything, begin with a 3-days’ supply and work your way up to 1 week and then 2 weeks as you adopt a preparedness lifestyle. Larger stockpiles provide you more flexibility across unpredictable events. For more detailed instructions on gathering supplies to support sheltering-in-place, individuals and families are encouraged to consult the and American Red Cross websites on basic emergency provisions. Businesses, schools, and organizations are similarly encouraged to adopt the Red Cross Ready Rating Program ( to evaluate and improve institutional preparedness, which includes acquiring and maintaining adequate emergency supplies.

What steps can building managers and safety officers take to help protect tenants and employees? People who oversee large properties can help raise the awareness of occupants about how and where a building or building complex can best protect them in the event of a nuclear detonation. Building managers can bring this FAQ to the attention of their tenants (whether businesses, organizations, or individual residents). Moreover, building managers can work with the safety officers of tenant organizations to determine the best areas for occupants to shelter, combining an understanding of the protection factor of building materials with knowledge of building codes and structures as well as ventilation systems. Tenant organizations can educate employees about sheltering, designate sheltering locations within buildings, post signs, and incorporate this information into disaster plans. Operators of large buildings with a high protection factor should also consider how to facilitate the availability of shelter to members of the public seeking refuge following a detonation.


  1. Buddemeier BR, Dillon MB. Key Response Planning Factors for the Aftermath of Nuclear Terrorism. LLNL‐TR‐410067. Livermore, CA: Lawrence Livermore National Laboratory; August 2009.

  2. National Security Staff Interagency Policy Coordination Subcommittee for Preparedness and Response to Radiological and Nuclear Threats. Planning Guidance for Response to a Nuclear Detonation. 2d edition; 2010. Accessed August 5, 2011.

  3. Ferlic KP. Fallout: Its Characteristics and Management. Armed Forces Radiobiology Research Institute Technical Report AFRRI TR83-5. Bethesda, MD; December 1983. Accessed August 6, 2011.

  4. National Council on Radiation Protection and Measurements (NCRP). Responding to a Radiological or Nuclear Terrorism Incident: A Guide for Decision Makers. NCRP report no. 165. Bethesda, MD: NCRP; 2010.

  5. What the planner needs to know about fallout. In: Department of Defense. DCPA Attack Environment Manual. June 1973. Accessed June 21, 2011.

  6. Building Assessment Checklist: Protection against Airborne Hazards. County of Los Angeles Public Health. August 2006.