Skip Navigation

Rad Resilient City


Tenet 1 | Tenet 2 | Tenet 3 | Tenet 4 | Tenet 5 | Checklist

Not all casualties due to a nuclear detonation are destined to happen; those that result from exposure to radioactive fallout can be prevented.

Fallout is made of highly radioactive particles mixed with dust and debris, and it can be spread quickly and widely by upper and lower air patterns.

The highly radioactive particles that make up fallout are generated when vaporized and irradiated earth and debris are drawn upward by the fireball’s heat and combine with the radioactive fission products created by the detonation. This cloud rapidly rises to an altitude of 2-5 miles for a 10-kiloton detonation, where, under some weather conditions, it assumes a mushroom shape.19 As the cloud cools, highly radioactive particles coalesce and fall to earth, with the heaviest and most dangerous falling first. Fallout will likely be visible as ash, rain, or particles the size of sand, but it may be present even if it is not visible. The distribution of fallout is determined primarily by upper level and surface wind patterns, which often travel in different directions from each other. Because wind patterns are so variable, fallout deposition cannot be predicted ahead of time. Even in real time, fallout patterns are difficult to predict because of microclimates, urban canyon effects, and other complications. Hence, actual measurements on the ground should augment plume models.

Fallout poses its greatest health effect in the hours immediately following the detonation, during which time high levels of penetrating radiation can lead to death.

The health hazard associated with fallout comes primarily from the human body’s exposure to penetrating radiation (similar to x-rays) discharged from fallout that has settled on the ground and building roofs. Exposure to high levels of radiation over a short period of time can cause acute radiation syndrome, in which people become very ill or die within minutes to months. The Fallout Preparedness Checklist focuses principally on the goal of saving the most lives in the immediate aftermath of a detonation by reducing the chances that people will develop acute radiation syndrome. This can be achieved if people take prompt protective actions against fallout exposure (Tenet 3 and Tenet 4). Outside the scope of the Fallout Preparedness Checklist is the delayed health effect that comes from exposure to lower doses of radiation over time, namely, an increased chance of developing cancer later in life. This delayed health effect is of secondary concern in the nuclear detonation context. In contrast, planning around nuclear accidents—usually slowly evolving events—focuses primarily on the goal of cancer avoidance by limiting individuals’ level of exposure to radiation to “as low as reasonably achievable.”

The strength of radiation drops sharply over time and distance from the nuclear detonation.

Radiation levels from fallout particles drop off rapidly with the passage of time, with more than half (55%) of the potential exposure occurring in the first hour and 80% occurring within the first day. The most dangerous concentrations of fallout particles (ie, potentially fatal to those outdoors) could extend 10 to 20 miles downwind from ground zero.5 This area is called the dangerous fallout zone (DFZ) (see Figure 1). Larger radioactive particles will settle out within 1-2 hours of the nuclear detonation, leaving behind the DFZ footprint.5 Most people in the DFZ will experience some level of exposure to fallout,7 but a series of decisions regarding shelter and evacuation may vastly reduce their chances of becoming sick or dying from high radiation levels. Outside the DFZ, fallout with lower levels of radiation will be spread up to hundreds of miles away. Radiation levels in this area, known as the radiation caution zone, are not high enough to cause immediate health problems. Nonetheless, protective actions such as sheltering/evacuation, controls on food consumption, and water advisories are warranted to prevent accumulated exposure to radiation that could result in a greater chance of cancer over a lifetime. Figure 2 illustrates how the radiation zones shrink dramatically over time.

Figure 1: Damage and Fallout Zones Modeled for 10-Kiloton Groundbursts

Figure 1

Adapted from Knebel AR, Coleman CN, Cliffer KD, et al. Allocation of scarce resources after a nuclear detonation: setting the context. Disaster Med  Public Health Prep 2011;5(Suppl 1):S20-S31.


Figure 2: Time Sequenced Size of Dangerous Fallout Zone and Radiation Caution Zone (0.01 R/h Boundary) for the 10 KT Groundburst Scenario

Figure 2

Adapted from: 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.

Go to Reference List | Go to Tenet 3