Scientific Background
Radioactivity and Radiation
The number of protons in the nucleus of a given element is always the same; a carbon atom, for example, always contains six protons. Individual atoms of a given element, though, can have different atomic masses because they contain different numbers of neutrons; atoms of a single element with different atomic masses are called isotopes. Most isotopes are stable, and remain in one form indefinitely. Radioactive isotopes, however, are unstable and more easily break down into other elements.
When a radioactive atom decays, it releases radiation. This radiation can be in the form of electromagnetic radiation, typically high-energy X-rays or even higher-energy gamma rays, or it can be in the form of particles, typically either alpha particles (composed of two neutrons and two protons) or beta particles (an electron or positron). Neutrons, which are uncharged particles, are produced in the decay of very heavy radioactive isotopes, such as uranium-235.
Radiation released from radioactive material is ionizing, which means that it can strip electrons from compounds with which it interacts, including living tissue. The more atoms in a given mass of material that decay each second, the higher the material’s radioactivity; the unit of radioactivity is the Curie, equivalent to 37 billion atomic disintegrations per second.
Radiation Dose
Ionizing radiation can damage living tissue with which it interacts, especially by disrupting cellular DNA. Radiation dose is measured in terms of how much energy is absorbed per kilogram of material, given by the scientific unit called the Gray (Gy). The equivalent dose includes an adjustment of the absorbed dose to reflect differences in the relative harm different kinds of radiation do to biological tissue. Alpha particles, for example, are weighted by a factor of 20 over gamma rays. Equivalent dose is measured in the unit called the Sievert (Sv). A third dose measurement, called the effective dose, is further adjusted based on the sensitivity of the specific tissue that received the radiation; effective dose is also given in Sieverts. Older terms corresponding to the Gray and the Sievert are the RAD and the REM, respectively; 100 RAD equals 1 Gray and 100 REM equals 1 Sievert.
Many factors affect the extent of injury a person might receive from exposure to a radioactive material, such as the type of radiation and whether the radiation is localized to a specific part of the body. The length of time over which the dose is received is also significant. Exposure to a large amount of radiation over a short time is generally more harmful than exposure to a smaller amount over a longer time, even if the total dose is the same.
The physical state of the material can be significant as well. Because alpha particles cannot penetrate clothing, the outermost layer of dead skin, or a few inches of air, alpha-emitting radioactive materials do little damage if they remain outside the body. If these materials are ingested or inhaled, or enter through a wound, they can quickly deliver a highly significant dose of radiation. Of note, a finely powdered radioactive material that is readily inhaled or ingested, or spread over a wide area of skin can be far more dangerous than if delivered in a form that results in only a localized exposure. X-ray, gamma, and neutron radiation readily penetrate tissue and the dose at a given depth depends on the energy of the radiation and length of exposure. Finally, some radioactive elements tend to concentrate in certain tissues and can be difficult to remove; radioactive strontium, for example, concentrates in bone and radioactive iodine collects in the thyroid gland.
Health Effects of Ionizing Radiation
The effects of external radiation on the body may appear within minutes or develop many years after exposure; higher doses produce symptoms more quickly. In the case of whole-body exposure to high doses of radiation, doses greater than 1 Gray (1 Gy) can result in early, transient nausea and vomiting. At doses between approximately 1 Gy and 6 Gy, damage to the hematopoietic system will result in immunosuppression and infection, and bleeding and anemia will begin weeks after exposure. Without appropriate therapy, death may result within 60 days. Appropriate medical care could enable the majority of patients to survive. At doses higher than 6 Gy, significant damage to the gastrointestinal tract might result in prolonged severe nausea, vomiting, diarrhea, ulceration of the intestinal mucosa, and systemic infection leading to sepsis. Death may occur within the first 2 weeks and most victims will die within 60 days. At very high doses of more than 20 Gy the central nervous and cardiovascular systems will be acutely damaged and no known medical interventions can prevent death, which may occur within 2 days. Doses of radiation lower than approximately 1 Gy would produce no short-term effects, although symptoms may appear within weeks or months of exposure. Very low doses are unlikely to produce any clinically important symptoms other than late-presenting symptoms related to an increased risk of cancer.
Radiological Threat Scenarios
Terrorists might use radiation and radioactive materials in an attack in many different ways, with outcomes that vary widely in severity. Terrorists could conceal a gamma or X-ray emitter in a public place to expose people to radiation, or place radioactive materials in food or water supplies. However, these actions would be unlikely to cause large numbers of medically significant exposures. Explosion of a Radiation Dispersal Devise (RDD) would spread radioactive material over a wide area. The threat from such a device would depend on the material used; in most cases, it is believed that few people would receive a high dose of radiation, although many people could be contaminated internally or externally. Terrorists might also attempt to attack a nuclear reactor or high-level waste repository in order to release radioactive material and contaminate a large geographic area.
By far the worst scenario would be the detonation of a nuclear explosive device. To accomplish such an act, terrorists would have to obtain an already manufactured military weapon or enough fissile material—either plutonium or highly enriched uranium—to make an "improvised nuclear device." A nuclear explosion releases heat energy sufficient to destroy large portions of a city plus an immediate burst of gamma radiation; highly radioactive products of the fission reaction would also spread over a large geographic area as "fallout" that would continue to expose survivors.
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