What is ionizing radiation?
When radioactive elements decay, they produce energetic emissions (alpha particles, beta particles, or gamma rays) that can cause chemical changes in tissues. The average person in the United States receives a "background" dose of about one-third of a rem* per year—about 80% from natural sources including earth materials and cosmic radiation, and the remaining 20% from man-made radiation sources, such as medical x-rays. There are different types of radioactive materials that emit different kinds of radiation:
Gamma and X-rays can travel long distances in air and can pass through the body exposing internal organs; it is also a concern if gamma-emitting material is ingested or inhaled.
Beta radiation can travel a few yards in the air and in sufficient quantities might cause skin damage; beta-emitting material is an internal hazard if ingested or inhaled.
Alpha radiation travels only an inch or two in the air and cannot even penetrate skin; alpha-emitting material is a hazard if it is ingested or inhaled.
* A rem is a measure of radiation dose, based on the amount of energy absorbed in a mass of tissue. Dose can also be measured in Sieverts (1 Sievert=100 rem).
Immediate Impact to Human Health
Most injuries from a dirty bomb would probably occur from the heat, debris, radiological dust, and force of the conventional explosion used to disperse the radioactive material, affecting only individuals close to the site of the explosion. At the low radiation levels expected from an RDD, the immediate health effects from radiation exposure would likely be minimal.
Health Effects of Radiation Exposure
Health effects of radiation exposure are determined by the:
- Amount of radiation absorbed by the body.
- Radiation type (as detailed in the box to the right)
- Means of exposure—external or internal (absorbed by the skin, inhaled, or ingested).
- Length of time exposed.
The health effects of radiation tend to be directly proportional to radiation dose. If a reasonable estimate can be made of a person's dose, a lot is known about the health effects at that dose.
Acute Radiation Syndrome (ARS)
ARS is not likely to result from a dirty bomb. It is a short-term health effect that begins to appear when individuals are exposed to a highly radioactive material over a relatively small amount of time. This chart shows that an estimated 10% of the population may exhibit signs of ARS if they are exposed to large radiation doses of 100 rems or more. Principal signs and symptoms of ARS are nausea, vomiting, diarrhea, and reduced blood cell counts.
Psychological effects from fear of being exposed may be one of the major consequences of a dirty bomb. Unless information about potential exposure is made available from a credible source, people unsure about their exposure might seek advice from medical centers, complicating the centers' ability to deal with acute injuries.