A “dirty” bomb, or radiation dispersal
device (RDD), is a conventional, explosive
bomb to which radioactive material
has been added. The blast of the weapon
not only kills and injures directly, but
also spreads the radioactive material to
the surrounding area and via airborne
spread. The size and sophistication of
the bomb, the type of radioactive material
used, and weather conditions dictate
the extent of the contaminated area,
while the speed of evacuation dictates
the level of human exposure.
The real threat of a RDD is one of fear
and disruption. The immediate casualties
would be those of the initial blast,
but panic over potential radiation exposure
could cause additional victims and
disrupt rescue and evacuation efforts.
The area’s remaining off-limits for
several months of expensive clean up,
possibly including building demolition
and soil replacement, would cause further
disruption.
The most likely radioactive materials
to be used are cobalt-60, strontium-90,
cesium-137 and americium-241, which
are often poorly protected and readily
available from military, medical, academic,
research, and industrial sources.
As an example, cobalt-60 is used in food
irradiation, while americium is used in
smoke detectors and oil exploration.
These materials are already believed to
be in the possession of major international
terrorist groups. Military-grade
plutonium and uranium would be more
deadly, but are signifi cantly harder to
obtain, handle and safely transport.
[For information on these two isotopes,
as well as radioactive iodine, see the
“Nuclear Blast Fact Sheet.”]
Health Risks
Other than the trauma associated with being caught in the explosion itself, the primary
health risk from a “dirty bomb” is cancer from long-term exposure to residual radiation.
However, the radiation dose from such a bomb is likely to be relatively small. As an
example, a bomb with a radioactive cobalt-60 rod used for food irradiation would
deliver an average dose of a few tenths of a rem (see Units of Radiation) for people
within a half-mile radius. The average person receives 0.3-0.4 rem per year from
natural radioactive sources, and 5 rem is both OSHA’s and the NRC’s annual dose
limit for nuclear and radiation workers. At such low doses it is impractical, if not
impossible, to calculate long-term cancer risks, and both the Health Physics Society
and International Council on Radiation Protection recommend against quantitative
estimation of health risks below an individual annual dose of 5 rem, or a lifetime dose
of 10 rem, above that of background radiation. These groups cite evidence that cancer
risks from radiation exposure do not follow the linear, no threshold hypothesis used by
the U.S. EPA. Instead, there appears to be a threshold (in excess of 10 rem delivered
at high dose rates) above which the risk of cancer develops. In addition, biological
mechanisms, including cellular repair of radiation injury, which are not considered by
the linear, no-threshold model, decrease the chance of cancers and genetic effects.
[Note: The EPA, Federation of American Scientists, and others support the linear, no
threshold model of radiation exposure. This view presents a worst-case perspective
in which any exposure at all to radiation can cause cumulative biological damage,
with less damage occurring at lower doses and more damage at higher doses along a
linear progression. These projections lead to speculation that a RDD using cobalt-60,
exploded at the southern tip of Manhattan under the right weather conditions, would
render all of Manhattan uninhabitable until razed and rebuilt at a cost of 2 trillion
dollars. Naturally, the media has picked this scenario to promote, adding to potential
for panic and disruption.]
From a statistical perspective, for radiation, tumor induction is the most important
long-term sequelae for a dose of less than 100 rem. These statistics, however, are
extrapolations from known data on exposures greater than 100 rem, and, as stated
above, such extrapolations are questionable. There is, however, reliable data showing
that exposure to 10 rem causes a 0.8% increase in the lifetime risk of death from cancer.
Thus, out of 5000 people with such an exposure, 40 may develop a fatal cancer.
Other known potential sequelae of radiation exposure, including cataract formation,
decreased fertility and fetal teratogenesis, are unlikely to occur following the explosion
of a RDD.

Treatment
Inhaled particles less than 5 microns in size will end up in the alveolar area, while the
mucociliary apparatus will clear larger particles. Soluble particles are then directly
absorbed into the blood stream or moved into the lymphatic system. Insoluble
particles, until cleared, will continue to irradiate surrounding tissues. In the alveoli,
the localized infl ammatory response can produce fi brosis and scarring.
Absorption of ingested radioactive material depends on the solubility and chemical
makeup of the contaminant. For example, cesium is rapidly absorbed; cobalt, radium,
and strontium are not. The target organ for ingested radionuclides that pass unchanged
in the feces is the lower GI tract. Gastric lavage and emetics can help empty the
stomach promptly, while purgatives, laxatives, and enemas can reduce radioactive
materials in the colon. Ion exchange resins limit gastrointestinal uptake of ingested or
inhaled radionuclides. Prussian blue (Ferric ferrocyanide, an investigational new drug
from Oak Ridge Affi liated Universities, Oak Ridge, TN) and alginates have been used
in humans to accelerate fecal excretion of cesium-137.
The skin is impermeable to most radionuclides, but wounds and burns allow particulate
contamination to bypass the epithelium. Also, fl uid in the wound may hide weak beta
and alpha emissions from detectors. Because of this, all contaminated wounds must
be meticulously cleaned and debrided.