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Radionuclides that distribute to the whole-body include sodium (Na)-24, Cs-137 (which
mimics potassium) and tritium (which is incorporated into water). Bone is the critical
organ for radioactive minerals that the body uses as calcium. Similarly, radioactive
iodides (e.g., I-131) are rapidly concentrated in the thyroid gland. Ultimately, these toxins
will be eliminated through the body’s normal excretory mechanisms.
Life Cycle of Internal Contamination
The life cycle of a radioisotope consists of its several stages of transit through the body:
• Intake.
• Uptake.
• Deposition.
• Elimination.
Intake. Radioactive materials can enter the body through inhalation, ingestion, and skin
penetration.
Inhalation pathway. Inhalation is the most efficient route of absorption for most toxins,
including radionuclides in insoluble particulate aerosols. Large particles are deposited
only in the upper airways, from which the mucociliary system clears insoluble particles.
The “respirable fraction” is 1–5 microns in size, which is the size associated with
efficient deposition in the terminal bronchioles and alveoli. The most hazardous
biowarfare agents (e.g., anthrax spores) and industrial dusts (e.g., silica) are in this size
range.
Insoluble radioactive particles continue to irradiate surrounding tissues until cleared from
the respiratory tract. Soluble isotopes are absorbed quickly and completely from the
entire respiratory tract. Insoluble particles are cleared from the upper bronchi within 1
hour, before much injury occurs. The clearance time from the alveoli can be 100 to 1,500
days or more for some compounds. Radioisotopes deposited within alveoli are cleared to
regional lymph nodes, where they may remain or be transported systemically. Alpha
radiation is the most damaging to alveolar tissue, causing fibrosis and scarring to a
spherical volume of tissue around the particle, in addition to any local inflammatory
response to the foreign particles.
Ingestion pathway. Swallowed radioactive material enters the digestive tract and is
handled like any other ingested material. This is true for material originating from
contaminated food or water, as well as for material cleared from the respiratory tract. As
with inhalation, absorption from the GI tract depends on the isotope chemistry, including
solubility. Most radionuclides are insoluble, with GI absorption