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exposure of miners is commonly expressed in the unit Working Level Month (WLM). One WLM
corresponds to exposure to a concentration of 1 WL for the reference period of 170 hours, or more
generally
WLM = concentration (WL) x exposure time (months) (one “month” = 170 working hours).
D.3.2 Dosimetry Models
Dosimetry models are used to estimate the dose from internally deposited to radioactive substances. The
models for internal dosimetry consider the amount of radionuclides entering the body,
the factors
affecting their movement or transport through the body, distribution and retention of radionuclides in the
body, and the energy deposited in organs and tissues from the radiation that is emitted during spontaneous
decay processes. The dose pattern for radioactive materials in the body may be strongly influenced by the
route of entry of the material. For industrial workers, inhalation of radioactive particles with pulmonary
deposition and puncture wounds with subcutaneous deposition have been the most frequent.
The general
population has been exposed via ingestion and inhalation of low levels of naturally occurring
radionuclides as well as radionuclides from nuclear weapons testing.
The models for external dosimetry consider only the photon doses (and neutron doses, where applicable)
to organs of individuals who are immersed in air or are exposed to a contaminated object.
D.3.2.1 Ingestion.
Ingestion of radioactive materials is most likely to occur from contaminated
foodstuffs or water or eventual ingestion of inhaled compounds initially deposited in the lung. Ingestion
of radioactive material may result in toxic effects as a result of either absorption of the radionuclide or
irradiation of the gastrointestinal tract during passage through the tract, or a combination of both. The
fraction of a radioactive material absorbed from the gastrointestinal tract is variable, depending on the
specific element, the physical and chemical form of the material ingested, and the diet, as well as some
other metabolic and physiological factors. The absorption of some
elements is influenced by age, usually
with higher absorption in the very young.
D.3.2.2 Inhalation.
The inhalation route of exposure has long been recognized as being a major
portal of entry for both nonradioactive and radioactive materials. The deposition of particles within the
lung is largely dependent upon the size of the particles being inhaled. After the particle is deposited, the
retention will depend upon the physical and chemical properties of the dust and the physiological status of
the lung. The retention of the particle in the lung depends on the location of deposition, in addition to the
physical and chemical properties of the particles. The converse of pulmonary retention is pulmonary
clearance. There are three distinct mechanisms of clearance which operate simultaneously. Ciliary
clearance acts only in the upper respiratory tract. The second and third mechanisms
act mainly in the
deep respiratory tract. These are phagocytosis and absorption. Phagocytosis is the engulfing of foreign
bodies by alveolar macrophages and their subsequent removal either up the ciliary "escalator" or by
entrance into the lymphatic system. Some inhaled soluble particles are absorbed into the blood and
translocated to other organs and tissues.
D.3.3 Internal Emitters
An internal emitter is a radionuclide that is inside the body. The absorbed dose from internally deposited
radioisotopes depends on the energy absorbed per unit tissue by the irradiated tissue. For a radioisotope
distributed uniformly throughout an infinitely large medium, the concentration of absorbed energy must
be equal to the concentration of energy emitted by the isotope. An infinitely large medium may be
approximated by a tissue mass whose dimensions exceed the range of the particle.
All alpha and most
beta radiation will be absorbed in the organ (or tissue) of reference. Gamma-emitting isotope emissions
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are penetrating radiation, and a substantial fraction of gamma energy may be absorbed in tissue. The dose
to an organ or tissue is a function of the effective retention half-time, the energy released in the tissue, the
amount of radioactivity initially introduced, and the mass of the organ or tissue.
D.4 BIOLOGICAL EFFECTS OF RADIATION
When biological material is exposed to ionizing radiation, a chain of cellular events occurs as the ionizing
particle passes through the biological material. A number of theories have been proposed to describe the
interaction of radiation with biologically important molecules in cells and to explain the resulting damage
to biological systems from those interactions. Many factors may modify the response of a living
organism to a given dose of radiation. Factors related to the exposure include the dose rate,
the energy of
the radiation, and the temporal pattern of the exposure. Biological considerations include factors such as
species, age, sex, and the portion of the body exposed. Several excellent reviews of the biological effects
of radiation have been published, and the reader is referred to these for a more in-depth discussion
(Brodsky 1996; Hobbs and McClellan 1986; ICRP 1984; Mettler and Moseley 1985; Rubin and Casarett
1968).
D.4.1 Radiation Effects at the Cellular Level
According to Mettler and Moseley (1985), at acute doses up to 10 rad (100 mGy),
single strand breaks in
DNA may be produced. These single strand breaks may be repaired rapidly. With doses in the range of
50–500 rad (0.5–5 Gy), irreparable double-stranded DNA breaks are likely, resulting in cellular
reproductive death after one or more divisions of the irradiated parent cell. At large doses of radiation,
usually greater than 500 rad (5 Gy), direct cell death before division (interphase death) may occur from
the direct interaction of free-radicals with essentially cellular macromolecules. Morphological changes at
the cellular level, the severity of which are dose-dependent, may also be observed.
The sensitivity of various cell types varies. According to the Bergonie-Tribondeau law, the sensitivity of
cell lines is directly proportional to their mitotic rate and inversely proportional to
the degree of
differentiation (Mettler and Moseley 1985). Rubin and Casarett (1968) devised a classification system
that categorized cells according to type, function, and mitotic activity. The categories range from the
most sensitive type, "vegetative intermitotic cells, " found in the stem cells of the bone marrow and the
gastrointestinal tract, to the least sensitive cell type, "fixed postmitotic cells," found in striated muscles or
long-lived neural tissues.
Cellular changes may result in cell death, which if extensive, may produce irreversible damage to an
organ or tissue or may result in the death of the individual.
If the cell recovers, altered metabolism and
function may still occur, which may be repaired or may result in the manifestation of clinical symptoms.
These changes may also be expressed at a later time as tumors or cellular mutations, which may result in
abnormal tissue.
D.4.2 Radiation Effects at the Organ Level
In most organs and tissues the injury and the underlying mechanism for that injury are complex and may
involve a combination of events. The extent and severity of this tissue injury are dependent upon the
radiosensitivity of the various cell types in that organ system. Rubin and Casarett (1968) describe and
schematically display the events following radiation in several organ system types. These include: a
rapid renewal system, such as the gastrointestinal mucosa; a slow renewal system, such as the pulmonary
epithelium; and a
nonrenewal system, such as neural or muscle tissue. In the rapid renewal system, organ
injury results from the direct destruction of highly radiosensitive cells, such as the stem cells in the bone
marrow. Injury may also result from constriction of the microcirculation and from edema and