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Safety Manual
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 Biological
Effects of Ionizing Radiation
Injury due to irradiation
is caused mainly by ionization within the tissues of the body. When
radiation interacts with a cell, ionizations and excitations are produced
in either biological macromolecules or in the medium in which the
cellular organelles are suspended, predominantly water. Based on the
site of interaction, the radiation-cellular interactions may be termed
as either direct or indirect.
- Direct
action occurs when an ionizing particle interacts with and
is absorbed by a macromolecule in a cell (DNA, RNA, protein, enzymes,
etc.). These macromolecules become abnormal structures which initiate
the events that lead to biological changes.
- Indirect
action involves the absorption of ionizing radiation in the
medium in which the molecules are suspended. The molecule which
most commonly mediates this action is water. Through a complex
set of reactions the ionized water molecules form free radicals
that can cause damage to macromolecules.
The most important
target for radiation in the cell is DNA in the nucleus. Biological
effects result when DNA damage is not repaired or is improperly repaired.
Extensive damage to DNA can lead to cell death. Large numbers of cells
dying can lead to organ failure and death for the individual. Damaged
or improperly repaired DNA may develop into lymphoma and cancers in
somatic cells. Two kinds of effects may result.
- Acute,
or nonstochastic, effects are health effects, the severity
of which varies with the dose and for which a threshold is believed
to exist. Radiation-induced cataract formation is an example of
a nonstochastic effect (also known as a deterministic effect).
- Delayed,
or stochastic, effects, are health effects that occur randomly
and for which the probability of the effect occurring, rather
than the severity, is assumed to be a linear function of the dose
without threshold. Genetic effects and cancer incidence are examples
of stochastic effects.
Tissue
and Cell Sensitivity to Radiation
Radiation
Safety Manual Table of Contents
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