occur with any type of matter, whether living or nonliving; but in a living cell, the ionized
or excited atoms and molecules may be highly reactive chemically. Under these
circumstances, secondary reactions will occur, resulting in changes in cellular structure,
damage to essential constituents, and observable biological injury.
c. In addition to the direct action of radiation, molecules damaged by radiation
can also produce cellular injury. In fact, the formation of free radicals from water is the
primary means of cell injury by ionizing radiation. The most frequently formed water
radical, the hydroxyl radical attacks neighboring molecules that are important for
homeostasis. The resulting damage to the genetic material of the cell is considered to
be the major cause of cell death.
d. It is the combination of these primary and secondary reactions that results in
acute and chronic radiation injury. The interrelationship of causes and effects is
extremely complex and is dependent not only on the energy of the radiation, but also on
the total dose, dose rate, presence of oxygen, sex, nutritional status, and other
physiological factors which affect the body.
e. These factors in combination lead to the observation that the cells of the body
which seem most radiosensitive are those which reproduce most rapidly and are in a
state of high-metabolic activity. Regions of the body such as blood-forming organs,
gonads, and hair follicles show injury at much lower dosages than slow or
nonreproducing tissues such as nerve tissue.
1-17. ACUTE EFFECTS
a. The effects of a radiation exposure may be grouped into two categories--
those appearing within days or weeks and those developing over a period of months to
years. Effects appearing early result from massive cell-killing and as a group are
associated into the acute radiation syndrome. In man, this rapid depletion of
radiosensitive cells produces some effects within hours of the exposure and is usually
resolved during the first month after the exposure. A convenient method of describing
these effects is through the time course of their development.
b. Prodrome. Occurring in the first 24 to 48 hours after an exposure to ionizing
radiation, nausea, vomiting, fatigue, and malaise are symptoms frequently expressed by
the victim. As a group, these symptoms are rarely observed in individuals exposed to
less than 50 rems but are almost always seen in those exposed to a dose greater than
200 rems. During this period of prodrome, the blood count will change in response to
the radiation dose, the severity of the change depends on the actual dose. Fifty rems
are considered a minimum dose for the appearance of changes in the blood count.
c. Acute radiation syndrome. After the prodromal period and a period of latency
whose duration will depend upon the dose, occurs the period of manifest illness.
Beginning toward the end of the first week following the exposure, a person receiving a
modest dose of ionizing radiation will experience some continued nausea, diarrhea,