are not thought to have any adverse
side effects. (See also radiation hazards;
radiation sickness;radiation units.)
radiation hazardsHazards from radia-
tionmay arise from external sources of
radiation or from radioactive materials
taken into the body. The effects depend
on the dose, the duration of exposure,
and the organs exposed.
With some forms of radiation, damage
occurs when the radiation dose exceeds
a certain limit, usually 1 sievert (Sv)
(see radiation unit). This damage may
include radiation dermatitis, cataracts,
organ failure (which may occur many
years later), or radiation sickness.
For other types of radiation damage,
the risk that damage will occur increases
with increasing doses of radiation. Can-
cercaused by radiation-induced mutation
is the major example of this type of
damage. Radioactive leaks from nuclear
reactors can cause a rise in mutation
rates, which may lead to an increase in
cancers, such as leukaemias; to birth
defects; and to hereditary diseases. Can-
cer usually develops years after exposure.
Radiation damage can be controlled
by limiting exposure. People exposed to
radiation at work have their exposure
closely monitored to ensure that it does
not exceed safe limits. People of repro-
ductive age or younger should have their
reproductive organs shielded when hav-
ing X-raysor radiotherapy.
There is no evidence of radiation haz-
ards with visual display units (VDUs).
radiation sicknessThe term applied to
the acute effects of ionizing radiation on
the whole, or a major part, of the body
when the dose is greater than 1 gray
(1 Gy) ofX-raysor gamma rays, or 1
sievert (1 Sv) of other types of radiation.
The effect of radiation depends on the
dose and the exposure time. Total-body
doses of less than 2 Gy are unlikely to
be fatal to a healthy adult. At doses of
1–10 Gy, transient nausea and occa-
sional vomiting may occur, but usually
disappear rapidly and are often fol-
lowed by a 2–3 week period of relative
well-being. By the end of this period, the
effects of radiation damage to the bone
marrow and immune system begin to
appear, with repeated infections and
petechiae (pinpoint spots of bleeding
under the skin). Some people are suc-
cessfully treated with a bone marrow
transplantor by isolation in a sterile envi-
ronment until the bone marrow recovers.
With a dose of 10–30 Gy there is also
an early onset of nausea and vomiting,
which tends to disappear a few hours
later. However, damage to the gastroin-
testinal tract, which causes severe and
frequently bloody diarrhoea(called the
gastrointestinal syndrome), and over-
whelming infection due to damage to
the immune system is likely to result in
death 4–14 days after exposure.
Acute exposures of more than 30–100
Gy cause the rapid onset of nausea,
vomiting, anxiety, and disorientation.
Within hours, the victim usually dies due
to nervous systemdamage and oedema
of the brain;these effects are called the
central nervous system syndrome.
radiation unitSeveral different inter-
nationally agreed units (called SI units)
are used to measure ionizing radiation.
For example, the roentgen (R) measures
the amount of radiation in the air, and
the becquerel is the SI unit of sponta-
neous activity of a radioactive source
such as uranium. For medical purposes,
the most commonly used units are the
gray (Gy) and the sievert (Sv).
The gray is the SI unit of radiation that
is actually absorbed by any tissue or
substance as a result of exposure to
radiation. 1 Gy is the absorption of 1
joule of energy (from gamma radiation
or X-rays) per kilogram of irradiated mat-
ter. The gray supersedes an older unit
called the rad (1 Gy = 100 rads).
Because some types of radiation affect
biological organisms more than others,
the sievert is used as a measure of the
impact of an absorbed dose. It uses
additional factors, such as the kind of
radiation and its energy, to quantify the
effects on the body of equivalent
amounts of different types of absorbed
energy. The sievert replaces an older
unit, the rem (1 Sv=100 rems).
radical surgeryExtensive surgery that
is aimed at eliminating a major disease,
usually cancer, by removing affected
tissue and surrounding tissue that
might be diseased.RADIATION HAZARDS RADICAL SURGERY
R