radiationThe emission of energy (as
electromagnetic waves) or matter (as par-
ticles) from unstable atoms, which turns
them into a more stable form. Some
types of radiation are harmful to life;
other types are essential (for example,
light and heat energy radiated from the
sun). Even harmful radiation may be
used for beneficial purposes; for exam-
ple, in treatment byradiotherapy, the
biologically-damaging effects of radia-
tion are used to destroy cancerous cells.
Four significant types of harmful radia-
tion are gamma radiation, X-rays, alpha
particles, and beta particles. Gamma
radiation and X-rays are types of elec-
tromagnetic waves, and are similar to
more energetic forms of light. All 4
types cause damage by ionization – the
waves or particles knock out electrons
from atoms in the matter that they pass
through, turning them into highly reac-
tive ions. In the case of living tissue, the
ions formed cause biological damage.
Radioactive substances that emit any
of these types of radiation constitute a
health hazard. However, alpha particles
cannot penetrate the skin, so sources of
alpha radiation are only dangerous if
ingested or inhaled. Gamma radiation
can travel large distances through many
substances, and even distant gamma
sources can pose a risk to humans.
Most sources of radiation are natural.
Natural sources of ionizing radiation
include cosmic rays from space, and
radioactive minerals. In some areas, the
gas radon, found in soil, rocks, or build-
ing materials, is a major source. Artificial
sources of ionizing radiation include X-
ray machines, radioactive isotopes used
in diagnosis and treatment (see radio-
nuclide scanning), and nuclear reactors.
Less energetic types of radiation, such
as ultraviolet light, may also cause biolog-
ical damage by mechanisms other than
ionization. Ultraviolet radiation from the
sun does not penetrate the body deeply,
but can damage genetic material in
cells and may lead to skin cancer.
Other types of nonionizing radiation to
which people are subjected are ultra-
sound, used in medicine for diagnosis
and treatment, and radio waves that are
generated during MRI. These techniquesrabiesAn acute viral infection of the
nervous system, once known as hydro-
phobia, that primarily affects dogs but
can be transmitted to humans by a bite
or a lickover broken skin. The virus trav-
els to the brain; once symptoms develop,
rabies is usually fatal.
The average incubation periodis 1–3
months, depending on the site of the
bite. The symptoms are slight fever and
headache, leading to restlessness, hyper-
activity, and, in some cases, strange
behaviour, hallucinations, and paralysis.
The victim develops convulsions, arrhy-
thmias, and paralysis of the respiratory
muscles and is often intensely thirsty,
but drinking induces painful spasms of
the throat. Death follows 10–14 days
after the onset of symptoms.
Following an animal bite, immuniza-
tionwith human rabies immunoglobulin
and a course of rabies vaccine is neces-
sary; this may prevent rabies if given
within 2 days. If symptoms appear, they
are treated with sedative drugs and
analgesic drugs. The main emphasis is on
preventing the disease through quaran-
tine regulations and human and animal
immunization. (See also bites, animal.)
rachiticA term used to describe abnor-
malities associated with ricketsor to refer
to people or populations with rickets.
radA unit of absorbed dose of ionizing
radiation (see radiation unit), which has
been superseded by the gray (Gy). “Rad”
stands for radiation absorbed dose.
radial nerveA branch of the brachial
plexus. The radial nerve, one of the main
nerves of the arm, runs from the shoulder
to the hand. It controls muscles which
straighten the wrist, and conveys sensa-
tion from the back of the forearm; the
thumb, 2nd, and 3rd fingers; and the
base of the thumb. The nerve may be
damaged by a fracture of the humerus
or by persistent pressure on the armpit.RABIES RADIATIONR