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THE
ORCBS > Radiation
Safety
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Safety Manual
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 External
and Internal Radiation Exposures
There are two potential
primary exposure types connected with work involving radioisotopes:
external and internal exposure to radiation. Each must be carefully
evaluated prior to working with radioactive materials, and precautions
must be taken to prevent these exposures.
External
Radiation Exposure
External hazards arise when radiation from a source external to
the body penetrates the body and causes a dose of ionizing radiation.
These exposures can be from gamma or x-rays, neutrons, alpha particles
or beta particles; they are dependent upon both the type and energy
of the radiation.
Most beta particles
do not normally penetrate beyond the skin, but when sufficiently
intense, can cause skin and/or eye damage. Very energetic beta particles,
such as those emitted by 32P, can penetrate several millimeters
into the skin. Shielding is needed in order to reduce the external
radiation exposure. Typically, a maximum of 1/2 inch thick sheet
of Plexiglas is an effective shield for most beta particles.
Alpha particles,
because of higher mass, slower velocity, and greater electrical
charge compared to beta particles, are capable of traveling a few
inches in air and rarely penetrate the outer dead skin layer of
the body. Therefore, alpha particles typically are not an external
radiation hazard.
X and gamma
rays, along with neutron radiation, are very penetrating, and are
of primary importance when evaluating external radiation exposure
and usually must be shielded. The onset of first observable effects
of acute radiation exposure, diminished red blood cell count, may
occur at a dose of approximately 100 rads of acute whole body radiation
exposure. The LD50 for humans (lethal dose where 50%
of the exposed population may die from a one time exposure of the
whole body) is about 500 rads, assuming no medical intervention.
Exposure to
external radiation may be controlled by limiting the working
time in the radiation field, working at a distance from the source
of radiation, inserting shielding between the worker and the source,
and by using no more radioactive material than necessary.
Internal
Radiation Exposure
Radioactive materials may be internally deposited in the body when
an uptake occurs through one of the three routes of entry: inhalation,
ingestion and skin contact. These exposures can occur when
radioactive material is airborne; is inhaled and absorbed by the
lungs and deposited in the body; is present in contaminated food,
drink or other consumable items and is ingested; or is spilled or
aerosolizes onto the skin and absorbed or enters through cuts or
scratches. Internal deposition may also result from contaminated
hands, with subsequent eating or rubbing of eyes.
Internal exposures
arise when radiation is emitted from radioactive materials present
within the body. Although external hazards are primarily caused
by x-rays, gamma rays, high energy betas and neutrons, all forms
of radiation (including low energy betas, gammas and alphas) can
cause internal radiation exposures. Alpha particles create a high
concentration of ions along their path, and can cause severe damage
to internal organs and tissues when they are inhaled, ingested or
are present on the skin. Once these particles get into the body,
damage can occur since there is no protective dead skin layer to
shield the organs and tissues. Internal exposures are not limited
to the intake of large amounts at one time (acute exposure). Chronic
exposure may arise from an accumulation of small amounts of radioactive
materials over a long period of time.
It is known
that many substances taken into the body will accumulate in certain
body organs, called target organs. For example, iodine will accumulate
in the thyroid gland. When iodine is inhaled or ingested, the body
cannot distinguish stable iodine from radioactive iodine; a significant
portion of the inhaled iodine will be deposited in the thyroid gland
within 24 hours.
Other elements,
such as calcium, strontium, radium and plutonium accumulate in the
bones. Here, high doses to bones can occur over very long periods
of time, since the body eliminates these materials very slowly once
they are incorporated into the bone structure. The blood forming
organs, such as the bone marrow, are very radiosensitive, since
bone marrow cells are in the S-phase of mitotic activity more often
than other cells. Hence, if there is a significant long-term exposure
to radioisotopes, chronic diseases such as leukemia and/or osteosarcoma
can occur. The induction time for the onset of these types of diseases
is typically in excess of 20 years.
A rule of thumb
used to assist in biological risk assessment for radiation is the
Law of Bergonie and Tribondeau. It states that most mature cells
are radioresistant; all immature cells are very radiosensitive.
It is very important for radioactive materials users to be aware
of the target organs for the nuclides they handle. Precautions may
then be taken to prevent exposures.
Laws
and Regulations Concerning Radiation
Radiation
Safety Manual Table of Contents
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