RADIATION
SAFETY TRAINING MANUAL
CHAPTER 5
SAFETY HAZARDS ASSOCIATED WITH COMMONLY USED
RADIONUCLIDES
CHAPTER
5 Table Of Contents
A. INTERNAL RADIONUCLIDE
HAZARDS
- 1. INHALATION
2. INGESTION
3. ABSORPTION
4. PUNCTURE
B. EXTERNAL EXPOSURE TO
RADIONUCLIDES
- 1. RADIONUCLIDES
ON THE SKIN
- TABLE 5.1
2. EXTERNAL
SOURCE OF BETA-EMITTERS
3. EXTERNAL
SOURCE OF GAMMA-EMITTERS
TABLE 5.2
TABLE 5.3
4. RADIATION
EXPOSURE FROM STORED RADIONUCLIDES
TABLE 5.4
Working with
radioactive materials involves some potential
risks. Therefore, precautionary measures must be
taken to ensure the safety of personnel working
with such materials as well as the public. The
following summarizes the procedures and methods
used in protection against internal or external
radiation hazards.
The hazard from
radionuclides can be divided into internal and
external exposures. The severity of the hazard
depends on a number of factors including the
energy of the radionuclide, the type of radiation
emission (e.g. beta or gamma radiation), the
activity, and the chemical form.
A. INTERNAL RADIONUCLIDE HAZARDS
The possibility of
a radioisotope inadvertently entering the body
exists. Once this occurs, the protection
techniques are somewhat limited; so emphasis has
to be placed on preventing radionuclides from
entering the body. The possible pathways into the
body are inhalation, ingestion, absorption, and
puncture.
1. INHALATION
Airborne
radioactive materials can enter the body through
inhalation. In biomedical applications, this is
not a major problem since most isotopes are used
as bound chemicals. However, the use of HTO
(tritiated water), 35S in some
labeling reactions, and Na125I can
create potential problems. Adequate protection
can be obtained by performing operations
involving potential airborne radioactivity in
approved fume hoods. These hoods are designed to
maintain a negative air flow and have a face
velocity of at least 100 linear feet per minute.
The air from these hoods is vented to the
outside. The evaluation by the Radiation Safety
Office of the procedure ensures that the air flow
is sufficient to keep environmental
concentrations well within acceptable limits.
Some sterile hoods are not suitable for use with
volatile radioisotopes because they operate under
a positive pressure.
2. INGESTION
Ingestion is
possible when unsealed sources of radioactive
materials are used. Ingestion can arise from
direct consumption of a radionuclide (!), by
placing contaminated fingers in or close to the
mouth, or from the consumption of contaminated
food. Food can be contaminated by coming in
contact with the radionuclides or with other
contaminated items such as plates, utensils, or
even hands. This potential can be eliminated by
following the guidelines listed below.
a.
Do not store food or beverages where
radioactive materials or contaminated items
are stored or used.
b.
Do not eat, drink, smoke, or apply cosmetics
in areas where radioactive materials are
used.
c.
Wear disposable gloves when handling
radioisotopes or contaminated articles.
d.
Label all containers used for radioisotopes
or contaminated items.
e.
Segregate and clearly label radioactive or
non-radioactive waste.
Table 5.1 gives
the Annual Limit of Intake (ALI) of radionuclides
that an individual may ingest without exceeding a
body dose of 5000 mrem/yr, a bone surface dose of
50 rem/yr, and a thyroid dose of 15 rem/yr. As a
simple rule of thumb; the more energetic the
particle, the less that can be ingested. Finally,
iodide uptake is clearly the major hazard which
is why bioassays are required of users of 131I
and 125I.
3. ABSORPTION
Disposable gloves
should be worn because some radionuclides can be
absorbed through the skin.
4. PUNCTURE
Radionuclides can
also enter the body via punctures in the skin so
it is important that sharp objects which could be
contaminated with radionuclides be handled with
utmost care. Any wounds received while working
with radioisotopes should be checked for possible
contamination immediately.
B. EXTERNAL EXPOSURE TO
RADIONUCLIDES
1. RADIONUCLIDES ON THE
SKIN
A matter of
interest to users of radioactive materials is
having contamination on the skin. Approximately
50% of the ionizing radiations will be captured
by the body. Low energy beta-emitters, such as 3H,
have such a short path length that they do not
penetrate dead skin. Thus, unless the skin is cut
or abraded there is no direct risk from skin
irradiation by 3H. Slightly higher
energy emitters such as 14C, 35S,
and 45Ca pose a slight risk since only
10-40% can cross the dead layer of skin.
In Table 5.2 the
properties of the common beta-emitters are
listed. The estimated dose in mrad/hr is
calculated for a situation in which one uCi of
radionuclide is deposited on one square cm of
skin. Note that these calculations only give the
radiation dose to which the basal cells are
subjected. High energy emitters can damage
internal organs. How long would it take to reach
the yearly limit if the radionuclide remained on
the skin?
The
yearly limits allowed by the State are:
| |
Rems/year |
| Whole
body |
5 |
| Hands,
forearms, feet, ankles |
50 |
| Lens
of eye |
15 |
TABLE 5.1
Annual
Limits of Intakes (ALI) 10CFR20 App B
| Radionuclide
|
Form
|
Target
Organ |
ALI
uCi Ingestion |
| 3H |
Water
|
Total
Body |
80,000 |
| |
5-3H-CdR
|
Hematopoietic,Stem
Cell Nuclei & Spermatogonia
|
|
| |
All
other DNA |
Hematopoietic,
Stem Cell |
|
| |
&
RNA & RNA |
Nuclei
& Spermatogonia |
|
| |
Precursors |
|
|
| |
|
|
|
| 14C |
Soluble
|
Total
Body |
2000 |
| |
Inorganic |
|
|
| |
DNA
Precursors |
|
|
| |
RNA
Precursors |
|
|
| |
|
|
|
| 22Na |
Soluble |
Total
Body |
400 |
| 32P |
Soluble
incl |
Total
Body |
600 |
| |
DNA
Precursors |
|
|
| 35S
|
Soluble |
Total
Body |
10,000 |
| 36Cl
|
Soluble |
Total
Body |
2000 |
| 45Ca |
Soluble |
Bone
surfaces |
30,000 |
| |
|
|
|
| 51Cr |
Soluble |
Total
Body |
40,000 |
| 86Rb
|
Soluble
|
Total
Body |
500 |
| 99Tc |
Soluble
|
Total
Body |
4000 |
| 111In |
Soluble |
Total
Body |
4000 |
| 125I
|
Soluble |
Thyroid |
40 |
If the
radionuclide were on the extremities, the limit
could be reached in about 24 hours; if on skin of
the remainder of the body, 6 hours. If the
contamination is detected, as it should be, the
radionuclide should be removed as soon as
possible. The danger comes from inadvertent
contamination. High energy radionuclides should
be readily detected by lab monitors. Wearing
disposable gloves coupled with careful
surveillance procedures after experiments will
avoid skin contamination.
2. EXTERNAL SOURCE OF
BETA-EMITTERS
The risk to
external exposure from low energy beta-emitters
is small when they are handled away from the
body. Beta- particles have a finite range in air.
Thus, when at least two feet separates the user
from 14C, 35S, and 45Ca,
there is no exposure. If these radionuclides are
contained in a vial, very little radiation will
escape through the walls.
Higher energy
beta-particles such as 32P are a
different case. They have a range of 20 feet in
air. The dose rate from a 1 mCi source of 32P
at 1 cm is 200 rad/hr. As illustrated in Table
5.2, 32P will penetrate about 1 cm
through biological tissue. The major risk from
external exposure is to the eye, a radiosensitive
organ for which the maximum permissible dosage is
15 rem/yr. If the 1 mCi source were held 1 cm
from the eye, the maximum dose would be reached
in 4.5 min. Use of a lucite shield (1 cm thick)
will practically eliminate the exposure hazard.
3. EXTERNAL SOURCE OF
GAMMA-EMITTERS
The exposure rate
for gamma emitters can be calculated for each
radionuclide using the Specific Gamma Ray
Constant which has units of R/hr per mCi at 1 cm.
Data for three of the most commonly used
gamma-emitters are given in Table 5.3, and for a
much wider range of emitters in Table 5.4.
Consider two
examples:
- a.
125I has a Gamma Constant of
0.7 (Table 5.3) which means a 1 mCi
source would produce 0.7 R/hr at 1 cm, a
2 mCi source, 1.4 R/hr at 1 cm, and a 1
mCi source would produce 0.007 R/hr at 10
cm.
-
- b.
A 1 mCi 60Co source will
produce an exposure of 13.2 R/hr at 1 cm
and 0.132 R/hr at 10 cm, in the absence
of shielding.
The best safety
rule to apply when using gamma-emitters is to
maximize distance, minimize the time of exposure,
and use shielding, as possible. Fortunately, the
hands are the body part that most often come near
to an unprotected gamma-emitter source, and the
hands are relatively radiation-resistant.
TABLE 5.2
Properties of Some
Commonly Used Beta-Emitters
| |
3H |
14C |
35S |
45Ca |
32P
|
90Sr |
| Half
Life |
12.3y |
5730y |
88d |
165d |
14.3d |
28.1y |
| Maximum
beta E (Mev) |
0.018 |
0.154 |
0.167 |
0.254 |
1.71 |
2.24 |
| Range
In Air (ft) |
0.02 |
1 |
1 |
2 |
20 |
29 |
| Range
in Unit DensityMaterial (cm) |
0.00052 |
0.029 |
0.032 |
0.06 |
0.8 |
1.1 |
| Half
value layer (cm) unit density
material |
- |
0.0022 |
0.0025 |
0.0048 |
0.10 |
0.14 |
| Dose
Rate/100beta/cm2-sec
(mrad/hr) |
- |
64 |
60 |
43 |
12 |
11 |
| Fraction
transmittedthrough dead layer of skin
(0.007cm) |
- |
0.11 |
0.16 |
0.37 |
0.95 |
0.97 |
| Dose
rate to basal cells of epidermis from
1 uCi/cm2 mrad/hr |
- |
2600 |
3600 |
5900 |
4300 |
3900 |
TABLE 5.3
Properties
of Some Beta-Gamma Emitters
| |
125I |
131I
|
60Co
|
| Half
Life |
60d |
8.1d |
5.25y
|
| Maximum
Beta Energy (Mev) |
|
0.61 |
0.31 |
| Average
Beta Energy (Mev) |
0.022 |
0.188 |
0.093 |
| Gamma
Energies (Mev) |
0.035(7%) |
0.364(80%) |
1.17(100%) |
| |
0.027-0.032(136%) |
0.638(8%) |
1.33(100%) |
| Gamma
Half Value Layer |
|
|
|
| Lead
(cm) |
0.0037 |
0.3 |
1.1 |
| H2O
(cm) |
2.3 |
5.8 |
11.0 |
| |
|
|
|
| Dose
Rate/100 photons/cm2-sec
(mrad/hr) |
0.020 |
0.065 |
0.45 |
| Specific
Gamma Ray Constant (R/hr per mCi at 1
cm) |
0.7 |
2.2 |
13.2 |
4. RADIATION EXPOSURE
FROM STORED RADIONUCLIDES
University of
California, San Francisco (UCSF) policy limits
the exposure rate to 2 mrem/hour at 1 foot from
stored radioactive materials or radioactive
waste. Adherence to this limit ensures that no
authorized user who regularly works close to
stored isotopes or waste can exceed permissible
dose limits. For example, 2000 working hr/yr
divided into the limit of 5000 mrem/yr equals 2.5
mrem/hr whole body dose.
These examples
have been chosen to illustrate that it is
possible to imagine situations in which maximum
permissible doses can be reached in UCSF labs.
The next section details procedures that, if
followed, will prevent such levels of exposure,
even when accidents happen.
TABLE 5.4
Physical
Data For Selected Gamma Emitters*
| NUCLIDE |
Gamma**
Factor |
Physical
Half-Life |
Shielding
Factors (cm Lead) 1/2-value
|
10th-value |
| Americium-241***
|
1.30 |
458 y |
<.001 |
0.003 |
| Barium-133 |
4.4 |
10.4
y |
<.1 |
0.5 |
| Cadmium-109 |
1.58 |
453 d |
0.001 |
0.004 |
| Carbon-11 |
5.9 |
20.3 |
0.55 |
1.6 |
| Cesium-137 |
3.4 |
30.0
y |
0.80 |
2.4 |
| Chromium-51 |
0.18 |
27.7
d |
0.2 |
0.7 |
| Cobalt-57 |
0.94 |
270 d |
0.01 |
0.05 |
| Cobalt-60
|
13.2 |
5.26
y |
1.5 |
4.5 |
| Gallium-67 |
0.80 |
78.1
h |
<.1 |
0.5 |
| Gold-198
|
2.3 |
2.69
d |
0.33 |
1.1 |
| Indium-111 |
3.19 |
2.81
d |
-- |
0.2 |
| Indium-113m |
1.68 |
99.4
m |
0.2 |
0.9 |
| Iodine-123 |
1.77 |
13 hr |
0.04 |
0.2 |
| Iodine-125 |
0.7 |
60.2
d |
0.002
|
0.006 |
| Iodine-131 |
2.2 |
8.06
d |
0.3 |
1.1 |
| Iron-59 |
6.2 |
45 d |
1.5 |
4.5 |
| Krypton-81m |
1.46 |
13 s |
-- |
0.1 |
| Mercury-203 |
1.48 |
46.5
d |
0.10 |
0.4 |
| Molybdenum-99 |
1.69 |
66.7
h |
0.65 |
2.55 |
| Potassium-42 |
1.36 |
12.4
h |
1.7 |
5.2 |
| Potassium-43
|
5.3 |
22.4
h |
0.5 |
1.8 |
| Radium-226***
|
8.25 |
1600
y |
1.4 |
4.6 |
| Rubidium-86 |
0.5 |
18.6
d |
1.4 |
4.1 |
| Scandium-47 |
0.53 |
3.40
d |
0.05 |
0.17 |
| Selenium-75 |
2.0 |
120 d
|
0.1 |
0.5 |
| Sodium-22 |
12.0 |
2.60
y |
0.9 |
3.6 |
| Sodium-24
|
18.3 |
15.0
h |
1.8 |
5.7 |
| Strontium-85 |
5.94 |
65.1d |
0.1 |
1.1 |
| Tantalum-182 |
6.8 |
115.0
d |
1.2 |
4.0 |
| Technetium-99m |
0.7 |
6.03
h |
0.02 |
0.08 |
| Thallium-201 |
0.31 |
74 h |
0.025 |
0.09 |
| Tin-113 |
0.96 |
115 d |
0.001 |
0.004 |
| Xenon-133 |
1.07 |
5.31
d |
0.003 |
0.015 |
| Zinc-65 |
3.1 |
243 d
|
1.4 |
4.1 |
*Many of these
nuclides also emit beta particles or
conversionelectrons which may contribute
additional dose.
**Gamma factor
is in R-cm2/mCi-hr
***Also emits
alpha particles.
|