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RADIATION
SAFETY GUIDE
 Learning
Objectives
Upon
successful completion of this course, you will be able to:
- Identify
and discuss key elements of how units of radiation are measured
- Define
common terms used in describing aspects of radiology including:
curie, roentgen, rad, and rem
- Identify
and explain the key aspects of radiation protection standards
including: time, distance, and shielding
- Explain
the physical and chemical effects of ionizing radiation
- Describe
the fundamentals of dosimetry
Introduction
The
following is a recommended sample Radiation Safety Guide.
While not all of its elements will apply to your situation,
this Radiation Safety Guide gives you a very clear idea of
what elements of radiation safety need to be dealt with by
both organizations and staff.
In
general, a radiation guide starts with some general “ground
rules” for the use of equipment by authorized personnel.
For example:
All
authorized machine use personnel must:
-
Post areas where radiation-producing machines are used and
stored. Rooms housing x-ray equipment must have warning
signs on entrances specifically for x-ray.
-
Maintain security/control of ionizing radiation-producing
equipment. Equipment itself or rooms housing x-ray equipment
must be locked when not in use.
-
Keep a log of dates, use parameters, and users' names, as
well as any performance checks done on equipment. The organization’s
Office of Environmental Health and Safety will also monitor
work areas where x-ray machines are used to detect leakage
or scatter radiation.
-
If you modify, transfer, dispose of, or purchase x-ray equipment,
your State Department of Health Services must be advised
by the Office of Environmental Health and Safety of these
facts. Notify Environmental Health and Safety of these changes.
-
Wear dosimetry (film badges and/or finger rings) to document
radiation exposures while working with x-ray equipment.
Machine
Use Authorizations (MUA)
Requests
to use radiation-producing equipment are separated into the
following categories:
1.
Analytical Use
2.
Diagnostic Human and Non-Human Use
Separate machine use applications are required for each machine,
and there may be requirements for additional shielding or
state certification.
Units
of Radiation Measurement
1.
Activity (Unit: Curie)
Since
the discovery by William Roentgen in 1895 that energetic electrons
impinging on a target of high atomic number produce rays that
easily penetrate matter and can expose photographic film (X
rays), the scientific community has adopted special units
to describe the amount and nature of ionizing radiation.
The
International Commission on Radiological Units (ICRU) was
formed to develop a system of units and nomenclature specific
to the needs of physicians and other persons working with
not only X rays, but other types of radiation found in nature
or produced by man. The units that have been developed were
named after pioneers in the field (Roentgen, Curie) or began
as descriptive terms that turned into acronyms, then into
units (rem-"roentgen equivalent man"). The ICRU
designated units on the basis of observed quantities. Thus
the special unit of activity, the curie, was equal to the
number of disintegrations taking place per unit time from
1 gram of radium. The curie (Ci) was later redefined as the
activity of that quantity of radioactive material in which
the number of disintegrations per second is 3.7E10 (a number
nearly the same as the number of disintegrations per second
from 1 gram of radium).
We
have since learned that a Curie of any radioisotope is a very
appreciable amount, too great for most laboratory applications,
so we commonly find activity expressed as millicurie (mCi,
1E-3 Curie) or microcurie (µCIF, 1E-6 Curie). It is
essential that one not confuse the symbol for micro with that
for milli. The 1,000-fold error that results may mean the
difference between an almost inconsequential radiation problem
and a major radiation hazard. A useful number to remember
is 2.22E6 disintegrations per minute per microcurie. Most
tracer applications require microcurie quantities, although
it is not unusual to find millicurie quantities of 3H, 32P,
and 125I in many laboratories.
2.
Exposure ( Unit: Roentgen)
The
ICRU defined the special unit of exposure in air to be the
Roentgen (symbolized by R). R = 2.58E-4 coulomb kg air This
unit is special in that it is defined only for X or gamma
radiation in air. Thus, the Roentgen is not applicable to
alphas, betas, or neutrons. Many survey instruments provide
output data in terms of mR/hr (mR, 1E-3R). The Roentgen is
not always useful for making accurate evaluations of energy
absorbed due to radiation impinging on material. It is the
absorbed energy that is a true index of biological damage.
If one knows how well a certain material can absorb radiation
as compared with air, the energy absorbed by that material
when exposed to 1 R can be calculated. It is very easy to
measure ionization in air with inexpensive equipment, so that
the Roentgen can be measured directly. It is not so easy to
measure the energy absorbed in material directly.
3.
Absorbed Dose (Unit: rad)
The
rad is the special unit of absorbed energy. It is defined
as that amount of ionizing radiation that deposits 100 ergs/gram
of material. The rad is applicable to all types of ionizing
radiation, yet it is difficult to measure directly. Normally
ionization in air or another gas is measured and the absorbed
dose in a particular material calculated. One Roentgen results
in 87.7 ergs being absorbed in 1 gram of air; if muscle tissue
is placed in the same radiation beam, 1 R in air corresponds
to about 95 ergs/gram. For most applications of x rays and
gamma rays, it is reasonable to assume that 1 R = 1 rad. One
Roentgen is a large exposure, therefore, we more often see
the term millirad (mrad, 1E-3 rad).
4. Dose Equivalent (Unit: rem)
The
rem is the unit of dose equivalent. The dose equivalent accounts
for the difference in biological effectiveness of different
types of radiation. It is the product of the absorbed dose
(rad) times the quality factor (QF) of the radiation. The
quality factor for x, gamma, and beta radiation is 1, therefore
for these radiations 1 rad = 1 rem. The quality factor for
alpha radiation is 20 and the quality factor for neutron radiation
varies with energy from 2-11.
Radiation
Protection Standards
1.
Introduction
Radiation
protection standards apply to radiation workers or the general
population. Standards for the general population are of importance
since they serve as a basis for many of the considerations
applicable to the siting of nuclear facilities and the design
and implementation of environmental surveillance programs.
Included in this section are a brief history of the development
of radiation protection standards, a review of the goals and
objectives sought, and a description of the approach being
used to base such standards on the associated risk.
2.
History of the Basis for Dose Limits
Shortly
after the discovery of x-rays of 1895 and of naturally occurring
radioactive materials in 1896, reports of radiation injury
began to appear in the published literature (i). Recognizing
the need for protection, dose limits were informally recommended
with the primary initial concern being to avoid direct physical
symptoms. As early as1902, however, it was suggested that
radiation exposures might result in delayed effects, such
as the development of cancer. This was subsequently confirmed
for external sources and, between 1925 and 1930, it became
apparent for internally deposited radionuclides when bone
cancers were reported among radium dial painters (1).
With
the publication by H.J. Muller in 1955 (ii) of the results
of his experiments with Drosophila, concern began to be expressed
regarding the possibility of genetic effects of radiation
exposures in humans. This concern grew and dominated the basis
for radiation protection from the end of World War II until
about 1960, and led to the first consideration of recommendations
for dose limits to the public. With the observances of excess
leukemia among the survivors of World War II atomic bombings
in Japan, and the failure to observe the previously anticipated
genetic effects, however, the radiation protection community
gradually shifted to a position in which somatic effects,
primarily leukemia, were judged to be the critical (or governing)
effects of radiation exposures. This belief continued until
about 1970 when it was concluded that, although somatic effects
were the dominating effects, the most important such effects
were solid tumors (such as cancer of the lung, breast, bone,
and thyroid) rather than leukemia (iii). Finally, in 1977
the International Commission on Radiological Protection (ICRP)
initiated action to base radiation protection standards on
an acceptable level of the associated risk (iv).
This effort was provided additional support by the National
Council on Radiation Protection and Measurements (NCRP) with
the issuance of their updated "Recommendations on Limits
for Exposure to Ionizing Radiation" in 1987 (v).
3.
Basic Standards - Philosophy and Objectives
The
primary source of recommendations for radiation protection
standards within the United States is the National Council
on Radiation Protection and Measurements (NCRP). Recommendations
of this group are in general agreement and many of them have
been given legislative authority through publication of the
Code of Federal Regulations by the U.S. Nuclear Regulatory
Commission.
| a. |
Basic
Philosophy
As a general approach, the main purposes in the control
of radiation exposures are to ensure that no exposure
is unjustified in relation to its benefits or those of
any available alternative; that any necessary exposures
are kept as low as is reasonably achievable (ALARA); that
the doses received do not exceed certain specified limits;
and that allowance is made for future developments. |
| b.
|
Objectives
of the Guides
In general, the objective or goal of radiation protection
(and associated standards) is to limit the probability
of radiation-induced diseases in exposed persons (somatic
effects) and in their progeny (genetic effects) to a degree
that is reasonable and acceptable in relation to the benefits
of the activities that involve such exposures. |
Radiation-induced
diseases of concern in radiation protection are classified
into two general categories: stochastic effects and non-stochastic
effects.
|
i.
|
A
stochastic effect is defined as one in which the probability
of occurrence increases with increasing absorbed dose,
but the severity in the affected individuals does not
depend on the magnitude of the absorbed dose. A stochastic
effect is an all-or-none response as far as individuals
are concerned. Cancers (solid malignant tumors and leukemia)
and genetic effects are examples of stochastic effects. |
|
ii.
|
A
non-stochastic effect is defined as a somatic effect which
increases in severity with increasing absorbed dose in
the affected individuals, owing to damage to increasing
numbers of cells and tissues. Examples of non-stochastic
effects attributable to radiation exposure are lens opacification,
blood changes, and decreases in sperm production in the
male. Since there is a threshold dose for the production
of non-stochastic effects, limits can be set so that these
effects can be avoided. |
4.
Radiation Protection Standards
| a. |
Occupational
Dose Limits
Standards
provide for an upper boundary effective dose equivalent
limit of 50 mSv/year (5 rem/year). On a cumulative basis,
however, the newest NCRP recommendations have proposed
that the average cumulative effective occupational dose
equivalent not exceed 10 mSv (1 rem) times the age of
the worker.5 UC Davis guidelines limit exposure to roughly
one-half the state and federal limits. Two key changes
or factors to be noted relative to these recommendations
are:
i.
The dose limit applies to the sum of the doses received
from
both
external and internal exposures.
ii.
The standards are expressed in terms of the effective
dose
equivalent,
an approach which permits, on a mathematical
basis,
the summation of partial and whole body exposures. |
5.
Dose Limits for the General Population
For
a variety of reasons, dose limits for the general population
are set lower than those for radiation workers. Justifications
for this approach include the following:
a. The population includes children who might represent a
group of
increased
risk and who may be exposed for their whole lifetime.
b. It was not the decision or choice of the public that they
be exposed.
c. The population is exposed for their entire lifetime; workers
are exposed
only during their working lifetime and presumably only while
on the job.
d. The population in question may receive no direct benefit
from the
exposure.
e. The population is already being exposed to risks in their
own
occupations; radiation workers are already being exposed to
radiation
in their jobs.
f. The population is not subject to the selection, supervision,
and
monitoring afforded radiation workers.
g. Even when individual exposures are sufficiently low so
that the risk to
the individual is acceptably small, the sum of these risks
(as
represented by the total burden arising from somatic and genetic
doses) in any population under consideration may justify the
effort
required to achieve further limitations on exposures.
6.
Concept of Effective Dose Equivalent
a.
Basic Objectives:
The
objective in developing the concept of the effective dose
equivalent was to obtain a system that would provide a unit
for radiation protection standards that could be used to express,
on an equal risk basis, both whole body and partial body exposures.
In developing this approach, the ICRP sought to:
i. Base the limits on the total risk to all tissues as well
as the
hereditary
detriment in the immediate offspring (first two
generations);
ii. Consider, in the case of internally deposited radionuclides,
not only
the dose occurring during the year of exposure, but also the
committed dose for future years.
Having
stated this objective, the next goal of the ICRP was to set
the occupational dose limits at such a level that the risks
to the average worker incurred as a result of his/her radiation
exposure would not exceed the risk of accidental death to
an average worker in a "safe" non-nuclear industry.
Based
on a review of data on a world-wide basis (see Table I), the
ICRP concluded that, on the average, within a "safe"
industry about 100 workers or less would be killed accidentally
each year for one million workers employed. Thus, the associated
risk of accidental death to the average worker in a "safe"
industry would be about:
| 100/year/1,000,000
= 1E-4/year. |
b.
Risks of Death from Radiation Exposures:
Based
on epidemiological studies with human populations and biological
studies in animals, estimates can be made of the risk of a
fatality from cancer or a genetic death for given levels of
dose equivalent to various body organs. Some examples are
given below to illustrate the thinking that goes into formulation
of risk factors:
-
Studies of the survivors of the atomic bombings in Japan
at the close of World War II indicate that for a collective
dose of 10,000 person-Sv (1,000,000 person-rem) to the bone
marrow, there will be, after latency period, an average
of one excess case of leukemia occurring in the population
each year. Assuming that each such case ultimately results
in a death, and that the excess continues for a period of
20 years, there will be a total of 20 excess cases of leukemia
and, therefore, 20 excess deaths due to this exposure. Thus,
the risk of death due to leukemia resulting from exposure
of the bone marrow can be estimated to be:
20
excess person deaths/10,000 person-Sv = 2E-3/Sv
-
Similar studies among uranium miners have shown that there
will be approximately 20 excess cases of lung cancer (and
consequently 20 excess deaths, assuming all cases of lung
cancer are fatal) for each 10,000 person-Sv (1,000,000 person-rem)
to the lungs. Thus the risk of death from lung cancer can
be estimated to be:
20 excess deaths/10,000
lung-Sv = 2E-3/Sv
-
For breast cancer, epidemiological data have shown that
there is an excess of about 100 breast cancers per 10,000
person-Sv (1,000,000 person-rem) to the female breasts.
Assuming that breast cancer is fatal 50% of the time; and
assuming that the population being exposed consists of 50%
men and 50% women, then the risk of excess deaths due to
exposure to the female breasts can be estimated to be:
100
excess cancers/10,000 breast-Sv x (0.5 fatality rate) x
(0.5 of population being female)= 2.5E-3 / Sv
-
For thyroid cancer, epidemiological data have shown that
there is an excess of about 100 thyroid cancers per 10,000
Sv (1,000,000 rem) to the thyroids in humans. However, the
fatality rate for thyroid cancer is only about 5%, so the
risk of death due to cancer of the thyroid resulting from
exposure to ionizing radiation is:
100 excess cancers/10,000
thyroid-Sv x (0.05 fatality rate)=5E-4/Sv
c.
Similar calculations can be made to estimate the excess deaths
due to exposures of other body organs, as well as genetic
deaths due to exposure of the reproductive organs.
Accidents
in Different Professions
Table
1:
Fatalities From Accidents in Different Occupations
(x 10,000 Per Year) |
| Category |
Occupation |
Fatalities
Per Year |
| Safe |
Trade |
0.5 |
| Safe |
Manufacturing |
0.6 |
| Safe |
Service |
0.7 |
| Safe |
Government |
0.9 |
| Less
Safe |
Transportation
& Utilities |
2.7 |
| Less
Safe |
Construction |
3.9 |
| Less
Safe |
Agriculture |
4.6 |
| Less
Safe |
Mining,
Quarrying |
6.0 |
| Less
Safe |
Sports |
15 |
| Less
Safe |
Deep
Sea Fishing |
30 |
| Less
Safe |
High-rise
Steelworkers |
50 |
| Less
Safe |
Farm
Machinery Workers |
80 |
|
Accidents
in Different Professions
1.
Physical and Chemical Effects of Ionizing Radiation
| a. |
Ionizing
radiation is so named because its initial interaction
with matter is the ejection of an orbital electron from
an atom, forming a pair of ions with opposite charges.
Radiation passing through living cells will ionize or
excite atoms and molecules in the cell structure. This
produces ions and radicals within the cell (mostly from
water molecules). When these radicals and ions interact
with other cell materials, damage can result. Certain
levels of cellular damage can be repaired by the cell.
Further levels can result in cell death.
i.
May directly involve and damage biologically important
molecules in the cell - Direct Effects. Damage to the
DNA molecule or a chemical change in other cellular material
are the primary results. Damage to the DNA molecule can
result in somatic mutations that may show up years after
the exposure or genetic mutations that require several
life spans to appear.
ii. May initiate a chain of chemical reactions, mediated
through cellular water, leading to ultimate biologic damage
- Indirect Effects. An hydroxyl poisoning effect on the
cell membrane results in a change in its permeability.
Inactivation and release of enzymes is the primary result. |
| b. |
The
unit of radiation dose is the rad which equals 100 ergs
of energy absorbed per gram of tissue. |
| c. |
Biological
effects of all types of ionizing radiations are similar.
Some radiations are more efficient than others, however,
and produce more biological damage per rad dose.
i. The rem is the unit of biological dose called the
units of Dose equivalence) which takes into consideration
the differing efficiencies of the different radiations.
ii. The Dose Equivalence in rems is obtained by multiplying
the dose in rads by the Quality Factor (QF) of the particular
radiation. The QF is related to its ionization density.
|
1
for most gamma and x-rays, beta particles
2 - 11 for neutrons
20 for alpha particles
2.
Cellular Effects of Ionizing Radiation
a.
Cell killing is responsible for acute somatic effects of radiation.
It occurs by two mechanisms:
i.
Inhibition of mitosis which results from moderate doses and
leads to
delayed
cell death.
ii.
Immediate cellular death which results from very high doses.
b.
Alteration of cellular genetic material consistent with continued
cell
proliferation: Usually manifests no visible change in cellular
appearance
but
a point (recessive) mutation is formed, which may or may not
be
passed
to future generations.
3.
Systemic Biological Effects of Ionizing Radiation
a. Somatic effects:
Abnormality may become manifest only after many generations
of cell
replication: proposed mechanism for long-term somatic effects
of
radiation - carcinogenesis, nonspecific life shortening. (These
are non-
stochastic effects.)
b.
Genetic effects:
If
involves gonadal cells, mutations are passed on to offspring.
Increase in number of "recessive" mutations in population
pool leads to
increased probability of abnormalities in offspring due to
chance mating
of individuals carrying same mutation. (These are stochastic
effects.)
4.
Acute Somatic Effects of Radiation Exposure in Humans
a.
Related to killing of cells, generally in tissues where cells
are rapidly
proliferating.
Observed effects usually occur 1-3 weeks after radiation
exposure.
b. Systems of primary involvement:
i. Hematopoietic system - (fever, infections, hemorrhages)
Chief
organ: bone marrow
Symptom
latency: days to weeks
Death
threshold: less than 500 rem
Characteristic
symptoms: Malaise, fever, fatigue, infection,
hemorrhage, and anemia. Low counts of platelets, lymphocytes
and
erythrocytes result in low resistance to infection and a decreased
clotting
ability.
ii.
Gastrointestinal system - (abdominal pain, vomiting, severe
diarrhea,
fluid and electrolyte imbalance)
Chief
organ: small intestine
Symptom
latency: hours to days
Death
threshold: 500-2000 rem
Characteristic
symptoms: Malaise, nausea, vomiting, diarrhea, fever,
dehydration, G.I. malfunction, and electrolyte loss. The intestinal
epithelium is destroyed.
iii.
General systemic effects "radiation sickness" -
(central nervous
system
syndromes).
Chief
organ: brain
Symptom
latency: minutes to hours
Death
threshold: 2000-5000 rem
Characteristic
symptoms: lethargy, tremors convulsions,
encephalitis,
meningitis, and edema. Acute inflammation and vascular
damage results in neuronal functional impairment.
5.
Dose relationships:
a.
0-150 rem - none to minimal symptoms. Perhaps long-term effects
many years later.
b. 150-400 rem - moderate to severe illness due to hematopoietic
derangement.
c. 400-800 rem - severe illness. LD50 in man probably about
500 rem. GI
damage at higher doses.
d. Above 800 rem - 100% fatal, even with best available treatment.
6.
Partial body exposure
Effects
depend on particular tissue or organ exposed, but significant
acute changes are usually seen only after a fairly high radiation
dose (>1000 rem).
7.
Long-Term Effects of Exposure to Ionizing Radiation
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