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SECTION III
BIOLOGICAL
EFFECTS OF IONIZING RADIATION
A.
UNITS OF EXPOSURE AND DOSE
Not
all of the energy in a radiation field gets deposited in the living
tissues of an individual who is in that field. Therefore, the distinction
must be made between the amount of ionizing radiation that an individual
is exposed to and the amount of energy that actually gets deposited
in the body. Exposure is often expressed in units of roentgens
which is a measure of the amount of ionization that the radiation (x- or
gamma rays) could produce in air. The radiation energy that is absorbed in
the body is referred to as the "dose" and is often measured in rads
or grays. Doses from different types of radiation are not always
additive. This is primarily because they deposit their energy differently
as they pass through living tissue. A radiation that deposits its energy
over a short range, like alpha, has the potential to cause more biological
damage than a radiation that deposits its energy over a longer range, like
x-rays. For radiation protection purposes, a factor is used to adjust for
the quality of different radiations so doses can be added. The absorbed
dose in rads is multiplied by this quality factor to yield
dose equivalent units in rem or sieverts. Rem can be added
to ensure that an individual has not received a dose in excess of the
maximum permissible limits.
Roentgen
(R) --
A
unit of exposure to ionizing radiation. It is that amount of gamma
or X-rays required to produce ions carrying 1
electrostatic unit of electrical charge in 1 cubic centimeter of dry
air under standard conditions. Named after Wilhelm Roentgen, German
scientist who discovered X-rays in 1895.
Rad --
Acronym
for radiation absorbed dose. The basic unit of
absorbed dose of radiation. A dose of one rad means the absorption of
100 ergs (a small but measurable amount of energy) per gram of
absorbing material.
Gray
A
joule per kilogram of absorbed energy.
Rem
Acronym
for roentgen equivalent man. The unit of dose
of any ionizing radiation that produces the same
biological effect as a unit of absorbed dose of ordinary X-rays.
Rem = Rad x Quality Factor (QF).
Sievert
A
Gray x Quality Factor.
Quality
Factor
The
factor by which the absorbed dose is to be multiplied to obtain a
quantity that expresses, on a common scale for all ionizing radiation,
the biological damage to exposed persons. It is used because some
types of radiation, such as alpha particles and neutrons, can be more
biologically damaging than other types, such as beta particles and
x-rays.
Effective
Dose Equivalent
The
Dose Equivalent weighted for the relative probability of specific
biological end points (fatal cancer and genetic damage). The weighting
factor is designated WT. Factors have been established for
several types of tissue such as breast, gonadal, bone marrow, lung,
thyroid, bone surfaces, and other organs. The units of Effective Dose
Equivalent are either Rems or Sieverts.
B.
ACUTE EFFECTS
The
biological effects from exposure to ionizing radiation can be classified
into two basic categories, acute and delayed. Acute effects are caused by
relatively high doses of radiation delivered over a short period of time.
These effects are dependent on how much and what area of the body is
exposed over what time. The following two tables classify acute radiation
injuries for different exposure levels to the skin (Table 2) and whole
body (Table 3).
|
TABLE
2: ACUTE RADIATION INJURY TO THE SKIN |
|
Dose in mds |
Effect |
|
|
|
200-300 (2-3 Gy) |
Epilation |
|
>300 ( >3 Gy) |
Radiation
dermatitis and erythema |
|
1000-2000 (10-20 Gy) |
Transepidermal
injury |
|
>2000 ( >20 Gy)
(single
exposure) |
Radio
necrosis
|
|
>5000( >50 Gy)
(over
extended period) |
Chronic
dermatitis
|
|
TABLE
3: SUMMARY OF EFFECTS ON HUMANS
OF
SHORT-TERM WHOLE BODY EXTERNAL EXPOSURE TO RADIATION |
|
Dose
(rads) |
Effects
on humans |
|
0
to 25
(0 - 0.25 Gy) |
No
detectable clinical effects. Delayed effects may occur. |
|
25
to 100
(0.25 - 1 Gy) |
Slight transient reductions in lymphocytes and neutrophils.
Disabling sickness not common; exposed individuals should be able to
proceed with usual tasks. Delayed effects possible, but serious
effects on the average person very improbable. |
|
100
to 200
(1 - 2 Gy) |
Nausea
and fatigue, with possible vomiting above 125 rads in about 20-25%
of people. Reduction in lymphocytes and neutrophils, with delayed
recovery. Delayed effects may shorten life expectancy (on the order
of 1%). |
|
200
to 300
(2 - 3 Gy) |
Nausea
and vomiting on first day. Latent period up to 2 weeks, perhaps
longer. After latent period, symptoms appear but are not severe:
loss of appetite, general malaise, sore throat, pallor, petechia,
diarrhea, moderate emaciation. Recovery likely in about 3 months
unless complicated by poor health or superimposed injury or
infection. |
|
300
to 600
(3 - 6 Gy) |
Nausea,
vomiting, and diarrhea in first few hours. Latent period with no
definite symptoms perhaps as long as 1 week. Epilation, loss of
appetite, general malaise, and fever during 2nd week, followed by
hemorrhage, purpura, petechia, inflammation of mouth and throat,
diarrhea, and emaciation in 3rd week. Some deaths in 2-6 weeks;
possible eventual death to 50% of those exposed at about 450 rads;
convalescence of others about 6 months. |
|
600
or more
( >6 Gy) |
Nausea,
vomiting, and diarrhea in the first few hours. Short latent period,
with no definite symptoms, in some cases during first week.
Diarrhea, hemorrhage, purpura, inflammation of mouth and throat, and
fever toward end of first week. Rapid emaciation and death as early
as the second week, with possible eventual death of up to 100% of
those exposed. |
From Saenger EL, ed, Medical Aspects of Radiation Accidents,
p 9. US Atomic Energy Commission, Washington, DC, 1963.
C.
DELAYED EFFECTS
Delayed
effects of radiation are those which manifest themselves years after the
original exposure. Delayed radiation effects may result from previous
acute, high-dose exposure or from chronic low-level exposures over a
period of years. It should be emphasized that there is no unique disease
associated with the long-term effects of radiation; these effects manifest
themselves in humans simply as a statistical increase in the incidence of
certain already-existing conditions or diseases. The delayed effects thus
far observed from radiation exposure have been:
|
TABLE
4: DELAYED EFFECTS OF RADIATION |
|
Genetic |
Effects
passed on from generation to generation due to mutation of genetic
material |
|
or |
|
Somatic |
Effects
manifested in exposed individuals themselves such as:
Cancer
Cataracts
Developmental
abnormalities in the fetus
Growth
retardation |
Actual
risk estimates for genetic and somatic effects from radiation exposure are
described below in Tables 5 and 6. These risks can be compared to other
health and occupational risks as tabulated in Tables 7 and 8.
|
TABLE
5: GENETIC RISKS OF LOW-LEVEL IONIZING RADIATION |
|
One
rem before conception is expected to produce 5-75 additional serious
genetic disorders per 1 million live- births (First generation). |
|
This
is small in relationship to the usual incidence of genetic disorders
of about 4.5% of live born off-spring (45,000/106
live-births)
|
|
TABLE
6: CANCER MORTALITY RISK ESTIMATES WHOLE-BODY LOW-LEVEL IONIZING
FOR
WHOLE-BODY LOW-LEVEL IONIZING RADIATION (BEIR V) |
400
Excess Cancer deaths over a life-time are predicted per
million persons exposed to 1 rad of radiation
|
|
4
x 10-4 Excess Cancer deaths per person per rad over a
lifetime |
6
x 10-6 Excess Cancer deaths per persons per rad per
year
|
|
In
a population of 10,000 persons four excess cancer deaths over a
lifetime would be expected from an exposure of 1 rad to each person. |
|
The
expected deaths from cancer for 10,000 persons over a lifetime is
normally 2500. |
|
TABLE
7: ESTIMATED LOSS OF LIFE EXPECTANCY FROM HEALTH RISKS
|
|
HEALTH
RISK |
ESTIMATED
AVERAGE DAYS OF
LIFE
EXPECTANCY LOST |
|
Smoking
20 cigarettes/day |
2370
(6.5 years) |
|
Overweight
(by 15%) |
777
(2.1 years) |
|
All
Accidents combined |
366 |
|
Auto
accidents |
207 |
|
Alcohol
consumption (US avg) |
365 |
|
Home
accidents |
74 |
|
Drowning |
24 |
|
Natural
background radiation, calculated |
9.3 |
|
Medical
radiation (63 mRem/yr avg), calculated |
6.2 |
|
All
catastrophes (earthquake, etc.) |
4.8 |
|
1
rem occupational radiation dose, calculated |
1.5 |
|
1
rem/yr from age 18-65, calculated |
51 |
|
TABLE
8: ESTIMATED LOSS OF LIFE EXPECTANCY FROM INDUSTRIAL HAZARDS |
|
INDUSTRY TYPE |
ESTIMATES OF DAYS OF LIFE EXPECTANCY
LOST
ESTIMATES OF DAYS OF LIFE EXPECTANCY
LOST
|
|
All Industry |
60 |
|
Trade |
27 |
|
Manufacturing |
40 |
|
Service |
27 |
|
Government |
60 |
|
Transportation and Utilities |
160 |
|
Agriculture |
320 |
|
Construction |
227 |
|
Mining and Quarrying |
167 |
|
Nuclear Industry (Avg radiation dose of
0.45 rem/yr) from age 18-65, calculated |
23 |
Information
in Tables 7 and 8 from "Catalog of Risks Extended and Updated",
Bernard L. Cohen, Health Physics Vol 61 No 3, September 1991.
Section IV
Table of Contents
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