RHB@MIT-MC (02/04/83)
From: Robert H. Berman <RHB @ MIT-MC>
In 1980, I wondered about x-radiation from a CRT I use at MIT.
I compiled the following information at MIT, which I pass on to
this list. This is a long message. It contains information about
units as well as biological effects.
Bob Berman (RHB@MIT-MC)
---
This file contains a lot of detailed information about the
biological effects of radiation as a partial response to the question
I asked about long-term exposure to the low level doses from CRTs.
This discussion also supplements the discussion in common;x-ray hnt
from HUMAN-NETS last spring (1980) about the workings of CRTs.
I went into this detail because it seemed informative to
assess the risk and effects of short-term high exposures with what
appears to be the unrisky exposures from CRTs. In other words, THE
LEVEL OF RADIATION FROM THE CRT'S IS NOT SIGNIFICANTLY DISTINGUISHABLE
FROM BACKGROUND RADIATION. What remains uncertain still is whether
there is in fact any important time-integrated effect. The consensus
of opinion is that, while there may be, it is a statistical effect that
has to be detrminable by sampling over a large population of exposed
people and that more work needs to be done in this area. The exposures
of several TV's is about a factor of 100 under the maximum permitted
dose allowed by OSHA (EPA) for people who work a 40 hr week.
We invited an engineer from the Radiation Safety office to
measure the radiation from our CRTs with his geiger counter. We found
for several of the Plasma TV terminals as well as Lisp machine
terminals, both in BW and WB modes, no significant radiation above
(approximately) .05 mrem/hr (see below for a discussion of the units)
and most of the time about .02 mrem/hr. This was measured by placing
the geiger counter tube next to the glass of the CRT screen as well as
moving it around the cabinet. There is a surface area diminution of
the radiation strength the further you get away from it. (It isn't
exactly 1/r^2 because of the finite size of the source.)
Color TVs produce a somewhat higher radiation dose because the
electrons have a larger energy (about 2-3 times larger) and because
color TVs produce particles with larger atomic numbers. (There is
metal in the TV screen in addition to phosphers.)
As several people pointed out, one of the serious health risks
of working many hours in front of a CRT is stress, tension, eyestrain,
lack of circulation, lack of exercise, etc. On the whole, this may be
worse than any radiation damage.
A great deal of factual information was obtained for what
follows from a standard textbook by J. Lamarsh "Introdunction to Nuclear
Engineering" published by Adision-Wesley in 1975. This book was
kindly made available to me by JGA@MC
------ Definitions of Radiation Exposure and Dose -------
First, some definitions and units. These units are formulated
by the International Commission of Radiation Units and Measurements
(ICRU). The term "exposure" denotes gamma and x-radiation incident on
a target at any point. Radiation produces ions and electrons in air.
Thus, the presence of radiation is measured by the number of ions
(electrons) produced in the atmosphere adjacent to a target. Finite
size effects of the source, or preferential absortion effects by the
target are not The biological effect of radiation is a function of the
ionization produced in the target. Thus, exposure X is defined as X =
dq/dm, where dq is the total (positive) ion charge produced in air
when all the electrons liberated in a volume of air whose mass is dm
are completely stopped. The unit of exposure is the Roetgen (R). Thus,
by definition, 1R = 2.58 x 10 ^(-4) coul/kg. The biological effect of
radiation is also a function of the total energy deposited in the
target. The imparted energy is the difference E(in) - E(out) of the
kinetic energies of all particles or radiation, less any change in the
rest mass of particles within the target. The absorbed dose D is the
imparted energy per unit mass of the target. The unit of absorbed dose
is the Rad (Radiation Absorbable Dose). By definition, 1 rad = .01
J/kg = 100 ergs/g.
The biological effect of radiation is not directly proprotional
to the energy deposition because it depends not only on the dose, but also
on the way the energy is distributed. Thus, for the same dose, more damage
is done by alpha particles, which produce dense tracks of ionization,
than by gamma rays which are less heavily ionizing. The fact that
radiations of different types and energies give different biological effects
can be described by a factor called the Relative Biological Effectivenss
(RBE). The ICRU has also introduced a quality factor Q for radiation to
summarize the RBE. Thus, for x-rays, Q=1, while for alpha particles, Q=10.
The dose equivalent, H, is defined by the ICRU has the product
of the absorbed dose and the quality Q. Thus, H = D Q. The idea is that
equal dose equivalents should reproduce roughly the same biological
effect. Of course, the effect of a dose to the hand is quite different than
the same dose to the blood forming organs or gonads. The unit of dose
equivalent is the rem. Thus, if the quality factor is unity for some
radiation, an absorbed dose of 1 rad gives a dose equivalent of 1 rem.
----- Some Cell Biology -------
Second, some cell biology. There are approximate 10^13 cells
per average adult. Cells are broadly divided into two groups --
somatic cells and germ cells. The former make up the bones, liver,
blood, etc. The later function only in reproduction. The germs cells
are the ones that carry heredity traits (23 chromosomes) that when
joined with the germ cells of an opposite sex person make children.
The chromosomes are made up of DNA molecules.
There are two fundamental mechanisms by which radiation can
affect cells. First, radiation may actually break (ionize) molecules.
Secondly, radiation may produce new chemicals, such as oxy (O) or
hydroxyl (OH) radicals. which interact chemically with cells.
The end effect of this depends on which of the molecular
structues inside the cell are affected. If, say, one of the several
thousand mitocondria in a cell is damaged, there will probably be no
overall problem to the cell as a whole. Alternatively, if the
radiation disrupts a DNA molecule in a chromosome, the result may be a
mutation. If this happens in a somatic cell, no macroscopic effect
occurs unless a large number of cells are affected. This is because
mutations usally interfere with the production of proteins for the
proper functioning of the cell. Accordingly, the mutant cell and its
progeny will porbbaly die out rather quickly. If the mutation happens
in a germ cell, and if the cell can be fertilized, and if the cell is
fertilized, and if it develops, the mutation is carried into the
offspring.
Another effect is the onset of cancer. Although the detailed
mechanism for the orgin of cancer is not known at this time, there is
evidence that it is caused by a virus particle that suddenly becomes
biologically active.
There are other indirect clinical effects of radiation.
Consider exposure of the intestines to x-rays. The lining of the
intestine is continually renewed by cells from just under the lining
surface. Radiation can slow the renewal rate. When the dose is large
enough, the surface can not be maintained and it disintegrates. As a
consequence, various bodily fluids enter the intestine, while bacteria
and toxic material from the intestine can pass into the blood stream.
The overal effect on an individual can be diarrhea, dehydration,
infection, and blood posioning.
---- Biological Effects at Large Doses ------
Studies about the effects of different levels of radiation doses
from World War II and experiments on laboratory animals can be summarized:
1. There is well-documented information on large, short-term
doses larger than 10 to 20 rem.
2. There is only limited data showing effects of
a) short-term doses up to 10 rem and unrepeated.
b) short doses of a few rem and repeated occasionally
c) chronic doses on the order of millirems/day.
This is this point -- 2c. That is the question I asked about long-time
exposure to CRTs.
The effects of large short-term doses can summarized in the following table
due to S. Glasstone and A Sesonske (1963).
Dose (rems) Effect
0-50 No observable effects
50-100 Slight blood changes
100-200 Vomiting within 3 hrs. fatigue, loss of appetite.
Recovery in all cases within a few weeks.
200-600 >300, all victims exhibit vomiting within
2 hrs. Severe blood changes with hemorrhage
and infection.
Loss of hair within 2 weeks. Recovery
for 20 to 100 percent from 1 month to 1 yr.
600-1000 Vomiting within 1 hr. severe blood changes,
loss of hair. 80 to 100 percent succumb within
2 months. Survivors will be convalescent over
a long period.
There is also a large body of evidence to demonstrate late effects in
persons who received a large short-term dose of radiation. These
include cancer (leukemia), cataracts, fertility, mutations,
degenertive effects, and life shortening. Both cancer and radiation
cataracts appear to be threshold effects. People with doses less than
100 rem appear to be statistically equivalent to the normal population
for leukemia. Cataracts do not occur for doses below 200 rem.
Fertility, for both male and female can be summarized:
Probable Effect on Fertility of Single Doses of
Gamma-rays to the Human Sex Organs
Dose, rads Probable effect
150 brief sterility
250 sterility 1 to 2 yr
500-600 permanent sterility, many persons
800 permanent sterility, everybody.
---- What Are Small Doses, Background ? -----
Lamarsh asserts that up to 1974, no deleterious effects are
observed from doses of radiation of a few millirems/day accumulating up
to a few rems a year. The typical hazard due to natural and man-made
radiation sources can be summarized:
Sources of radiation
Source Dose, mrem/yr
Cosmic rays 45
Gamma rays 60
Medical + dental 72
x-rays
Global fallout 4
The values for cosmic and gamma rays are for New York City. They are
approximately twice as high in Denver. The total annual dose with
other estimated sources is around 210 mrem. This is theroughly what people
mean by "background" radiation.
The standards for radiation exposure are set by Environemntal
Protection Agency (OSHA) and in other countries by the International
Comisssion on Radiation Protection.
----- What are the US government Standards? ------
One problem on setting radiation standards is that while there
is no evidence for deleterious effects of radiation at low doses,
most reasonable scientists suspect there may be some effect.
Therefore, one assumes:
1. there is a linear dose-effect relationship for all radiation
effects from high dose levels from hundreds of rads to zero dose.
2. there is no threshold effect for radiation doses.
3. all low doses are completely additive.
4. there is no biological recovery from radiation at low doses.
None of these assumptions is strictly correct, but they are a
conservative basis for formulating standards. Perhaps, one could formulate
other standards if one had more data about low level chronic radiation.
The curernt standards for radiation protection were formulated in 1971
and are summarized:
Type of exposure Maximum permissiable dose equivalent
Occupational
Whole Body
Prospective annual limit 5 rem
retrospecive 10-15 rem
long-term to N years 5(N-18) rems
of age.
Skin 15 rem
Hands 75 rem
Forarms 30 rem
Other Organs 15 rem
Fertile women .5 rem when pregnant
Occasional
Individual .5 rem
Students .5 rem
Population as a Whole .17 rem
Emergency
Life-saving
Whole Body 100 rems
Hands, Forearms 200 rems
Less Urgent
Whole Body 25 rems
Hands, Forearms 100 rems, total
It should be noted that 5 rems/yr are equal to 100 mrem/wk.
Distributed over a 40-hour week, this is a dose equivalent rate of 2.5
mrem/hr.
As described above, we measured the Plasma TV's and a Lisp
machine TV with a geiger counter and found exposure rates of about .02
mrem/hr, which is about a factor of 100 under the maximum permissible
equivalent dose. Now, while it may not be possible for everyone to get
a geiger counter or a dosimeter, one can be assured a little bit about
the radiation level not getting out of line. The standards for
commerical TVs are on the same order as these CRTs. If you suddenly
start produced ten times, say, as energetic electrons because your picture
tube is broken, you will almost certain see a marked deterioration in
the quality of the image, if there is one at all when the electrons are
that energetic..