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]]]]]]]]]]]]]]]] THE MYTH OF PLUTONIUM TOXICITY [[[[[[[[[[
Bernard L. Cohen (1/3/1989)
By Bernard L. Cohen, Department of Physics, University of
Pittsburgh, Pittsburgh, Pennsylvania 15260.
(From Karl Otto Ott and Bernard I. Spinard, eds. Nuclear Energy
(New York: Plenum Press, 1985), pp. 355-365)
[Kindly uploaded by Freeman 10602PANC]
Plutonium is constantly referred to by the news media as ``the
most toxic substance known to man.'' Ralph Nader has said that a
pound of plutonium could cause 8 billion cancers, and former
Senator Ribicoff has said that a single particle of plutonium
inhaled into the lung can cause cancer. There is no scientific
basis for any of these statements as I have shown in a paper in
the refereed scientific journal Health Physics (Vol. 32, pp.
359-379, 1977). Nader asked the Nuclear Regulatory Commission to
evaluate my paper, which they did in considerable depth and
detail, but when they gave it a ``clean bill of health'' he
ignored their report. When he accuses me of ``trying to detoxify
plutonium with a pen,'' I offered to eat as much plutonium as he
would eat of caffeine, which my paper shows is comparably
dangerous, or given reasonable TV coverage, to personally inhale
1000 times as much plutonium as he says would be fatal, or in
response to former Senator Ribicoff's statement to inhale 1000
particles of plutonium of any size that can be suspended in air.
My offer was made to all major TV networks but there has never
been a reply beyond a request for a copy of my paper. Yet the
false statements continue in the news media and surely 95% of the
public accept them as fact although virtually no one in the
radiation health scientific community gives them credence. We
have here a complete breakdown in communication between the
scientific community and the news media, and an unprecedented
display of irresponsibility by the latter. One must also
question the ethics of Nader and Ribicoff; I have sent them my
papers and written them personal letters, but I have never
received a reply.
Let's get at the truth here about plutonium toxicity. We
begin by outlining a calculation of the cancer risk from intake
of plutonium (we refer to it by its chemical symbol, Pu) based on
standard procedures recommended by all national and international
organizations charges with responsibility in this area, and
accepted by the vast majority of radiobiomedical scientists.
1. ESTIMATE OF PLUTONIUM TOXICITY FROM STANDARD PROCEDURES
The first step is to calculate the radiation dose in rem (the
unit of dose) to each organ of the human body per gram of Pu
intake. According to ICRP (International Commission on Radiation
Protection) Publication No. 19, about 25% of inhaled particles of
the size of interest (0.5-5 [micro]m in diameter) deposit in the
lung, and 60% of this is eliminated only with a 500-day
half-life. From this information and the known rate and energy
of [alpha]-particle emission, we can calculate the radiation
energy deposited in the lung, which is directly convertible to
dose in rem.
According to ICRP Publication 19, 5% of inhaled Pu gets into
the bloodstream from which 45% gets into the bone and an equal
amount collects in the liver; the times required for elimination
from these are 70 and 35 years, respectively. This is all the
information needed to calculate doses to bone and liver in rem
per gram of Pu inhaled.
If Pu is ingested with food or water in soluble form, the ICRP
estimates that 3 x 10^-5 (30 parts per million) gets through the
intestine walls into the bloodstream. From this and the
information given above, calculation of rem to the bone and liver
per gram of Pu ingested is straightforward. In addition, there
is dosage to the gastrointestinal tract calculable by ICRP
prescriptions.
Once the dose in rem is calculated, the next step is to
convert this to cancer risk using the BEIR Report, the standard
reference in this area produced by the National Academy of
Sciences Committee on Biological Effects of Ionizing Radiation.
It recommends a model in which there is a 15-year latent period
following exposure during which there are no effects, followed by
a 30-year ``plateau'' period during which there is a constant
risk of 1.3 x 10^-6 (1.3 chances per million) per year per rem
for lung cancer and 0.2, 1.0, and 0.3 x 10^-6 per year per rem
for bone, gastrointestinal tract and liver* cancer, respectively.
For children less than 10 years old, these are divided by
five, and for an older person, there is a calculable probability
that death will result from other causes before the cancer
develops. With this information we can calculate the cancer risk
as a function of age at intake. Averaging over ages, we obtain
the average cancer risk per gram of Pu intake.
* In the BEIR Report, liver cancer is included among ``all
other'' for which the risk is 1.0 x 10^-6, the value used here is
based partly on other information.
TABLE I
Cancer Doses in Micrograms (Defined as the
Inverse of Risk per Microgram)
-----------------------------------------------------------------
Entrance Mode 239-Pu Reactor-Pu
-----------------------------------------------------------------
Inhalation (dust in air) 1300 200
Ingestion with food or water 6.5 x 10^6 1 x 10^6
-----------------------------------------------------------------
The results are given in Table I for the most important
isotope of Pu, 239-Pu, which contains 1 curie of radioactivity
for each 16g, and for the mixture of Pu isotopes that would be
commonly found in power reactors, which is 6 times more intensely
radioactive (1 curie in each 2.5 g). We refer to the latter as
``reactor-Pu'' and use it in our discussions where appropriate.
Table I shows the inverse of the risk, which we call the
``cancer dose.'' For example, we see that the risk of inhaling
reactor-Pu is 1/200 per [micro]g, so if one inhales 10 [micro]g,
he has one chance in 20 of developing cancer as a result.
Another application is that in a large population we may expect
one cancer for every 200 [micro]g inhaled, so if a total of 1000
[micro]g is inhaled by people, we may expect 5 cancers
(regardless of the number of people involved).
Estimates of cancer doses of Pu have also been derived using
different methods by the British Medical Research Council in its
report ``The Toxicity of Plutonium,'' and by Dr. C.W. Mays (who
developed some of the important basic information in his
experiments on dogs) in a report published by the International
Atomic Energy Agency (IAEA-SM-202/806), and they agree closely
with Table I. We see from Table I that Pu is dangerous
principally when inhaled as a fine dust. It is not very toxic
when ingested with food or drink because of its very small
probability of passing through the intestine walls into the
bloodstream. Pu forms large molecules, which have great
difficulty in passing through membranes.
In addition to causing cancer, intake of plutonium can also
cause genetic defects among progeny in the next 5-10 generations,
but the total number of eventual genetic defects before they are
bred out is only 20% of the number of cancers. For simplicity we
restrict our discussion to cancers, but the genetic effects can
always be included by applying the 20% addition.
The estimates in Table I are based on data from radiation
effects on humans as analyzed in the BEIR Report. These include
Japanese A-bomb survivors, miners exposed to radon gas, people
treated for various maladies with radium or with X-rays, etc.
None of these effects were from Pu -- there is no evidence for
any injury to humans from Pu toxicity. However, there is a
considerable amount of data from animal studies with Pu, and this
is summarized for lung cancer in Fig. 1 where the line shows the
estimate from our calculation. In general the agreement is quite
reasonable.
[Omitted: ``FIGURE 1. Data from animal studies with Pu,
summarized for lung cancer.'' The graph shows 40 data-points,
with confidence intervals, from animal studies (dogs, mice, rats,
rabbits) with a calculated line over them. The x-axis, which is
logarithmically scaled, is labeled ``Dose to Lung (millions of
millirem)'' and the y-axis is labeled ``Incidence of Lung Cancer
(%)''. Taking representative points from the calculated line in
the figure, we get: (~0.3 Mmrem, ~1%), (~1.0 Mmrem, ~5%), (~10.0
Mmrem, ~38%), (~11.0 Mmrem, ~65%). Mmrem: millions of millirem.]
There has been a great deal of publicity about the high point
for beagle dogs (the highest point in Fig. 1) but we see that our
curve passes within the error bars given by the authors. One
aspect of the experiment that is frequently overlooked is that
the latent period for development of the cancers increased with
decreasing dose, and in fact the dogs contributing to the point
under discussion developed cancer rather late in life. If this
effect is extrapolated to lower doses, the latent period for most
doses usually considered would greatly exceed life expectancy, so
the effects we derive in this paper would be substantially
reduced.
2. CRITICISMS OF STANDARD PROCEDURES
There have been several criticisms of treatments like the one
we have given. The best known of these is the ``hot-particle''
theory, which gives greatly increased effects (by a factor of
100,000) due to the fact that the Pu is not evenly distributed
over the lung but is concentrated in particles, which give much
higher than average doses to a few cells. This theory has been
studied and rejected by the following groups:
o A Committee of the U.S. National Academy of Sciencesb
especially assembled for this study in a report entitled
``Health Effects of Alpha-emitting Particles in the
Respiratory Tract
o U.S. National Council on Radiation Protection and
Measurement (NCRP), a very distinguished group composed of
about 70 in our nation's leading radiobiomedical research
scientists, in NCRP Publication No. 46
o British Medical Research Council in ``The Toxicity of
Plutonium''
o U.K. National Radiological Protection Board in its Report
R-29 and Bulletin No. 8 (1974)
o U.S. AEC in a very elaborate study, WASH-1320, authored by
three of the world's leading researchers on Pu toxicity
o U.S. NRC in Federal Register, Vol. 41, No. 76
o U.K. Royal Commission on Environmental Pollution -- Sixth
Report -- Nuclear Power and the Environment
One easily understood aspect of these criticisms is that there
were about 25 workers at Los Alamos who inhaled varying amounts
of Pu about 30 years ago, and according to the ``hot-particle''
theory each should have experienced about 200 lung cancers,
whereas there have been no lung cancers as yet among them.
According to our estimates in Table I, there is a 40% chance that
one of them would have had lung cancer, so this is experimental
evidence that Table I does not grossly underestimate the cancer
risk from Pu intake. [For more on the Los Alamos workers see
George L. Voelz, Robert S. Grier, Louis H. Hempelmann, ``A
37-Year Medical Follow-Up Of Manhattan Project Pu Workers'',
Health Physics, Vol. 48, No. 3 (March 1985), pp. 249-259.]
Another criticism of the ``hot-particle'' theory is that there
are experiments on animals in which two groups were exposed to
the same total amount of Pu but in one of them it was much more
in the form of hot particles -- and that group experienced fewer
cancers. It was also pointed out that particles in the lung do
not stay in one place but are constantly moving about so that
their exposure does not fall on only a few cells.
After these rejections of the ``hot-particle'' theory
appeared, John Gofman, a former research scientist who has spent
the past several years as the full-time leader of an antinuclear
organization, came out with a new theory ascribing enhanced
toxicity to Pu. His paper was not written for a scientific
journal but was inserted in the congressional Record by Senator
Gravel. His basic premise was that smoking destroys the cilia,
the fine hairs that stop dust particles from entering the
bronchial region -- this much was well established -- and that Pu
particles therefore remain in that region for a very long time,
allowing their radiation to cause bronchial cancers. This allows
him to ignore the animal data as animals do not smoke. He also
manages to explain the lack of lung cancers among the 25 Los
Alamos workers by a combination of four improbable hypotheses,
the failure of any one of which would destroy his theory.
There have been at least seven individual critiques of the
Gofman theory. Perhaps the most telling criticism is that there
was a series of experiments at New York University in which a
number of graduate students inhaled a controlled amount of
radioactive dust and the rate at which this dust was cleared from
the bronchial region was directly determined by placing radiation
detectors over their chests and measuring the radiation intensity
as a function of time. It was found that there was no difference
between smokers and nonsmokers, and the experimenters concluded
that smokers do more coughing and have increases mucous flow,
which compensates for their lack of cilia. In fact, if dust
accumulated in the bronchial region of smokers in the manner
postulated by Gofman, their bronchial tubes would be completely
closed and they would die by suffocation.
There were many more weak points in the details of the Gofman
paper. He misuses the BEIR Report, he miscalculates the area of
the bronchial region by a factor of 17 and thereby incorrectly
increases the toxicity by that factor, he misuses the ICRP lung
model, etc. He even suggests that the great increase in lung
cancer in recent years may be due to Pu, but this increase has
been steady since the 1930s whereas Pu-induced cancers should not
have occurred until 1960. Moreover, the lung cancer increases
have been in areas with chemical industry and high air pollution,
and there has been no increase in areas downwind from the Nevada
test site where Pu would have its maximum effect.
A relatively less publicized attack on the conventional
approach to evaluating Pu toxicity is the ``warm-particle''
theory of Edward Martell. He hypothesizes that natural radiation
is one of the principal causes of lung cancer, but this idea has
not been accepted by the cancer research community.
K.Z. Morgan has proposed that the relative biological
effectiveness (RBE) for Pu in bone might be 250 times larger than
the usual value. C.W. Mays, on whose experiments much of
Morgan's hypothesis is based, reanalyzed Morgan's work and
concluded that if his approach is correct, the increase should be
only by a factor of 10. There is experimental information on
this from some supposedly ``terminally ill'' patients injected
with Pu in 1945-46 to study Pu metabolism. Four of these are
still alive and one who was injected with a rather large quantity
died of unrelated causes only in 1968. If the RBE of Pu were 10
times the present value, there is a better than even chance that
one of these five would have gotten bone cancer, but none did.
As our calculated inhalation effects are dominated by lung
cancer, a factor of 10 increase in bone cancer risk would only
double the total inhalation risk.
S.M. Wolfe, and employee of a Nader-sponsored group, drew
far-reaching conclusions from the fact that 11 of the first 30
deaths in the US Transuranic Registry (a registry of people who
have worked with plutonium) revealed cancers on autopsy, whereas
based on listed cause-of-death for all U.S. males, only 6.2 of
each 30 deaths is from cancer. His paper, which was never
published in the scientific literature, received very wide
publicity in the news media. However, it turned out that
autopsies were done preferentially on people who had died of
cancer, and that explained the entire effect. In addition, it
was pointed out that Pu is expected to cause cancers of the lung,
bone, and liver, whereas among the 11 cases there were no bone or
liver cancers, and less than the expected number of lung cancers
for a normal population. Needless to say, the news media never
bothered to report that the Wolf paper was based on an incorrect
premise.
In evaluating all of the criticisms outlined above, it is
important to realize that they are actively considered every year
by a committee of the ICRP and that they have repeatedly been
rejected. Likewise, the EPA, which has jurisdiction in the U.S.,
studied the matter and decided not to modify its standards. No
standard-setting or official study group in any country has given
credence to any of these criticisms of the standard procedures we
used in deriving Table I.
3. CONSEQUENCES OF PLUTONIUM DISPERSAL
It is clear from Table I that Pu is dangerous principally as
an inhalant, so we now consider the consequences of a dispersal
of Pu powder in a populated area. The calculations are done with
a standard meteorological model, in which the dust cloud moves
with the wind dispersing in the downwind, crosswind, and vertical
directions. Meteorologists have determined the extent of
dispersal as a function of wind velocity and atmospheric
stability. Figure 2 shows the results of calculations assigning
the atmospheric stability most characteristic of each wind
velocity. This is different between day and night, so separate
curves are given for each.
These curves give the area within which various fractions,
q/Q, of the dispersed Pu are taken in by a person inhaling at an
average rate. For example, we see from Fig. 2 that for a typical
daytime 8 m/sec wind velocity, only in an area of 500 m^2 is as
much as 10^-6 (one millionth) of the dispersed Pu inhaled. A
typical city population is 10^-2 people/m^2, so there would
typically be about 5 people in this area. Similarly, from Fig.
2, about 60 people would inhale 10^-7, 700 people would inhale
10^-8, etc. of the dispersed Pu.
As we know the cancer risk per microgram of Pu inhaled from
Table I, it is straightforward to calculate the total number of
cancers expected per gram of Pu dispersed. When corrections are
applied for the fraction of typical Pu powders that are in
particles of respirable size, the efficiency of dispersal, the
protection afforded by being inside buildings, and decreased
breathing rates at night, the result is that we may expect about
one eventual cancer for every 24 g of Pu dispersed, or about 19
fatalities per pound.
If there is a warning, as in a blackmail scenario, people can
be instructed to breathe through a folded handkerchief or a thick
article of clothing, with a resulting decrease in fatalities to 3
per pound dispersed.
Eventually, the Pu settles to the ground but it may then be
blown up by winds. Meteorologists have also developed methods
for calculating these effects (``deposition'' and
``resuspension''). Within the first few months, this causes
about one-third as many cancers as inhalation from the initial
cloud. Beyond this time period, resuspension is of much less and
continually decreasing importance as the Pu becomes part of the
soil.
[Omitted: ``FIGURE 2. Area over which the ratio of inhaled to
dispersed Pu has values shown for q/Q versus wind velocity under
typical day and night atmospheric conditions.'' The x-axis,
which is logarithmically scaled, is labeled ``Wind Velocity
(meters/sec)'' and the y-axis is labeled ``Area (meter^2)''.]
Of course, 239-Pu lasts for tens of thousands of years, so let
us consider its effects over this time period. We know the
amount of uranium in soil and we know now how much there is as
dust in the air, so we can estimate how much is inhaled per year
-- it calculates out to be 1.3 x 10^-11 of that in the top 20 cm
of soil. If this factor is applied to the Pu after it becomes
part of the soil, we find that over the 25,000-year half-life
there will eventually be about one fatality per 2500 g of Pu
dispersed. Thus, we see that the long half-life is almost
irrelevant; nearly all of the damage eventually done occurs very
soon after dispersal.
A summary of all these effects of Pu dispersal is given in
Table II. It also includes plant uptake into food. There is a
great deal of information on uptake of Pu by plants both from
laboratory experiments and from several areas where an
appreciable amount of Pu has gotten into the soil from bomb tests
or from various research activities. Plant uptake is small for
the same reason that Pu does not easily pass through the walls of
the intestines -- it forms large molecules, which do not easily
pass through membranes. From Table II we see that the total
eventual effect of Pu dispersal in a city is one fatality per 18
g dispersed without warning, or 25 fatalities per pound.
TABLE II
Summary of Fatalities per Gram
of Reactor-Pu Dispersed
-----------------------------------------------
Inhalation from cloud 0.042 (1/24)
Resuspension 0.014
Long Term 0.0004 (1/2500)
Plant uptake into food 0.002
Total 0.058 (1/18)
-----------------------------------------------
4. DANGERS OF PLUTONIUM DISPERSAL
The fear is sometimes expressed that the world may become
``contaminated'' with 239-Pu. To evaluate this potentiality, we
calculate that if all the world's present electric power were
produced by fast breeder reactors in an equilibrium situation
where Pu is consumed as fast as it is produced, the total amount
of 239-Pu existing in the world would be 2 x 10^8 curies.
By comparison, the radium (226-Ra) in each meter of depth of
the earth's crust is 1.2 x 10^9 curies, so there is as much Ra in
each 17 cm of depth as there would be 239-Pu in the whole world.
For ingestion, Ra is 40 times more toxic than Pu as it passes
through the intestine walls much more easily. For direct
inhalation, Ra is less hazardous than Pu, but it serves as a
source of radon gas, which comes up out of the ground and mixes
with the air we breathe, and therefore is a serious inhalation
hazard, so as material on the ground, Ra is a 40-fold greater
inhalation hazard than Pu.
Thus, as a long-term hazard either for ingestion or for
inhalation, Ra is 40 times worse than Pu; the total Pu in
existence for an all-breeder power system would then be as
dangerous as the Ra in each 4 mm of our soil. Of course, nearly
all of this Pu would be in reactors or in other parts of the
nuclear industry, well isolated from the environment.
There is now a legal requirement on the allowable releases of
Pu from nuclear plants, which is such that if all U.S. power were
nuclear and derived from fast breeder reactors (they use the most
Pu), the total releases would be about 0.6 g/year. If we use
table II, this would predict an average of 0.03 fatality/year,
but that would be valid only if nuclear plants were in cities; as
they are not, the expected effects are about 10 times less, or
one fatality in 300 years.
Some perspective on this problem may be obtained by comparing
the 0.6 g/year tht [sic] may some day be released by the nuclear
industry with the amount of Pu that has been dispersed in the
atmosphere in nuclear bomb tests, which is 5 million g.
Estimates on the same basis that we have been using predict about
200 U.S. fatalities to date from Pu releases in bomb tests, and
4000 in the world. It also predicts about 200 fatalities
worldwide from the reentry burn-up in 1964 of a space vehicle
carrying a SNAP-9A 238-Pu-powered energy source. It is important
to keep in mind that all of these estimates are theoretical.
These is no direct evidence for Pu toxicity having caused serious
injury to any human being, anywhere, ever.
The reason why the legal requirement on plutonium releases is
so stringent is not because Pu is so dangerous, but because the
technology is available for keeping the releases that low, and in
fact this technology is very close to present practice. Pu dust
particles tend to stick to each other and their containers, so Pu
is not easily dispersed. It is also very readily collected on
filters; anywhere Pu powder is used, the air is exhausted through
filters, which catch all but about one part per billion of the
dust suspended in air.
Of course, the control measures are expensive and they
increase the cost of nuclear electricity. As previously noted,
the reason they are required is not because Pu is so dangerous --
one fatality every 300 years is surely a trivial problem when
burning coal, our only viable alternative to nuclear energy, is
killing 10,000 people every year with its air pollution -- but
because the public is afraid of plutonium. Ralph Nader, former
Senator Ribicoff, John Gofman, and their like have done their
work well, and the public is paying the price in its electric
bills.
One often hears that in large-scale production of Pu we will
be creating unprecedented quantities of a poisonous material.
Because Pu is dangerous principally as an inhalant, we compare it
in Table III with quantities of other poisonous inhalants
produced in the U.S. We see that it is relatively trivial by
comparison. Moreover, it should be noted that Pu is not easily
dispersed whereas the others are gases and hence readily
dispersible. Of course, Pu released to the environment will last
far longer than these gases, which would be decomposed
chemically, but recall from our earlier discussion that nearly
all of the damage done in Pu dispersal is by the initial cloud of
dust; all of the later resuspension and the thousands of years
spent in the soil do far less damage. It is thus not unfair to
compare Pu with the poison gases, and we see from Table III that
it will always be far less of a hazard.
TABLE III
Lethal Inhalation Doses Produced Annually in the
U.S. (x 10^12)
----------------------------------------------------------
Chlorine 400
Phosgene 18
Ammonia 6
Hydrogen cyanide 6
Pu if all U.S. power were from fast breeder reactors 1
----------------------------------------------------------
It is often argued that there is a great deal we do not know
about Pu toxicity. While this may be true, one would be
hard-pressed to name another public health issue that is as well
understood and controlled. Surely it would not be air pollution
from burning coal, which is a million times more serious a
problem. Surely it is not food additives or insecticides or such
[the dangers from these have also been greatly exaggerated] that
may well be doing real harm to our health. Pu hazards are far
better understood than any of these, and the one fatality per 300
years they may someday cause is truly trivial by comparison.
In spite of the facts we have cited here, facts well known in
the scientific community, the myth of Pu toxicity lingers on.
The news media ignore us, and prefer to continue scaring the
public at every opportunity. They don't recognize the difference
between political issues on which everyone is equally entitled to
an opinion, and scientific issues, which are susceptible to
scientific investigation and proof. The myth may linger forever.
* * *
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