Most of us don't have any way to judge the hazard to us from a variety of things that may adversely affect us as we go about our daily life. Many things are widely accepted as hazardous but few of us have the expertise to judge just how dangerous they may be. It is especially difficult when the things that we are concerned with are measured in unfamiliar units with the person telling us about them jumping wildly among milli, micro, Curies, Becquerels, rad, rem, Sievert, Gray or other arcane (to the listener) descriptions of the thing being discussed. This is compounded by the problem that many people have thinking about what big numbers really mean. Comparison of risks to other risks is often doomed to failure. Very few people can relate the hazard of a cigarette to the risk per mile of a car trip let alone the risk of exposure to quantities of something unfamiliar expressed in completely alien units.
There is however one hazard for which we all have a good intuitive sense. That is the hazard of high places. That is, any location that is at an elevation that is greater than an adjacent location.
Everyone recognizes that if a person were to step off the top of an eight story building there would be a severe health effect ... very soon. There are radiation doses that are large enough to produce as severe an effect except that "very soon" is days to months rather than seconds. If someone stepped off something 1/1000th as high as that building the effect would be much less. (At this point I'll ask you just how high that would be and you can take a moment to think about it.) If you were here with me I would provide the answer by gesturing about 1 inch (a couple of centimeters). We all accept the risk of changes in elevation of an inch or more many times a day and in very nearly every case there is no adverse effect. When someone does stumble because of a small difference in the height of the surface that they are walking on, they probably are not hurt or may very infrequently scrape an elbow, break an arm or suffer some other unpleasant but repairable injury. The same is true with radiation. Exposures to moderate doses usually produce no injury to the person and if there is an injury in the vast majority of cases the injury is repairable. (The damage from radiation, if it occurred, would be at the cellular level and wouldn't be noticed by the person and the repair or replacement of the damaged cell would also be undetectable.)
In very exceptionally rare
cases one of
these lower "high places"
could
result in a severe injury which could be non repairable. The individual
could stumble on a minor irregularity in the pavement and strike their
head, leading to a concussion, or a hemorrhagic stroke, possibly
leading
to death. This all would probably take place over hours to days. In the
case of radiation there may be cellular changes which develop into
benign
or cancerous tumors which may be treatable or may lead to death. With
radiation
this takes place over 10 to 50 or more years. In either case we don't
expect
the most severe outcomes except in very rare circumstances and even
then
we expect that medical intervention will provide a favorable outcome in
many cases.
Now consider drops 1/1000 as high, that is 1/1,000,000 of that 8 story
building. How high is that? (Again I'll let you try to figure it out.)
An image may help, it's a fraction of the thickness of a sheet of
paper.
It is incredibly unlikely that such a drop would cause anyone to have
an
accident but even if we believed that a tumble was because of a drop
from
that height there would be no way that we would be able to prove that
or
demonstrate just which of the extraordinary large numbers of
irregularities
that high or higher that are in every surface we traverse is the cause
of our fall.
Now how does all this talk of falls from high and not so high places relate to radiation doses. The 8 story building is equivalent to about 1000 rads (10 Grays). Tumors are often given doses larger than this during radiation treatments. (By restricting the highest dose to the tumor, it can be killed while the rest of the person receives a lower dose). A radiation worker's annual radiation dose limit corresponds to a few inches. One year's natural background most places on earth is comparable to a fraction of an inch though some locations are as high as half a foot. A chest X-ray is similar to stepping off something as high as a pile of a few sheets of paper. A single coast to coast flight will be one sheet or so. A full abdominal series of X-rays on a overweight person could be like a drop of half a foot or more. Some radiation therapy treatments can expose the whole body to doses equivalent to a fall of more than one story. These points of comparison let a person think about the risks involved in a reasonable way. It also gives a jumping off point (pun intended) for a discussion of many other aspects of radiation risk. Check back to this paragraph (it is the meat of the analogy) as you read on to see how these values fit with the ideas below.
The following paragraphs give some directions that our discussion may go from here. I have never used them all when talking with one person and it would have done more harm than good if I had tried to. Based on the questions that you have you can jump to any of the points below. There is little dependence of any one on any of the others.
-------------------------
Variation
in effect
for the same exposure
Differences
between
individuals
Differences within an individual
Some
final thoughts
----------------------------------------------------
Measuring
radiation exposure
Our ability to measure either the height or the radiation dose is not
a problem. Both radiation and height are physical quantities and can be
measured very accurately. Depending on the situation
either can be measured to several significant figures. Measuring height
uses simpler tools but both can be accurately determined. Uneven
surfaces
that a person fell from or onto could make it more difficult to
precisely
determine the height of a fall just as dosage that is varies from one
location
to another would make that measurement harder.
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Variation
in effect for the same exposure
The accuracy with which we can determine the size of either the fall
or a radiation dose is far better than our ability to predict the
health
effect in any particular case. Individuals could be exposed to falls
from
exactly the same height and have radically different outcomes. The same
is true of radiation. The small chance of injury normally doesn't keep
us from moving around on surfaces where we are exposed to changes in
elevation
that range from minuscule to a foot or so. It would seem that radiation
could be viewed in the same way if it were as familiar and we could see
it as easily.
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Differences
between individuals
There is considerable variation in the effect of a fall that depends
on who is falling. An elderly woman who suffers from osteoporosis is
much
more likely to be injured by a fall than a healthy child or young
adult.
The response to radiation also varies among individuals. However, for
radiation,
age has the opposite effect. Because nearly all mechanisms of radiation
damage require cell division to make themselves apparent, they may not
appear for many years making them much more likely to be seen in the
younger
individual.
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Differences
within an individual
Even
within the individual there is a wide variation in the effect of
a radiation dose. The most radiosensitive tissues are those with
rapidly
dividing cells (blood forming and intestinal lining) and the most
resistant
are those that divide rarely or not at all (brain and bone). Since
there
isn't any way for a part of the body to fall without the rest of the
body
I have a problem with this part of the analogy but I will make an
attempt. (If you can suggest a better way of explaining this point
please let
me know.) Consider that rather than dropping a part of the
body
on a fixed surface
we drop something the same size and weight as that part of the body on
a person. It is clear that if a hand-like object struck your hand after
a fall of several stories it would hurt but it would be far less
significant
than your head being struck by a head-like object dropped from a
similar
height. In a similar way there are parts of the body in which radiation
is more likely to cause injury.
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Partial
Body Exposure
Different parts of the body can be exposed to different
amounts
of radiation. In fact in both accidental and intentional
exposures it is impossible to avoid because of the variation of dose
due to distance from the source, reduction in dose to one part of the
body because of absorption in another part and in the case of internal
sources, such as radioactive iodine, selective uptake by different
cells in the body. In most cases this variation can be taken
into
account in determining the effect of the exposure. During
cancer
therapy the doctors try to deliver a does to the tumor that will kill
all of its cells while limiting the damage to cells in the rest of the
body. With current medical procedures it is impossible to
completely spare healthy tissue during therapy. External
radiation beams of X or gamma rays will often produce reddening of the
skin where they enter and leave the body. Radiation scattered
from the primary beam will expose radiosensitive tissues.
Internal sources will also deliver dose to nearby cells that are not
cancerous. As a result the dose to the tumor can be less than
optimal in order to avoid life threatening doses from exposure to other
parts of
the body.
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Multiplication
of effect
A fall because we unexpectedly stepped off a curb can multiply the
effect
of that small change in elevation by increasing the impact of the event
resulting in more damage than simply stepping down. In the case of the
fall any injury is highly dependent on just which part of the body
absorbs
the energy. Landing on your hands, shoulder or backside is likely to
result
in much less significant damage than taking the whole force of the
impact
on your head. You can see a similar effect in radiation. If the
radiation
hits particular points in the cell there will be a higher probability
of
harm than if other places are hit. Points that can cause this sort of
increased
damage are rare just as are falls from curbs that cause serious injury.
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Exceptional
circumstances
In the case of both radiation and falls there have been cases where
people have survived when most people would not have. Children have
fallen
from fifth floor windows with their fall broken only by shrubs and
survived
with little medical intervention. In
Fort
Lauderdale May 11, 2005 a 69-year-old
woman survived a nine-story fall with only shoulder injuries when she landed
on a first floor canopy.
Like
these exceptional cases, people have been exposed to doses that would
be expected to kill most of us but yet recovered. In those
cases they usually have had the advantage of accidental shielding that
protected some of their blood forming organs providing a supply of
undamaged
cells to help them through the critical period following exposure. Bone
marrow transplants can provide another source of undamaged cells which
could improve their chances. Having an identical twin helps a lot if
you
are exposed to doses that would be expected to be lethal.
In either the case of a fall from a very high places or exposure to a
very high dose it takes a very fortunate set of circumstances for a
favorable outcome.
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Benefit
and risk
For an activity or situation to be reasonable it is important that its
benefits should outweigh its risks. It is apparent that not everyone
agrees
on where the balance point might be when we consider activities like
rock
climbing and skydiving. However I think that few would argue that the
benefits
of a curb separating the road from a sidewalk, providing a clear
demarcation
between vehicular and pedestrian areas as well as helping to control
rain
water flow, is greater than its risk due to its elevation. Similar risk
benefit considerations should enter into the evaluation of activities
that
involve radiation. An x-ray for diagnosis of a possible broken bone is
very likely to be justified while the same dose to see how your shoes
fit,
as was done in the 1950's, is probably not.
Another way activities can be evaluated is risk vs. risk. If two
activities
both accomplish a particular goal is one safer or otherwise better than
another.
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Legal
limits
In the United States and throughout much of the rest of the
world there are limits to the heights and the amount of radiation to
which
a member of the public can be exposed and there are less stringent
limits
for radiation workers. One obvious difference in terms of height is
that store employee
can be required to climb a ladder as part of their job while a member
of
the public in that same store is restricted to stairs or ramps. Workers
can be required to work on a stage, loading dock or the back of a truck
where a fall of several feet (a meter or so) is a distinct possibility
and those doing
residential roofing often work, with little or no protection, a couple
of
stories above the ground. Regulations require that in public
places people
must be protected from the possibility of falls from much lower
heights.
In the case of radiation there have been water and air quality
standards
promulgated that ensure that members of the public
receive
doses that are less than 1/300th of those to which a worker could be
legally exposed.
Part of the justification for the difference is the voluntary versus
involuntary
nature of the exposure. An employer is required to keep worker informed
of his or her radiation dose and actions by both the employer and
employee
can be taken to limit further exposure. The members of the public
however
are usually not aware of radiation to which they are exposed and
therefore
have no way to take action to reduce it.
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It's
natural
We are all exposed to radiation from a variety of natural sources just
like the changes in the roughness of the surfaces that we cannot avoid
in the course of our daily lives. You would expect that different
locations
(a cobblestone street or a basketball court) would not be equally
smooth.
Radiation levels also vary with location. You should expect to find
lower
radiation levels in a boat on the ocean than high up a granite
mountain.
It is important to realize that we are exposed to a variety of sources
of radiation both natural and man made and it is impossible to sort out
whether an effect was caused by one or the other (or something else
entirely)
for any particular case.
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Shielding
A very minor point that may be obvious, a handrail or wall is analogous
to a radiation shield in preventing an individual from being exposed to
possibly harmful situations.
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Stochastic??
Deterministic??
Some people with whom I have discussed this analogy have complained
that I have confused the issue by freely mixing stochastic (random
response
to low doses of radiation) and deterministic (guaranteed effects for
high
doses). What they fail to recognize is that in the case of a fall from
various heights exactly the same is true. This correspondence often
helps
people see radiation effects more clearly. They recognize that when a
person
falls from some great height death is virtually certain. However, a
fall
of 1 story or so, a healthy adult could reasonably expect to be hurt
but
wouldn't expect to be killed. This would correspond to about 100 rads
(1 Gray), of radiation where we would expect to see changes in blood
counts
and immune response but not the more serious reactions that would
likely
lead to death. For much smaller drops, say off a curb or a radiation
workers
maximum allowable annual dose, we are talking about negative effects
only
very very rarely. When the dose or height is much less than that there
are a lot
more verys than in that last sentence.
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Linearity
of effect
It may seem reasonable that the further you fall the more likely you
are to be injured and for reasonably large values this seems to be the
case. For radiation the assumption is often made, in the context of
establishing
regulations, that a dose 1/10th as great will produce an effect 1/10th
as great. This is usually referred to as linearity. But, if we consider
only those drops of less than a few millimeters, a quarter inch or so,
it is much more difficult to establish the truth of this apparently
reasonable
assertion. In fact it may be impossible to prove it even for drops of a
foot (30 cm.) or more. When we understand this problem for height it is
likely that
we will have an appreciation with the difficulty of proving, or even
expecting,
linearity for low radiation doses. As a side effect of this assumption,
one would expect to get the same result from many small "impacts" (from
elevation
or radiation) whether those "impacts" are to one or many individuals,
as
would be obtained by the same total "impact" given to one person. The
assumption
of linearity also makes no provision for the possibility that there
will
be recovery from damage that may have been caused by the first events
before
later ones occur. Whether or not these views seem reasonable to you
with
respect to elevations that are less than the height of a curb, they are
equivalent to the assumption on which many radiation regulations are
based.
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Thresholds
I am not talking about the thing that you step over when you leave a
house. What I would like you to consider is the possibility that there
is some height that we all can deal with that has no possibility of
harming
us. That would constitute a threshold for an effect (if we don't ever
go
past a change in elevation that exceeds that small value we will never
be exposed to injury from changes in elevation). If you accept the
possibility
that this may hold for some small value of elevation changes you may be
open to the possibility of a similar effect for radiation (that is,
there
is some small dose below which there will be no adverse effect.)
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Could it be
good for you?
The minor bumps, jars and stresses that we are exposed to in the normal
course of our lives when we deal with the stairs, curbs, and just
walking
around do seem to be good for us. Studies of people who have been
limited
in their exposure to these minor impacts, by enforced bed rest or
during
space flight, find significant negative changes in them especially in
the
strength of their bones and muscles. This analogy doesn't prove that
the
same is true of radiation. But there are scientific reports that seem
to
demonstrate for some situations (animal or group exposed, amount of
dose,
duration of exposure) that low doses could be beneficial. After reading
many of those reports and considering the proposed mechanisms I
consider
the jury to still be out.
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Real
radiation units
This section gets technical and is not part of
the high
places analogy
so feel free to skip it. It is included for those who want
to
know more about how radiation is measured.
The physical units,
Roentgens, rads and Grays.
There are differences between the Roentgen and rad but the numeric
values for each are nearly equal for most materials and common
radiation energies.
The Roentgen is defined as the amount of gamma or X-ray radiation
(only) that releases a particular charge in one kilogram of
air
(only). The rad is the quantity of radiation (of any sort)
that
deposits an energy of 100 ergs per gram of material (of any
sort). In the early days of radiation the Roentgen was
something
that a physicist could measure so it was the standard. Later
a
more universal unit was needed so the rad was defined. Still
later when SI (International System, metric) units were adopted, by
nearly everybody but the US, the Gray was defined as a secondary unit
equal to
1 Joule per kilogram. Converting units shows it to be equal
to
100 rads. All of these units were defined
independently of
any consideration of biology.
The
biological units,
rems and Sieverts
In studies of many different biological systems (cell cultures,
insects, animals, plants ...) there is some consistency in the relative
effects of the same dose of radiation but with different linear energy
transfer (LET) characteristics. A definition is in order
here. LET is the rate at which a particle loses energy along
its
path. It is defined only for particulate radiations not
X-rays or
gammas. An electron or beta particle only
ionizes an
occasional atom it passes, spreading its energy over a relatively long
path. An alpha particle of the same energy ionizes many more
and
deposits its energy over a much shorter distance and protons are
somewhere in-between. The LET depends on the energy of the
particle as well as what kind. Because X-rays and gammas
deliver
their energy by knocking electrons out of atoms and those electrons
have a low LET the effective LET for X-rays and gamma radiation is the
same as that of electrons.
Because of these differences the effect on an organism is usually much
larger from a high LET radiation dose than that from an equal dose
delivered by low LET particles. The ratio of doses that
produce
the same effect for different radiations is called the relative
biological effectiveness (RBE). The RBE for a particular pair
of
radiations is not the same for all biological systems but it tends to
be similar. After a large number of experiments involving a
variety of biological systems and radiations were conducted a
relationship between LET and RBE was noted. From that
relationship a set of values ,known as Quality Factors (QF), were
developed. They are 1 for X-rays and gammas, 20 for alphas,
and intermediate values for other radiations. Now it is easy
to
get the dose in rem or Sieverts (the effective dose). Just
multiply the dose in rads by the QF appropriate for the radiation
field. The SI unit for effective dose is the
Sievert which
like the Gray is 100 times as large as its counterpart in non SI
units. Because most of the penetrating radiation to which we
are
exposed is low LET, the dose in rems is normally very close to the dose
in rads.
One caution, the values of QF are only approximately true and then only
for relatively low doses. For doses above 10 rads or so the
dose
in rads is a much better predictor of biological effect than the dose
in rem.
Radioactive material units,
Curies and
Becquerels
The quantity of radioactive material is measured in Curies.
It
was originally defined as the number of disintegrations taking place
in 1 gram of radium but it was later redefined as exactly 3.7E10
(37,000,000,000) disintegrations per second when
it became easier to do the necessary physics than the
chemistry.
The corresponding SI unit is the Becquerel. It is 1
disintegration per second of any radioactive material.
Each disintegration releases photons and/or particles that carry energy
(measured in units of eV or MeV). How much dose results from
the
disintegration depends on where that energy is deposited.
Each
radionuclide produces its own mix of particles and photons of various
energies. If the disintegration produces only high
energy
gammas the energy will be spread over a large volume and the dose from
that event will be low. The same energy released in the form
of
particles with shorter ranges (higher LET) will concentrate the energy
in a smaller volume and make a higher dose there. So a particular
quantity of a high energy gamma emitter inside the body is less
significant than the same number of Curies of an alpha emitter since
some of the energy from the former won't remain in the person while all
of that from the alpha emitter will. If each of the radiation
sources is outside the body the short range of the alpha particles
makes them very much less significant than the
gammas. When
trying to understand the effect of radioactive materials inside the
body this gets very complicated but if the same amount of energy is
deposited in the same parts of the body over the same time it doesn't
matter whether the source of the radiation is inside or outside.
Multipliers
Any of the units that are mentioned in this section can be modified by
a prefix multipliers. "Milli" (as in millirem for example)
refers
to an amount that is 1/1000th as large as a rem. "Micro"
means
the
unit is 1/1,000,000th of the basic unit. When you see "kilo"
the
quantity is 1000 times as large as the unmodified unit and "mega" is
1,000,000 times as large. There are lots more prefixes for
both
larger and smaller multipliers but those are the common ones.
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Some final
thoughts
Over the past 40 years or so I have used this analogy many times and
it has grown steadily more complete and more complex. I have found that
in most cases the first part of this explanation gives you the
opportunity to recognize/explain that the dose that is the source of
their concern is equivalent of a sheet of paper or half an inch or
wherever
it falls on this scale. That image seems to be accepted much more
readily than other
ways of explaining it.
Some people may be concerned about using this analogy to explain the
risks of radiation exposure because it leaves open the possibility that
very small doses may lead to death. And it does leave that possibility
open, but most people will recognize that just as the tiny
irregularities
on a hardwood floor may cause a person to stumble which may result in a
fall, and that fall could cause a person to strike their head, and that
blow may cause a blood clot to form, which might be in a critical part
of the brain, and that could cause a person to die, that chain of
events
is so unlikely that it isn't worth much consideration as we walk around
and I find that most
are willing to accept that the same may well be true of equivalent
amounts
of radiation.
One can clearly
try to use this
analogy beyond where it is
applicable
but if you recognize its limits I hope that you will be able to use it
to help explain to yourself and to others one way to look at the risks
of radiation
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Alan