Ionising radiation and health

risk analysis, with particular attention to radioactivity

Introduction

This document is introductory only; very much more is known about radiation and can be learnt by study. The purpose of this document is to remove irrational fears and to present basic analysis of risk. The figures given in the document are often useful examples rather than the latest figures for your country or local conditions.

You live surrounded by ionising radiation, just as you live in a world surrounded by heat. If you were transported into the sun, or even a furnace, you would not last long. Heat is a form of energy; at normal background levels it is no big deal, but you sure don’t want to take up residence in the core of a nuclear reactor, any more than you would welcome living unprotected in a coal-fired furnace.

Them there rays…
Electromagnetic Radiation

In 1902, Ernest Rutherford found that three different kinds of radiation are emitted in the decay of radioactive substances. These he called alpha, beta, and gamma rays in sequence of their ability to penetrate matter. Be aware that alpha, beta and gamma particles are named in order of reducing size, but of increasing energy. Gamma particles also penetrate matter more because they interact less with other matter.

Alpha particles were found to be identical with the nuclei of helium atoms.

Beta rays were identified as electrons.

Gamma rays In 1912, gamma rays were shown to be much more penetrating and have all the properties of very energetic electromagnetic radiation, or photons. Gamma-ray photons, when they originate from radioactive atomic nuclei, are between 10,000 and 10,000,000 times more energetic than the photons of visible light. Gamma rays, with a million million times higher energy, make up a very small part of the cosmic rays that reach the Earth from supernovae, or from other galaxies. The origin of the most energetic gamma rays is not yet known.

High energy is not highly dangerous in isolation. I would rather a drop of rain fell on my head at 50 miles per hour, than a grand piano at 10 miles per hour. These particles are incredibly small. If I am hit by a small fast moving bullet, it is likely I’ll survive the experience; whereas a slower-moving, but physically larger, blast from a shotgun is liable to inconvenience me greatly. High-energy particles have the potential to damage the cells in your body, setting off cancers. But I emphasise, you live among such radiation, you just do not welcome a surfeit.

advertising
disclaimer

Here are some example approximate thicknesses:

Food: eating is bad for you!

The lifetime risk of cancer:

That is, the risk of eating at all is around 60 times the risks of all those additives that the media rattle on about. If you are really worried, perhaps you might like to starve! [1]

Now that is just the cancer risk from eating that dangerous stuff called food. You might also choke on the stuff, get mad cow disease (BSE), or some nasty, potentially fatal, e-coli bug like E.c. 0157:H7. Even if you don’t die from eating food, it is quite likely that you will still die! Be happy, don’t worry – well, at least not so much.

There is no such thing as a dangerous substance or a poison,
only a dangerous dose.

During radioactive decay, an unstable nucleus usually emits alpha particles, electrons, gamma rays, and neutrinos.

In nuclear fission, the unstable nucleus breaks into fragments, which are themselves complex nuclei, along with other particles such as neutrons and protons. The resultant nuclear fragments are often in a highly excited state, and then reach their low-energy ground state by emitting one or more gamma rays.

Because gamma rays have no electric charge, and thus do not interact with matter as strongly as do charged particles, they have great penetrating power. Because of their penetrating power, gamma rays can be used for radiographing holes and defects in metal castings and other structural parts. At the same time, this property makes gamma rays extremely hazardous. The lethal effect of this form of ionising radiation makes it useful for sterilizing medical supplies that cannot be sanitized by boiling, or for killing organisms that cause food spoilage. More than 50 percent of the ionising radiation to which humans are exposed comes from natural radon gas, which is an end-product of the radioactive decay chain of natural radioactive substances in minerals. Radon escapes from the ground and enters the environment in varying amounts.

Naturally produced background radiation

Naturally produced background radiation comes from a number of sources, as shown in the illustration above.
Some human-generated radiation sources are also indicated. A very high radiation source is Chernobyl, with people living in Control Zones near to the plant receiving about 10 mSv each year.

A comparison of the activities of selected radioactive materials

Note that plutonium is 153,000 times more active than depleted uranium. (2,298,000/15=153200)

U238, U235 and U234 predominantly emit alpha particles (over 95% are alpha particles). The alpha activity of natural uranium amounts to about 25 kBq/g. The progeny from the alpha decay of uranium themselves continue to decay, mostly by emitting beta particles. The activity of these progeny is added to that of uranium. The beta radiation of the progeny of natural uranium and depleted uranium have practically the same intensity, amounting to about 25 kBq/g.

U238 series

As a lump of radioactive material breaks down breakdown products start to accumulate in the lump. When these are radioactive they also start to break down according to the attributes of the particular material. This generates further radiation and so on. A typical breakdown series for u238 can be found here.

Uranium, together with its progeny, has an activity of 50 kBq/g (for instance, 50 000 decays take place per gram per second).

The very long half-life [read this linked section now, if you do not understand half-life] of U238 (4.5 billion years) yields a low decay rate per unit mass of uranium. Naturally occurring uranium, which mostly consists of U238, is one of the least radioactive substances containing unstable isotopes on the planet. It is classified by the International Atomic Energy Agency in the lowest hazard class for radioactive materials.

Exposure to the radiation emitted from uranium can occur if it is outside the body, or if it is ingested, inhaled or taken in by other means. It is useful to consider the exposure pathways with regard to average radiation exposure in the normal environment. As you can see in the illustration, gamma radiation can travel right through you from the environment and alpha radiation can be stopped by your skin. However, should you ingest radionucleides (radioactive materials) and they become lodged in the body, the alpha radiation will become a considerably greater problem because the radionuclides will not just wash or fall off.

Alpha rays are 20 times more effective than beta and gamma rays at causing tissue damage. To allow for this, the dose in grays is multiplied by an effectiveness factor and the new units are called sieverts (abbreviation Sv) and the dose is called the equivalent dose.

The Sievert (Sv) is the international measure of radiation expressed as a dose-equivalent. In the general population ingestion of uranium and its decay series in food and drink gives a committed effective dose of 0.11 mSv per year for adults, as compared to 0.0058 mSv through inhalation, excluding inhalation of radon (1.2 mSv).

This annual dose corresponds to 5% of the average annual dose due to internal and external exposure to natural sources of radiation (2.4 mSv). It relates essentially to progeny, Pb210 and Po210. U238 accounts only for 0.00025 mSv total dose and 0.000021 mSv for inhalation. External exposure from all natural U238 in soil is also negligible. U238 series, together with other primordial radionuclides, Th232 and K40, cause a world-average annual external exposure of about 1 mSv per year.

Risks

If I have a 1 in 1 million chance of being in an aeroplane crash and my risk doubles, I have then a 2 in 1 million chance of being in a crash. These rates are worked out by calculating, for example, how many people on average died in such crashes over the last 10 years. If none were killed in 9 of the years and 20 in the final year, that will give you an average of 2 each year.

Of course in the year that 6 die in a crash, the Daily Slime will ‘report’ as follows: “200% more killed in ’plane crashes this year, something must be done”. (2 is the 100% base average figure, 6 is three times 2 and is therefore 300% relative to the base rate, hence the rise of 200% above the base rate.) Although, knowing full well the ignorance of the reporters at the Daily Slime, they will probably say 300% because they can’t count either.

The chance of being killed on the roads of America  is approximately 1:8000 each year, over 80 years that is a risk of 1 in 100. This is now becoming a serious risk compared with figures for flying! A doubling of that road-kill risk would be serious news!

The risk of being killed while flying on large commercial jets is around 1 in million, if you fly 200,000 miles a year (the risks are higher on commuter flights and higher still in a private aircraft). Clearly you will not be killed in an aircraft if you do not fly in one; but you can still worry if you like; there is even about 1 chance in 25 million that an aeroplane will fall on you sometime in your life!

It is important to

• distinguish between base rates, and
• percentage or ratio changes from absolute rates.

It is further important to remember that

• all figures relate to particular conditions [2]
• averages do not tell you what will happen to you!

Risks from different types of ionising radiation exposure

• The atomic bomb
• X-rays
• High altitudes, including flight
• Plutonium
• Depleted uranium
• Decommissioning nuclear reactors

The atomic bomb

The atomic bomb, or atom or fission bomb, is a weapon whose explosive power comes from the fission (or splitting) of atomic nuclei. When the nucleus of a heavy atom, such as uranium-235, is split, a certain amount of mass is released as an equivalent amount of energy, powering the atom bomb. On a pound-for-pound basis, the U-235 in an atomic bomb can release on the order of one million times as much energy as TNT.

Effects of an Atomic Bomb Explosion

On Aug. 6, 1945, an atomic weapon of about 15 kilotons was exploded about 1,800 feet (550 meters) over the Japanese city of Hiroshima. On August 9, a plutonium-based weapon of about 20 kilotons was exploded about 1,800 feet (550 meters) above Nagasaki. The bombs devastated both cities. About 70,000 people died at Hiroshima and about 40,000 at Nagasaki, and many thousands more were injured.

The devastation of Hiroshima and Nagasaki resulted from three main types of effects:

• blast,
• thermal radiation, and
• nuclear radiation.

The blast effect of an atomic bomb is similar to that of a conventional explosive but much more intense and far-reaching. (Note: only the blast effect is significant for chemical high explosives.)
Thermal radiation, which results from the extremely high temperatures created by an atomic explosion, causes serious burns on exposed parts of the body and may ignite fires over a wide radius.
Nuclear radiation, which results from the neutrons and gamma rays associated with fission, causes death and injury as a result of damage to living tissue.

Among the survivors of the attacks on Hiroshima and Nagasaki, roughly equal numbers of injuries were caused by blast and thermal radiation but considerably fewer by nuclear radiation. Each of the three types of effects posed serious hazards to unprotected persons out to a distance of about a mile (1.6 km) from a point directly below the explosion.

X-rays

You may find some comments and references on medical x-ray radiation here.

X-rays are a type of penetrating radiation that, depending on the dose, can reduce cell division, damage genetic material, and harm unborn children. Exposure to x-rays is measured in units of radiation absorbed dose (rad).

Cells that divide quickly are very sensitive to x-ray exposure. Unborn children are particularly sensitive to x-rays because their cells are rapidly dividing and developing into different types of tissue. Exposure of pregnant women to sufficient doses of x-rays could possibly result in birth defects or illnesses such as leukaemia later in life. With most x-ray procedures, relatively low levels of radiation are produced. However, a doctor may decide to postpone or modify abdominal or lower back x-rays in a pregnant woman unless absolutely necessary. Women who receive x-rays before realizing they are pregnant should speak to their doctors. Some pregnant women may be exposed to x-rays in the workplace, so governments may establish limits to protect unborn children from radiation exposure in work settings.

The rate of risk of cancers from X-ray exposure are calculated from atomic bombs and other exposures, extrapolating for the smaller exposures given by X-rays.

High altitudes, including flight

There is some theoretical effect, but empiric results do not easily detect it.

An excellent summary regarding plutonium

Aspects of depleted uranium munitions [.doc format]
how dangerous is depleted uranium?

The person who wrote this document has a neat sense of humour. Obviously, they also have had to deal with moonbats!

“DU [depleted uranium] is used commercially in medicine (radiation shields).”
—
“The ATSDR [US Agency for Toxic Substances and Disease Registry] cites that "no human cancer of any type has ever been seen as a result of exposure to natural or depleted uranium" and further states that because of the low radiation from natural and depleted uranium, "no radiological health hazard is expected from exposure to natural or depleted uranium." ”

Decommissioning nuclear reactors

You will find a great deal of hysterical nonsense about vast monetary sums and time scales involved in ‘decommissioning’. As you will see from a careful reading of this document and its companion document on nuclear power, the ‘dangers’ are great exaggerations, based mainly in ignorance.

Naturally, the nuclear industry will quote huge sums for clean-up, just as other government contractors, and their political clients, will seek enormous profits in fields like aviation, space flight and pharmaceuticals, where most people are unable to judge the technology or to make reasonable assessments of costs.

All the waste from all nuclear activities since the beginning of the development of nuclear power would fit into the space of a few football fields; and, as you will see from these documents, this nuclear waste is not the dangerous horror as represented by some lobbyists.

In fact, a major reason that no large, deep-earth depository has yet been built is that the amount of waste currently available does not easily justify the cost of dealing with it. Delays and obstruction are aggravated by constant campaigns from those seeking publicity through scaremongering, combined with the usual NIMBY (not in my backyard) syndrome. These delays are tiresome because we end up with scattered and inadequately supervised dumps in many locations.

The wastes of the nuclear industry are quite trivial when placed against the last century or two of devastation, ruined landscapes, subsidence and externalised filth, attributable to the fossil fuel industries. The difference in waste level is hardly surprising as it is estimated that something like 10 million times the energy can be extracted from an atom of uranium as can be extracted from an atom of carbon in fossil fuels!

Some definitions

Bequerel (Bq)

The unit of measurement for radioactivity. An activity of one Bq means that one decay takes place per second.

dyne

unit of force in the centimetre-gram-second system of physical units, equal to the force that would give a free mass of one gram an acceleration of one centimetre per second per second, or approximately 2.248 10{sup -} pound. One dyne equals 0.00001 newton.

erg and joule

unit of energy or work in the centimetre-gram-second (cgs) system of physical units used in physics; to lift a pound weight one foot requires 1,356 x 10^7 ergs. It equals the work done by a force of one dyne acting through a distance of one centimetre and is equal to 10{sup -7} joule, the standard unit of work or energy.

gray

unit of absorbed dose of ionising radiation, defined in the 1980s by the International Commission on Radiation Units and Measurements. One gray is equal approximately to the absorbed dose delivered when the energy per unit mass imparted to matter by ionising radiation is 1 joule per kilogram. As a unit of measure, the gray is coherent with the units of measure in the SI system. The gray replaced the rad, which was not coherent with the SI system. One gray equals 100 rads.
A uniform dose of 3 to 5 Gy to the whole body will kill fifty percent of people exposed in one to two months. This is a large unit and the milligray (mGy), which is one thousandth of a gray, is more commonly used.

 

rad and rem

radiation absorbed dose: the amount of radiation absorbed per unit mass of material. (Rads are often converted to units of rem by multiplication with quality factors to account for biological damage produced by different forms of radiation. The quality factor for x-rays is 1, so rads and rems are equivalent.)

 

sievert

unit of radiation dose equivalent in the International System of Units. The sievert (Sv) has been recommended by the International Commission on Radiation Units and Measurements (ICRU) as a substitute for the rem, the long-standing special unit of biologic dose of ionising radiation. Like the rem, the sievert takes into account the relative biologic effectiveness (RBE) of ionising radiation, since each form of such radiation—e.g., X rays, gamma rays, neutrons—has a slightly different effect on living tissue.

Accordingly, one sievert is generally defined as the amount of radiation roughly equivalent in biologic effectiveness to one gray (or 100 rads) of gamma radiation. The sievert is inconveniently large for various applications, and so the millisievert (mSv), which equals 1/1000 sievert, is frequently used instead. One millisievert corresponds to 10 ergs of energy of gamma radiation transferred to one gram of living tissue. See also gray.

Bibliography

End notes

1. Danger ahead, p.80.
   
   
2. Even the books I have listed are often very sloppy on this matter.

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© abelard, 2004, 05 february

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the address for this document is http://www.abelard.org/briefings/ionising-radiation.asp

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