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Radiation

Dose

The biological effects of heavy particle ionising radiation are measured with a quantity called Absorbed Dose. However, while two differing particles may have the same amount of energy, they may have very different responses from our living, biological cells, so a factor called Relative Biological Effectiveness (RBE) is also used. RBE is a measure of the amount of cell damage a given type of particle causes as compared to a dose of x-rays with the same energy. The combination of RBE with Absorbed dose is measured in units called Sieverts.

Estimated dose (Micro-Sieverts) Activity
5 sleeping next to your spouse for one year
10 a year of watching TV at an average rate
10 a year of wearing a luminous dial watch
10 a year of living in the USA from nuclear fuel and power plants
10 a day from background radiation (average, varys a lot throughout the world)
20 having a chest x-ray
65 flying from Melbourne to London, via Singapore
300 Yearly dose due to body's potassium-40
460 maximum possible offsite dose from Three Mile Island Accident
400 - 1000 Average annual dose from Medical sources
7,000 having a PET scan
8,000 having a chest CT (CAT) scan
50,000 off-site dose from accident at Chernobyl Nuclear Power Plant (estimates vary widely)
2,000,000 Typical single dose to Cancer region from Radiation Therapy
700,000 - 13,000,000 staff and firefighters at the Chernobyl Nuclear Power Plant during and immediately after the accident
65,000,000 Typical total dose to Cancer region from Radiation Therapy

Background, naturally occurring environmental sources of radiation include cosmic radiation (which occurs when subatomic particles from outside the solar system interact with the Earth's atmosphere and produce a shower of gamma rays, neutrons and leptons), terrestrial radiation (due to naturally occurring radionuclides such as uranium, radium and thorium which are present in rocks, soil and water), internal radionuclides (most common is potassium-40 which emits beta and gamma particles as it decays, and may be found as a fraction of the total amount of potassium in the body). The largest source of our dose from background radiation is from radon-220 and radon-222 gases, which are the airborne products of the decay of terrestrial uranium. Inhalation of radon gas contributes approximately two-thirds of the dose that is received from natural, background sources.

Biological Effects of Radiation.

Thus we can see that during our normal daily activities we are continually exposed to ionising radiation both from natural and human-made sources. The passage of ionising radiation through the body may produce adverse biological effects. The effects of concern are primarily, although not solely, due to damage to the genetic material inside the cell; the DNA or `double-helix' molecule that carries genetic information. The current scientific data suggests that there is no safe minimum, or threshold, for adverse radiation effects on the DNA of biological systems and that even small doses can produce consequences for the organism. The low-dose response of biological systems it believed to be linear - that is, smaller doses of radiation produce a proportionately smaller risk of adverse effects like a straight-line graph.

The biological effects fall into the following categories;

Cell Death or Apoptosis

- biological systems are composed of many individual tiny cells; each with its own DNA (only red blood cells don't have any DNA - they lose it during development). If there is catastrophic damage to vital cell function by radiation energy absorption the cell may cease to function and `die'.

Cancer Induction

- in this case the DNA is damaged or altered but the alteration is not lethal to the cell. If the normal regulator genes which control the rate at which cells divide and die are rendered malfunctioned the cell may become `immortal' and multiply at an abnormal rate producing an out-of-control growth of a line of abnormal cells. This is a cancer. Most tissues can produce cancers with enough radiation damage but rapidly dividing tissue lines, such as blood-forming `haemopoietic' lines which may produce leukemias, are particularly vulnerable. The risk for cancer production in the general population is estimated to be approximately 0.06 cases per million Micro-Sieverts of absorbed dose.

Genetic Damage to Future Generations

- this can arise because mutations, or changes in the pattern of bases in the DNA, can occur in the DNA that ends up in sperm or eggs and becomes a permanent feature of any resulting babies. Most often, radiation induced damage to such egg or sperm DNA is incompatible with the life of the fetus in utero but there is a finite chance of a live baby being born with defects. This is of particular concern because the damage to the genetic material can then be passed on to all future generations and become a permanent feature of the gene-pool; damaging many individuals. Of course, such mutations are also a natural part of life and the evolution of biological systems. The concern is that we do not want to increase the mutation rate above the natural background rate. The estimated risk of permanent damage to a second generation (grandchild) individual is 0.02 cases per million Micro-Sievert of exposure.

Dose-Response Tissue Reactions

- or `radiation burns'. These are the sorts of effects seen immediately after the bombing of Hiroshima; but lesser effects can occur with smaller doses. However, there does seem to be a `threshold' for this kind of effect with very small doses encountered in normal life (apart from sun-burn!) not producing detectable damage.

More information on the biological effects of radiation can be found at the International Commission on Radiological Protection web site.

A Study on Childhood Cancers in Great Britain

A study on the incidence of childhood cancer around nuclear power plants in Great Britain by the Health Protection Agency for the committee on Medical Aspects of Radiation in the environment concluded that there have been no increase in childhood cancers for children living less than 25km from a nuclear power plant. The report was published in 2005.

Download the report here.

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