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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.