How to evaluate the doses resulting from ingestion or inhalation of radioactive substances ? How to translate activities measured by detectors - but difficult to understand - to effective doses that are of concern for the man in the street? In other words, how to convert becquerels into millisieverts ?
When radioactive atoms are ingested, the resulting effective dose is found by multiplying the activity of the amount of ingested atoms by a ‘dose conversion factor’. One calculates in this way the radioactive potential toxicity of a substance, called ‘radiotoxicty, which is the dose to which a person or a group of persons would have been exposed had they absorbed all the radioactive atoms of the sample.
There are also tables of dose factors for inhalation, whose use is less straightforward because these factors depend on the size of the aerosols which carry the radioactive atoms.
The relevant effective dose obtained is a ‘committed dose', i.e. a dose evaluated for a person’s lifetime. This evaluation depends on the method of ingestion and the path taken by the radiation through a person’s body.
One calculates "equivalent" and "effective" committed doses resulting from ingestion or inhalation of a radioactive substance. One has to estimate, nearly on the scale of a human life, the final dose that will be suffered by a tissue (or organ) or by the whole body. The assumed time span is 50 years for adults and 70 years for children in order to reflect the phasing out of radioactive substances or their non-elimination for some of them.
Alpha emitters (such as uranium, plutonium, and the actinides produced by nuclear reactors) are far more dangerous in this regard than beta emitters (such as the products of nuclear fission). We can also see that the inhalation of alpha emitters (and their consequent presence in the lungs) is several tens of times more hazardous than their ingestion.
The paths of radioelements in the human body differ according to their chemical properties. Radioactive atoms will undergo the same metabolic processes as their neighbours in the periodic table. This is why radium can be found alongside calcium in the bones, and why iodine-131 as well as its stable isotope both target the thyroid gland.
The thyroid gland plays a particularly important role before adulthood, and as a result children and adolescents are far more sensitive than adults to radioactive iodine 131. A toddler will be over ten times more sensitive to radioactive iodine that makes its way to the thyroid.
In order to evaluate dose factors the most accurately possible, radiobiologists use models which take into account the type and strength of radiation involved, as well as the way in which energy is deposited in the body and the age of the recipient. These models, though inevitably imperfect, make use of the most up-to-date research in the field. Graphs and figures are then published and updated annually to include the more recent data.
The values of dose factors, which convert ‘activities’ in becquerels to ‘doses’ in sieverts, are very tiny because the Sievert (Sv) is a large unit and the Becquerel(Bq) a very small one. The largest dose factors of heavy elements such as plutonium are around one ten-millionth of Bq/Sv, whereas light elements such as tritiated water have much smaller factors of closer to one hundred-billionth Bq/Sv.
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