(mSv—one thousandth of a sievert) are commonly used. Strictly speaking the unit of the sievert should only be used for referring to low doses of up to about a few hundred millisieverts, above this and in some other special circumstances a unit called the 'gray' should be used. For simplicity this convention will be ignored and the unit of the sievert will be used throughout this report.
To try to put this unit into perspective the average natural background radiation dose received by everyone in Hong Kong is about 2 mSv per year. A radiation worker in the nuclear industry will typically receive a few mSv per year but, may legally receive up to 50 mSv per year. A dose of 5 000 mSv received over a short period could be fatal without medical care, a dose in excess of 10 000 mSv received acutely would almost certainly be fatal.
FIG. 3.1
RADIOACTIVE DECAY
ACTIVITY
1/2
1/4
1
1/8
1
2
3
5
HALF-LIVES
3.5 Biological Effects
When radiation impinges on a cell in the body the electrically charged particles, called 'ions' produced by the radiation can cause chemical changes to the cell's structure. The most significant damage that the radiation can cause is to the DNA in the cell's nucleus. DNA carries the genetic blueprint for the reproduction of the cell. Damage to the DNA may be unimportant or be repaired by the cell, or the damage may kill the cell. Generally speaking cell death is unimportant unless very large numbers of cells are involved. There also appears to be a small chance that the damaged cell, or perhaps incorrectly repaired cell, may in later generations lose its biological mechanism which controls the rate of reproduction. This can lead to a rapidly reproducing group of cells—-
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