Calculating Risk
Before we do something potentially dangerous we either consciously or unconsciously calculate the risk involved. You would certainly not worry too much about crossing the street where you lived but would you walk across a busy motorway?
We can calculate risk using this formula:
Risk = Probability x Consequences
When teachers plan school trips they must now use this to identify and minimize dangers
So what are the risks of ionising radiation? Why is ionising radiation dangerous?
When ionising radiation such as an alpha particle hits a human body cell it leaves a trail of destruction.
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The atoms of the molecules it strikes may be permanently
damaged. These molecules do important jobs in the cell and if enough are
damaged then the cell may be destroyed.
Cells which divide rapidly (skin, testis) are most prone to damage. If the chromosomes inside the nucleus are damaged this may cause cancer. The cell divides rapidly and the cancerous cells take over the body. Radiation may also cause mutations if it interferes with chromosomes in the cells of the testis. |
When body tissue is exposed to radiation if we know the energy of the particles involved and the count rate we can calculate the absorbed dose in Grays.
An absorbed dose of 1 Gray is when 1 kg of tissue absorbs 1 J of energy
We must also take into account the type of radiation involved and the tissue which is exposed. We do this by multiplying by a quality factor to find the equivalent dose in Sievert.
Equivalent Dose (Sv) = Absorbed Dose (Gy) x Quality factor
| Background radiation | 2 mSv in 1 year |
| A chest X ray | 200 µSv |
| Working in nuclear power station | 40,000 µSv in 1 year |
so what does this mean in terms of risk?
A rough estimate (by the world health organisation) is that an absorbed dose of 1 Sv carries with it a probability of cancer of about 5%.
| The risk of getting cancer due to background radiation? | = 2 x 10-3 x 5% = 0.01% or 1
in 10,000 per year
If you live 80 years it is 1 in 125 |
| The risk due to working in a nuclear power station? | = 40 x 10-3 x 5% = 0.2% or 1
in 500 per year
However if you work there 40 years it is about 1 in 20 |
