People in Ireland are exposed to ionising radiation every day from many sources the majority of which are natural in origin but there is a small contribution from man-made sources such as medical examinations. Regardless of origin, the effects of man-made and naturally occurring radiation are the same. The effects are determined by the radiation dose.
The radiation dose received by an individual is expressed in a unit called the sievert, which is abbreviated by the symbol Sv. The sievert is a large unit, and in practice it is more usual to measure radiation doses received by individuals in terms of fractions of a sievert.
1 sievert = 1000 millisievert (mSv) = 1,000,000 microsievert (μSv)
A very high dose of radiation over a short period of time to the whole body destroys cells in the body at a faster rate than they can be replaced which can cause serious illness and even death. Scientists know fairly accurately the risk of a fatal cancer at this end of the scale.
Exposure to the same dose to a limited part of the body or over a prolonged period of time may have no immediate effects due to the body’s ability to repair itself. However, the effects may be seen later in life with the risk it may cause cancer. These risks are actually low and in general the health effects are dependent on the dose received.
At low doses the risk of fatal cancer is not precisely known, but it is assumed that there is a direct relationship between dose and risk all the way down to zero. That means, at zero dose there is zero risk of harm, and at around 6,000,000 microsievert (µSv) (6 Sv) death is almost certain - a straight line between the two points gives the relationship between dose and risk.
Based on this direct relationship, we can estimate that a dose of 10 microsivert (μSv) may increase the lifetime risk of fatal cancer by about one in 2,000,000. This compares with the existing lifetime risk of fatal cancer of approximately 1 in 4.
In a population of four million, one million can be expected to eventually die of cancer. If all four million people were exposed to an additional dose of 10 microsievert (μSv), an additional 2 cancer deaths could be expected.
On average a person in Ireland is exposed to 3950 microsieverts (μSv) of radiation each year from both natural, and man-made sources.
These account for approximately 86 per cent of our annual average radiation dose and include the following:
Radon is a naturally occurring radioactive gas produced from the uranium that is present in all rocks and soils. Radon enters buildings from the ground and can sometimes build up to unacceptable levels. Exposure to radon increases the risk of lung cancer and is linked to 250 lung cancer cases in Ireland every year.
Like radon, thoron is a naturally occurring radioactive gas. The principal source of thoron in indoor air is building materials. Radiation doses are normally much lower than those from radon.
The natural radioactivity present in soils and the sea is transferred in small amounts to food which results in a small radiation dose which cannot be avoided.
The earth is constantly bombarded by high-energy radiation from outer space. Radiation doses are greater at higher altitudes and those who fly regularly receive an additional dose.
Radionuclides of uranium, thorium and potassium are abundant in rocks and soils. The gamma radiation emitted from these radionuclides gives a radiation dose.
These account for approximately 14 per cent of our annual radiation dose and are dominated by the use of radiation in medicine.
Many procedures carried out in medical diagnosis involve exposure to radiation such as dental and chest X-rays, mammography, CT scans, etc.
These include the fallout from atmospheric nuclear weapons testing, the Chernobyl nuclear accident and the routine discharge of radionuclides from nuclear installations. Liquid discharges from the Sellafield remain the dominant source of artificial radioactivity affecting the Irish Sea. Once present in the environment, radionuclides are available for uptake by fish, shellfish, crops and animals, and so make their way into the food chain.