Monday, March 14, 2011

Radiation - Putting doses into perspective

Sources of radiation

The Irish population is exposed to radiation from several sources, which are present either naturally in the environment or have been produced artificially by man.
By far the biggest proportion of our exposure to radiation comes from natural sources, particularly radon. Other natural sources include cosmic radiation from outer space, radioactivity in the food and water we eat and drink and radiation from the ground.  The beneficial use of radiation in medicine is the principal source of artificial radiation to which we are exposed (diagnostic X-rays, radiotherapy for cancer treatment, etc.).
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Effects of ionising radiation

A very high dose of ionising radiation over a short period of time to the whole body can cause death within a period of days or weeks.  If the same dose were received over a limited part of the body, it might not prove fatal but other early effects could occur.
If the same dose were received over a prolonged period – weeks or months - there would be more opportunity for the body to repair itself and there may be no early signs of injury.  However, late effects could occur from the damage to the tissue which may only become apparent later in life or in the person’s descendents.  The most important of these late effects is cancer.
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Long-term radiation risk

In Ireland, there is little risk of exposure to doses likely to cause early effects and most people will be more concerned about the risks associated with low doses of radiation and the long-term effects of these doses.
Although the cause of most cancers is poorly understood, exposure to agents such as tobacco smoke, asbestos and radiation are known to induce them. Radiation may induce cancer by causing mutations in DNA. These mutations allow a cell in normal tissue to begin abnormal growth that can sometimes lead to a malignancy. There can be a prolonged gap between the initial biological damage caused by ionising radiation, and the appearance of clinical symptoms. This can range from several years up to 40-plus years.
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How are doses calculated?

The risk from exposure to ionising radiation is determined by calculating the effective dose (commonly shortened to dose) that takes into account
  • The amount of radiation
  • The type of radiation
  • Its effect on specific organs
  • The specific organs exposed
  • The overall risk of harm.
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What units are used to measure doses?

The effective dose (or dose) 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)
= 1,000,000,000 nanosievert (nSv)
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Cancer risks

The effects of high and very high doses of radiation are reasonably well known from scientific studies and scientists know fairly accurately the risk of a fatal cancer at this end of the scale.
At lower 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 µSv (6 Sv) death is almost certain - a straight line between the two points gives the relationship between dose and risk.
In simple terms, a small dose carries a small risk, a medium dose carries a medium risk, and a high dose carries a high risk.
Based on the direct relationship between dose and risk, we can estimate that a dose of 10 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 one in four. This means that 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 10μSv of ionising radiation, an additional 2 cancer deaths could be expected.
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Putting doses into perspective

The following list gives the radiation doses commonly received every year by the average person in Ireland. Also given, at certain doses, is the lifetime risk of a fatal cancer.
  • 1 microsievert (μSv) – the average annual dose to a ‘heavy’ consumer of seafood from the Irish Sea 
  • 8 μSv – the dose received on a return flight from Dublin to London 
  • 20 μSv – the dose from a single chest X-ray 
  • 20 μSv - 1 in 1,000,000 lifetime risk of fatal cancer
  • 45 μSv – the annual average dose from airline travel 
  • 240 μSv – the annual average dose from radioactivity in food 
  • 300 μSv – the annual average dose from gamma radiation from the ground 
  • 350 μSv – the annual average dose from cosmic radiation 
  • 540 μSv – the annual average dose from medical examinations
  • 1000 μSv (1 mSv) - 1 in 20,000 lifetime risk of fatal cancer
  • 2230 μSv (2.23 mSv)– the annual average dose from radon in the home and workplace
  • 3950 μSv (3.95 mSv) – the average total annual dose from all sources of ionising radiation
  • 10,000 μSv (10 mSv) – 1 in 2000 lifetime risk of fatal cancer
Doses at and above 1,000,000 μSv (1Sv) received over a short period of time are given below to illustrate the doses at which immediate harm to the body is evident.
  • 1,000,000 μSv (1 Sv) – Onset of early radiation effects 
  • 2,000,000 μSv (2 Sv) – Threshold for early death 
  • 4,000,000 μSv (4Sv) - 50 per cent chance of survival 
  • 6,000,000 μSv (6Sv) – Early death.  

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Minimising your risk

Radon is the largest source of radiation to which most irish people are exposed and it is one of the sources that is most readily controlled.  You can minimise the health risk from radon gas, by measuring it in buildings and, if necessary, reducing its concentrations in your home and in your workplace.
The RPII regulates the use of artificial radiation in certain workplaces to minimise the risk to workers and the public.

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