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L'înformâtion et les sèrvices publyis pouor I'Île dé Jèrri

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Risks from x-ray radiation

​​About ionising radiation

CT scans and x-rays use ionising radiation. Ionising radiation has enough energy to cause damage to cells, which can increase the risk of cancer later in life. However, these risks to health are very low and ionising radiation is widely used in cancer therapy. In general the health effects of ionising radiation are dependent on the dose received.

CT scans and radiation

The dose of x-ray radiation needed for a CT scan is much more than for a single x-ray picture, but is still generally quite a low dose.

The risk of harm from the dose of radiation used in CT scanning is thought to be very small, but it is not totally without risk. As a rule, the higher the dose of radiation, the greater the risk. So, for example, the larger the part of the body scanned, the greater the radiation dose. And, repeat CT scans over time cause an increased risk of harm. Also, the younger you are when you have a CT scan, the greater the lifetime risk of developing cancer or leukaemia. However, all medical radiation doses are kept as low as reasonably possible and all requests for any form of x-ray have to be justified so the benefit outweighs any possible harm. 

Radiation from natural sources

Every day all over the world people are exposed to ionising radiation, mainly from natural sources in the environment and from space. In the UK the Health Protection Agency (HPA) (now part of Public Health England) has worked out that on average people are exposed to about 2.7 millisieverts (mSv) of radiation a year. A millisievert is a measure of radiation dose which allows for the fact that ionising radiation can affect different parts of the body to differing degrees.

The 2.7 mSv dose that people in the UK are exposed to comes from a number of sources. Many building materials contain low degrees of natural radioactivity, as does granite and radon gas seeping from the ground into buildings, so the largest exposure is from naturally occurring radiation in homes and workplaces. There are also natural contributions from food and cosmic radiation. A table comparing different sources of radiation is shown below. 

Dose comparisons for ionising radiation

One way to think about the risk is to compare the radiation dose from a CT scan or x-ray to other activities, such as an aeroplane flight. The table below compares different events to the average amount of background radiation you would receive.​

(The table is adapted from information found on the Public Health England website. The doses quoted are an average for the UK and our local medical doses are often lower than these.)
 Source of exposure
Dose (mSv)
Equivalent period of natural
background radiation (1)
Lifetime additional risk of
fatal cancer per examination (2)
Dental x-ray        
 less than 1.5 days
 1 in a few million
135g bag of brazil nuts            
 less than 1.5days
 1 in a few million
Chest x-ray
 3 days 
 1 in a million
Transatlantic flight
 11 days 
 1 in 300,000
​Cervical spine (neck) ​0.08 ​ 14 days ​ 1 in 200,000
Nuclear power station worker average annual occupational exposure     
 27 days
 1 in 125, 000
Thoracic spine ​0.7 ​ 4 months ​ 1 in 30,000
​Hip ​1.0 ​ 5.5 months ​ 1 in 22, 000
​Lumbar spine ​1.3 ​ 7 months ​ 1 in 15,000
UK annual average radon exposure
 7 months
 1 in 15,000
​Barium swallow ​ 1.5 ​ 8 months ​ 1 in 13,000
CT scan of the head
 8 months
 1 in 13,000
UK average annual radiation dose
 15 months
 1 in 7,350
​Barium follow through ​ 3 ​ 16 months ​ 1 in 6,700
USA average annual radiation dose
 32 months
 1 in 3,700
CT scan of the chest
 36 months
 1 in 3,200
Average annual radon dose to people in Cornwall
 3.5 years
 1 in 2,500
Whole body CT scan
 4.5 years
 1 in 2,000
Annual exposure limit for nuclear industry employees
 9 years
 1 in 1,000
Level at which changes in blood cells can be readily observed
 45 years
 1 in 200
Acute radiation effects including nausea and a reduction in white blood cell count
 450 years 
 1 in 20
Dose of radiation which would kill about half of those receiving it in a month
 2,250 years

1. UK average = 2.7mSv per year: regional averages range from 1.5 - 7.5 mSv per year
2. Approximate lifetime risk for patients 16 to 69 years old: for paediatric patients multiply risks by about two; for geriatric patients divide risks by about five
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