Most of the background radiation dose we receive in the UK is from natural sources (about half is inhaled as radon gas). About 15% of our annual dose comes from human activities such as X-rays in hospital or at the dentist or emissions from nuclear establishments.

Thus the figure on the right is for 'Mr. Average in the UK'. Such data is worked out using the radiation doses received by a lot of people living and working in a lot of different locations.

The background dose comes from five main sources:

Food and drink

Human activity

Cosmic rays

Rocks and Soil

Radon

Some radiation sources are natural but human activities also raise the background count we are exposed to.

The radiation dose you get in a year depends on:

where you live (the rock type in your locality),

your state of health (whether you require treatment or investigation in hospital that involves ionizing radiation ),

your diet (salt substitutes contain potassium rather than sodium and the percentage of potassium that naturally occurs in the radioactive form is higher than that of sodium),

your occupation (whether you work in an environment that exposes you to nuclear radiation - such as a hospital or laboratory where nuclear isotopes are used)

whether you smoke or not (cigarette smoke is known to contain dangerous alpha emitters).

Measuring Backgrount Rate

The activity from background radiation is called the background rate. If a Geiger-counter is switched on in a room it will register ionising radiation. This will vary according to location.

Before an experiment with a radioactive source is carried out the background rate must be established. This is done by switching on the Geiger-counter when the source is not present and recording the average activity for twenty minutes or so.

This value must then be deducted from any readings taken with the source present so that the true activity of the sample can be found.