What background radiation is

1. A detector switched on in an ordinary room still records counts even though no radioactive source is present.
2. Those counts come from background radiation, which is always there in the surroundings.
3. The level is low, so it is far weaker than the radiation from a sealed source held close to a detector.
4. Most background radiation is natural and comes from radioactive materials in rocks, soil, air, food and the body, together with radiation arriving from space.
5. A smaller share comes from human activity, mainly medical uses of radiation.
6. A detector cannot tell background radiation apart from radiation coming from a source, so the background count rate must be measured and then subtracted from every reading.

Background radiation from Earth

1. Rocks and soil contain small amounts of naturally occurring radioactive isotopes, mainly uranium, thorium and potassium.
2. As those unstable nuclei decay they release ionising radiation into the surroundings, so the ground itself is a weak source.
3. Granite holds more uranium than most other rocks, so areas built on granite, such as parts of Cornwall and Aberdeenshire, have a higher background level than average.
4. Bricks, stone and concrete are made from rock, so building materials raise the background level measured inside a house.
5. Uranium in rock decays through a series of steps that produces radon, a radioactive gas that escapes from the ground into the air.
6. Radon collects in enclosed spaces such as cellars, mines and poorly ventilated rooms, and it is the largest single natural contributor to the average dose received in the UK.
7. Houses in high-radon areas are fitted with underfloor fans or sealed floors so that the gas is drawn away rather than allowed to build up indoors.
8. Food and drink contain radioactive potassium-40, so eating and drinking adds a small amount of radioactive material to the body.
9. All living things contain radioactive carbon-14, which means the human body is itself a weak source of background radiation.

Background radiation from space

1. Cosmic rays arrive at the Earth continuously from every direction in space.
2. When a cosmic ray strikes a nucleus high in the atmosphere it produces a shower of other fast-moving particles that travel down towards the ground.
3. The atmosphere absorbs much of this radiation, so only part of it reaches sea level.
4. The higher you go, the less atmosphere lies above you to absorb the radiation, so cosmic radiation increases with altitude.
5. People living in mountain regions, and aircrew who spend long working hours at cruising altitude, receive more cosmic radiation than people at sea level.
6. Cosmic rays also create carbon-14 in the upper atmosphere, which then passes into plants, animals and people through food chains.

Human-made sources

1. Medical procedures are the largest human-made contribution, including X-ray images, computed tomography scans and radioactive tracers used in hospitals.
2. Nuclear power stations and the industries that supply and reprocess their fuel release small, monitored amounts of radioactive material.
3. Fallout from past nuclear weapons testing and from nuclear accidents left long-lived radioactive isotopes in soil and dust across large areas.
4. Together these human-made sources add far less to the average dose than natural sources do.

Why the level varies

1. Background radiation is not the same everywhere, because the sources are not spread evenly over the Earth.
2. Places built on granite have more uranium beneath them, so more radon is released and the background count rate is higher.
3. Places at high altitude receive more cosmic radiation because less of it has been absorbed by the air above.
4. Poorly ventilated rooms, cellars and mines let radon collect instead of dispersing, so the level indoors can be higher than outdoors.
5. Some occupations raise exposure, for example mining, flying aircraft and operating X-ray equipment in a hospital.
6. Because the level varies from place to place, the background count rate must be measured in the same room, with the same detector, as the experiment it will be subtracted from.