Everyone on Earth is continuously exposed to low levels of ionising radiation from natural and artificial sources — this is called background radiation. Understanding how to measure radiation dose, what the sources are, and how risk depends on the type of radiation is essential for GCSE physics and for making informed judgements about safety.
What is background radiation?
Background radiation is the low-level ionising radiation that is present everywhere in the environment, from both natural and artificial sources. It is measured continuously and defines the baseline dose that every person receives, regardless of their occupation or lifestyle.
The average background radiation dose in the UK is approximately 2.7 millisieverts (mSv) per year, though this varies significantly with location, occupation, and altitude.
Natural sources account for about 85 % of background radiation:
- Radon gas — the single largest source (approximately 50 % of UK background dose). Radon-222 is produced by the radioactive decay of uranium-238 in rocks and soil. It seeps into buildings through floors and accumulates in poorly ventilated spaces. High radon areas include parts of Devon, Cornwall, and Derbyshire.
- Cosmic radiation — high-energy particles from space. Dose increases with altitude: airline pilots receive more than office workers; people at high altitude receive more than those at sea level.
- Gamma rays from the ground — from naturally radioactive rocks such as granite.
- Food and drink — radioactive isotopes such as carbon-14 and potassium-40 are present naturally in all food.
- Internal radiation — carbon-14 and potassium-40 are incorporated into the body and decay within it.
Artificial sources (about 15 %):
- Medical — X-rays and CT scans are the dominant artificial source for most people (a chest X-ray ≈ 0.02 mSv; a CT scan of the abdomen ≈ 6–10 mSv).
- Nuclear industry — very small dose for the general public in normal operation.
- Fallout — residual contamination from historical atmospheric nuclear weapons tests.
How is radiation dose measured?
Radiation dose is a measure of the biological harm caused, not just the quantity of radiation received. Two steps are required:
1. Absorbed dose (gray, Gy): The energy deposited per kilogram of tissue. 1 Gy = 1 joule per kilogram of tissue
2. Equivalent dose (sievert, Sv): Accounts for the different biological effectiveness of each radiation type. Equivalent dose = absorbed dose × radiation weighting factor (Q).
| Radiation type | Radiation weighting factor (Q) |
|---|---|
| X-rays, gamma rays | 1 |
| Beta particles | 1 |
| Neutrons | 2–20 (depending on energy) |
| Alpha particles | 20 |
Alpha particles cause 20 times more biological damage per unit of absorbed energy than gamma rays, because they transfer all their energy within a tiny volume — heavily ionising a small region of tissue.
Effective dose (also in Sv) further accounts for the sensitivity of different organs — the gonads, red bone marrow, and lungs are most sensitive.
At GCSE, the sievert is used as the unit of dose, and you need to know that alpha radiation is most damaging per unit of absorbed dose.
What factors determine the risk from radiation exposure?
Risk depends on several factors:
- Dose received — higher dose → greater risk. There is no proven "safe" threshold; risk is assumed to be proportional to dose (the linear no-threshold model used for radiation protection).
- Type of radiation — alpha is most ionising (weighting factor 20) and most damaging to DNA. Beta and gamma have a weighting factor of 1.
- Duration of exposure — acute (short, high-dose) exposure is generally more harmful than the same total dose spread over a long time, because the body can repair some radiation damage between exposures.
- Part of the body exposed — some tissues (bone marrow, gonads, thyroid) are more sensitive to radiation damage than others.
- Age — children are more sensitive than adults because their cells divide more rapidly.
- Whether the source is internal or external — an internal alpha emitter (such as radon gas inhaled into the lungs) is very dangerous because the alpha particles, despite being easily stopped by paper, deliver all their energy directly to lung tissue. An external alpha source is harmless because the skin stops alpha particles before they reach sensitive tissue.
How does dose compare across common activities and exposures?
| Exposure | Approximate dose |
|---|---|
| UK annual background radiation (average) | 2.7 mSv |
| Chest X-ray | 0.02 mSv |
| Flight London–New York (return) | ~0.16 mSv |
| CT scan (abdomen) | 6–10 mSv |
| Annual occupational limit (radiation workers, UK) | 20 mSv |
| Dose associated with measurable increase in cancer risk | ~100 mSv |
| Acute dose causing radiation sickness | >1000 mSv (1 Sv) |
These figures illustrate that everyday medical and background exposures are far below levels associated with deterministic (immediate) harm, though they do contribute a small statistical increase in lifetime cancer risk.
How do radiation workers control their dose?
The three principles of radiation protection are time, distance, and shielding:
- Time — reduce the time spent near a radioactive source.
- Distance — move further away. Radiation intensity decreases with the square of the distance (inverse square law).
- Shielding — use appropriate materials: paper stops alpha; aluminium or perspex stops beta; thick lead or concrete is required for gamma.
Radiation workers wear film badges or thermoluminescent dosimeters (TLDs) to monitor their cumulative dose and ensure they remain within annual limits.
Frequently asked questions
What is background radiation in GCSE physics?
Background radiation is the low-level ionising radiation present everywhere in the environment from both natural and artificial sources. Natural sources include radon gas (from uranium decay in rocks), cosmic rays, gamma rays from the Earth, and radioactive isotopes in food. Artificial sources include medical X-rays and residual nuclear fallout. The average UK background dose is about 2.7 mSv per year. Background radiation must be subtracted from any experimental measurements to find the count rate due to a specific source.
What unit is used to measure radiation dose and why?
The sievert (Sv) is the unit of equivalent radiation dose — it measures the biological harm caused rather than just the energy deposited. It is calculated by multiplying the absorbed dose (in gray) by a weighting factor that reflects how damaging the type of radiation is. Alpha particles have a weighting factor of 20 because they cause 20 times more biological damage per joule absorbed than gamma rays. At GCSE, you often use millisieverts (mSv; 1 mSv = 0.001 Sv) for everyday exposures.
Why is radon the largest source of background radiation in the UK?
Radon-222 is produced by the radioactive decay of uranium-238, which is present in many rocks, particularly granite. Radon is a noble gas, so it does not bond to rock — it seeps through floors and walls into enclosed spaces where it can accumulate. When inhaled, radon (and its radioactive decay products) undergoes further decay inside the lungs, releasing alpha particles that directly irradiate lung tissue. Because alpha particles are highly ionising, even relatively small activities of radon can cause significant DNA damage and increase lung cancer risk over long periods.
What is the difference between irradiation and contamination?
Irradiation means being exposed to radiation from a source outside the body — for example, standing near a gamma-emitting source. It stops as soon as you move away or the source is removed. Contamination means radioactive material is on or inside the body — for example, swallowing or inhaling radioactive particles, or getting them on the skin. Contamination continues to irradiate the body even after leaving the vicinity of the original source, and internal contamination with alpha emitters is particularly hazardous because alpha particles cannot be blocked by the body's surface once inside.
For Socratic GCSE physics with Professor Newton — reasoning from particle ionisation to biological risk before quoting any dose figure — visit aitutors.me.