Radiation units confuse even technically trained people because four very different things share the same field: how much radiation a source emits, how much energy a body absorbs, how much biological harm that energy is expected to do, and the old US customary equivalents of all three. Once you separate them, the picture clears up.
The Four Quantities
| Quantity | SI Unit | Old Unit | Conversion |
|---|---|---|---|
| Activity | Becquerel (Bq) | Curie (Ci) | 1 Ci = 3.7 × 10¹⁰ Bq |
| Absorbed dose | Gray (Gy) | Rad | 1 Gy = 100 rad |
| Equivalent dose | Sievert (Sv) | Rem | 1 Sv = 100 rem |
| Exposure | C/kg | Röntgen (R) | 1 R ≈ 2.58 × 10⁻⁴ C/kg |
| Effective dose | Sievert (Sv) | Rem | weighted whole-body Sv |
Dose in Everyday Context
Most people receive 2–3 mSv per year from natural background — cosmic rays, radon, food, and the body's own potassium-40. Flying from London to New York adds about 0.05 mSv. A single dental X-ray is roughly 0.005 mSv, a chest X-ray about 0.1 mSv, and a chest CT around 7 mSv. Living one year next to a normally operating nuclear power station adds well under 0.01 mSv. The headline-grabbing exposures — full-body CT at 10 mSv, interventional cardiology procedures at 15 mSv — are still far below the 100 mSv threshold where cancer risk becomes statistically measurable.
Why Equivalent Dose Matters
Different radiation types deposit energy differently. Gamma rays and X-rays scatter energy widely; alpha particles dump everything into a few cells. A weighting factor (1 for X-ray/gamma, 20 for alpha, 2–20 for neutrons depending on energy) converts gray to sievert and gives a quantity that better predicts long-term biological risk. Effective dose adds organ-by-organ weighting on top, since the same Sv to bone marrow is far worse than to skin.
- Background: ~2.4 mSv/year worldwide average.
- Dental X-ray: ~0.005 mSv.
- Mammogram: ~0.4 mSv.
- Annual public limit: 1 mSv above background.
- Annual worker limit: 20 mSv (5-year average).
Three Units, Three Different Questions
The most common mistake is treating becquerels, grays, and sieverts as interchangeable. They answer completely different questions, and only the last one speaks to health risk:
- Becquerel (Bq) — “How active is the source?” One decay per second. A smoke detector holds about 30,000 Bq; your own body runs near 4,000 Bq from natural potassium-40. High activity nearby does not automatically mean high dose.
- Gray (Gy) — “How much energy did the tissue absorb?” One joule per kilogram, with no regard for which radiation type delivered it.
- Sievert (Sv) — “How much biological harm is likely?” Gray multiplied by a radiation-weighting factor (1 for X-rays and gamma, up to 20 for alpha). This is the only unit you should compare against dose limits.
For authoritative dose figures and safety guidance, bodies such as the World Health Organization publish reviewed reference values. This guide is general information, not medical or safety advice.
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