Article — Radiation Converter (Sv, Rem, Gy, Bq, Ci)
Radiation converter: sieverts, rem, grays and becquerels
Radiation units come in three flavours: absorbed dose (gray and rad), equivalent dose (sievert and rem), and activity (becquerel and curie). The legacy CGS units (rad, rem, Ci) survive in older texts and US regulations; SI units (Gy, Sv, Bq) dominate modern usage. Conversion is exact: 1 Sv = 100 rem, 1 Gy = 100 rad, 1 Ci = 3.7 × 10¹⁰ Bq.
The categories aren't interchangeable. Gray measures physical energy deposited. Sievert applies a biological weighting factor to that energy. Becquerel counts radioactive decays. Confusing them can lead to dose estimates off by factors of 20 or more. This converter keeps each category separate.
Radiation units overview
Absorbed dose tells you how much energy was dumped into tissue per kilogram (1 Gy = 1 J/kg). Equivalent dose multiplies absorbed dose by a radiation weighting factor — 1 for X-rays, 20 for alpha particles — to account for biological damage. Activity measures the source's decay rate, independent of what (if anything) absorbs the radiation.
Sievert vs gray for radiation dose
For X-rays, gamma rays and beta particles, 1 Gy = 1 Sv. The weighting factor is 1 because these radiations damage tissue in proportion to their energy. For alpha particles, 1 Gy = 20 Sv. Alpha emitters internal to the body (inhaled radon decay products, ingested polonium) carry the weighting factor up dramatically.
The sievert was adopted by ICRP in 1977. Before that, dosimetry used the rem (Roentgen Equivalent Man), which is exactly 1/100 of a sievert. Older instruments often read rem. The conversion 1 Sv = 100 rem is exact by definition.
The sievert was named for Rolf Maximilian Sievert, a Swedish medical physicist who pioneered radiation protection research in the 1920s-50s. He spent his career studying low-dose radiation effects long before they became regulatory concerns.
Becquerel and curie for activity
Activity measures how often radioactive nuclei decay. One becquerel is one decay per second — an extremely small amount. A typical banana has about 15 Bq from natural potassium-40. The curie (Ci), in contrast, is defined as the activity of 1 gram of pure radium-226: 3.7 × 10¹⁰ decays per second. The two units span ten orders of magnitude apart.
Medical doses often quote MBq (megabecquerel) or mCi (millicurie). 1 mCi = 37 MBq. A typical F-18 FDG PET scan uses about 370 MBq (10 mCi) of tracer. Iodine-131 thyroid therapy might use 3,700-7,400 MBq (100-200 mCi).
1 Sv 100 rem1 Gy 100 rad1 Ci 37 GBq1 mCi 37 MBq1 mSv 100 mremRadiation dose reference
Putting radiation numbers in context matters because the scale spans many orders of magnitude. A dental X-ray gives about 5 µSv. A chest X-ray, 0.1 mSv. An abdominal CT, 10 mSv. The threshold for any detectable biological effect starts around 100 mSv. Acute radiation syndrome begins above 1 Sv (1,000 mSv).
- Natural background = 2-3 mSv per year worldwide
- Cross-Atlantic flight = 0.04 mSv (cosmic rays)
- Dental X-ray = 0.005 mSv
- Chest X-ray = 0.1 mSv
- Head CT = 2 mSv
- Abdominal CT = 10 mSv
- PET-CT scan = 25 mSv (with tracer)
- Annual occupational limit = 20 mSv (5-year average)
- Acute radiation syndrome threshold = 1,000 mSv (1 Sv)
Medical radiation exposures
Medical imaging is the largest controllable source of radiation exposure for most adults. A chest X-ray adds 0.1 mSv on top of annual background; the marginal cancer risk is minute (about 1 in 100,000 per scan). A chest CT adds 7 mSv — closer to 1 in 1,000 per scan over a lifetime. Abdominal and pelvic CTs deliver 10 mSv, equivalent to 3-4 years of natural background.
The medical principle is ALARA — As Low As Reasonably Achievable. CT scans should be ordered only when the diagnostic benefit clearly outweighs the cumulative radiation risk. Children get higher per-mSv risk and should receive paediatric protocols with reduced doses.
Ask the radiologist for your CT dose in mSv. Many modern scanners display it on the patient receipt. Tracking lifetime medical radiation helps you and your doctors make better risk-benefit choices for future imaging.
Occupational radiation limits
The International Commission on Radiological Protection (ICRP) sets occupational limits at 20 mSv per year averaged over five years, with no single year exceeding 50 mSv. Public limits are 1 mSv per year above natural background. Pregnant workers have a stricter 1 mSv limit for the duration of the pregnancy.
US Nuclear Regulatory Commission limits are slightly higher: 50 mSv/year for radiation workers. European regulators follow ICRP. Aircrew on polar routes can approach 5-6 mSv/year from cosmic rays alone — pilots are now classified as occupationally exposed in Europe.
Acute radiation syndrome
High whole-body doses cause acute radiation syndrome (ARS). Threshold symptoms (nausea, fatigue) start around 0.5-1 Sv. Above 4 Sv, half of unprotected adults die within 30 days without aggressive medical treatment — this is the LD50/30 dose. Above 6 Sv, death is near-certain even with treatment. Above 10 Sv, death within days.
The Chernobyl first responders received 4-16 Gy whole-body doses. Of 134 with confirmed ARS, 28 died within months. Modern bone marrow transplantation and supportive care can save patients in the 4-6 Sv range that would have been fatal in 1986.
For cancer risk, the prevailing model is linear no-threshold (LNT): risk scales linearly with dose and there is no safe minimum. Doubt has been expressed about LNT at very low doses, but regulators still use it for conservative protection. Cumulative dose matters; small doses add up.
Radiation weighting factors
ICRP defines tissue and radiation weighting factors that convert absorbed dose (Gy) to equivalent dose (Sv) and effective dose (Sv, weighted across organs). Radiation weighting factor w_R: 1 for X-rays, gamma rays, beta particles and electrons. 2 for protons. 5-20 for neutrons depending on energy. 20 for alpha particles and heavy ions.
Tissue weighting factors w_T account for differing organ sensitivity. The 2007 ICRP values: 0.12 for bone marrow, breast, colon, lung, stomach; 0.08 for gonads; 0.04 for bladder, oesophagus, liver, thyroid; 0.01 for skin, bone surface, brain, salivary glands. The sum across organs is 1.
Effective dose, the sum of weighted equivalent doses across all organs, is the single number that best represents stochastic cancer risk. A whole-body CT scan with 10 mSv equivalent dose corresponds to roughly 1 in 1,000 lifetime cancer risk per ICRP coefficients. The number is small in absolute terms but accumulates: ten CTs over a lifetime moves a person's cancer risk by about 1%.