Millisieverts, Becquerels & Co.: Radiation Units Finally Explained
News about radioactivity fires units at you: becquerels here, millisieverts there, sometimes a gray or — in the U.S. — rem and picocuries in between. In truth, two concepts get you through almost everything.
Becquerel: how much is decaying?
The becquerel (Bq) measures the activity of a source: 1 Bq means one atomic nucleus decaying per second. (The U.S. unit is the curie; 1 picocurie = 0.037 Bq — that’s why a radon level of 4 pCi/L equals about 148 Bq/m³.) The number alone says nothing about danger — what matters is which radiation is emitted and whether it reaches you at all. Your own body contains roughly 9,000 Bq (mostly potassium-40, see How radioactive is our food?) — and is not a safety hazard. For radon, concentration is given per liter or cubic meter of air.
Sievert: what does it do to me?
The sievert (Sv) measures the biological effect of radiation on humans — the effective dose. It accounts for the type of radiation and the sensitivity of the organs hit. Because a full sievert is enormous, everyday life runs on thousandths (millisievert, mSv) and millionths (microsievert, µSv). The old U.S. unit is the rem: 1 mSv = 100 mrem.
In between sits the gray (Gy) for pure absorbed energy — you’ll meet it in medical reports at most.
The everyday scale
| Dose | Example |
|---|---|
| 0.0001 mSv | one banana (on paper) |
| 0.005 mSv | single dental X-ray |
| 0.05 mSv | flight Europe–New York |
| 0.16 mSv | a year of 2 Brazil nuts daily |
| 1 mSv | annual public dose limit from licensed activities (international standard) |
| ~6 mSv | average annual dose, U.S. resident (radon, medicine, nature) |
| 20–50 mSv | annual occupational limits (EU / U.S.) |
| ~100 mSv and up | range where increased cancer risk becomes statistically demonstrable |
The one-line takeaway
Becquerels count, sieverts weigh. A big becquerel number sounds dramatic but can be harmless (your own body!). A dose in millisieverts, by contrast, can be compared directly against the table above. And for where to see live radiation readings near you, see Live radiation maps.