mSv

Millisievert (mSv) is a unit of effective dose used in radiation protection to quantify the health risk from exposure to ionizing radiation. One millisievert equals one thousandth of a sievert (Sv). The sievert is the SI unit for dose equivalent, which accounts for the type of radiation and its biological effect, while the gray (Gy) is the unit of absorbed dose (energy deposited per kilogram). Effective dose combines absorbed dose with radiation weighting factors and tissue weighting factors, yielding a single value in sieverts that reflects potential health risk. Because most exposures in medical and environmental contexts are small, millisieverts are commonly used.

In practice, mSv is used to set and communicate regulatory limits and to compare risks from different sources. For the general public, regulatory limits are typically around 1 mSv per year from planned exposure, while occupational exposure limits are higher, commonly around 20–50 mSv per year averaged over five years, with possible higher single-year limits in some regimes. The actual limits vary by country.

Typical values: natural background radiation varies by location, about 2–3 mSv per year on average. Medical exposures include dental X-rays (~0.01 mSv), chest X-rays (~0.02 mSv), mammography (~0.4–0.5 mSv), and CT scans ranging from about 1 to 10 mSv or more depending on the exam. Cumulative doses are important for risk assessment but depend on dose rate and patient factors.

Limitations: mSv is a simplified population-level risk metric; it does not predict individual risk and relies on models with uncertainties in tissue weighting and radiation quality.