Millisievert

The millisievert (mSv) is an SI unit used to quantify the biological effects of ionizing radiation on human tissue. It is one thousandth of a sievert (Sv). The sievert is a unit for dose equivalents that reflect not only energy deposited but also the potential for biological harm, using factors that weight radiation type and tissue sensitivity.

To relate physical energy to risk, the absorbed dose D is measured in gray (Gy). The dose equivalent H equals D multiplied by a radiation weighting factor wR. The effective dose E sums H across tissues with tissue weighting factors wT, offering an overall estimate of stochastic risk.

The millisievert is commonly used in medicine, radiation protection, and environmental assessments. In medical imaging, CT scans and X-rays are described in mSv; occupational dosimetry uses mSv for workers exposed to radiation; natural background and aviation exposures are also expressed in mSv, often in a few millisieverts per year.

One sievert equals 1000 millisieverts; one millisievert equals 1000 microSieverts. For small doses, μSv may be used. Dosimeters measure exposure, and reported values are typically in mSv per year or per procedure.

Regulatory guidance from bodies such as the International Commission on Radiological Protection (ICRP) and the International Atomic Energy Agency (IAEA) uses mSv and Sv to express limits and risk. Public annual exposure limits are commonly around 1 mSv from all sources, while occupational limits are higher, reflecting different risk tolerances and exposure scenarios.