Millisieverts

Millisievert (mSv) is the SI unit used to express the effective dose of ionizing radiation received by a person. It equals one thousandth of a sievert (1 Sv = 1000 mSv). The sievert is a dose quantity that combines the amount of energy deposited in tissue with the biological sensitivity of tissues and the type of radiation. As a result, the millisievert is commonly used to communicate potential health risk from low to moderate exposures, particularly in medical contexts and radiation protection.

In practice, mSv is used to report patient doses in diagnostic imaging and to communicate dose information for radiation safety to workers and the public. For x-ray and gamma radiation, the radiation-weighting factor is 1, so the effective dose in mSv is approximately equal to the absorbed dose in mGy.

Typical exposures vary widely. Natural background radiation adds about 2–3 mSv per year, depending on location. Medical imaging examples include chest X-ray around 0.02 mSv, dental X-ray a few microsieverts, mammography about 0.4 mSv, abdominal CT roughly 5–10 mSv, and whole-body CT potentially tens of mSv. Nuclear medicine studies and PET-CT can range from a few to several tens of mSv depending on the procedure.

Regulatory guidance often distinguishes occupational and public exposure. A common framework sets occupational exposure at about 20 mSv per year, averaged over five years, with no more than 50 mSv in any single year; public exposure from artificial sources is typically limited to about 1 mSv per year.