Sieverts

The sievert, symbol Sv, is the SI derived unit used to express the biological effect of ionizing radiation. It quantifies radiation dose in terms of the potential for harm to living tissue, rather than just the energy deposited.

The base unit for energy deposited is the gray (Gy), defined as one joule of energy absorbed per kilogram of matter. The sievert incorporates the type of radiation through a radiation weighting factor (W_R), so the same absorbed dose from different radiation types can correspond to different biological impacts.

Equivalent dose to a tissue T is defined as H_T = D_T × W_R, where D_T is the absorbed dose in that tissue. The effective dose E is the sum over all tissues of W_T × H_T, with W_T being tissue weighting factors that reflect the varying radiosensitivities of different tissues. The effective dose provides a single value intended to represent overall risk from a radiological exposure.

In practical use, doses are typically expressed in millisieverts (mSv). Natural background radiation is about 2–3 mSv per year. Medical imaging exposures vary widely: many X-ray procedures are on the order of a few hundredths to a few tenths of a millisievert, while CT scans can range from several to tens of millisieverts. Occupational exposure limits recommended by ICRP are about 20 mSv per year averaged over five years, with no more than 50 mSv in any single year; public exposure limits are about 1 mSv per year. The dose–risk relationship used for regulation and planning is commonly described by the linear no-threshold model, acknowledging uncertainty at low doses.