Sievert — unit of equivalent and effective ionizing radiation dose

Definitions and relationships

Absorbed dose is measured in grays (Gy) and denotes energy deposited per unit mass. The sievert is used for equivalent dose and effective dose. Equivalent dose (symbol H) adjusts absorbed dose by a radiation weighting factor (w_R) to account for differences between, for example, alpha particles and gamma rays. Effective dose (symbol E) further combines equivalent doses across body tissues using tissue weighting factors (w_T) to produce a single value reflecting overall health risk.

How it is calculated

• Equivalent dose for a tissue: H_T = sum over radiation types (D_T,R × w_R).
• Effective dose: E = sum over tissues (w_T × H_T).

These weighting factors are set by radiological protection bodies and are designed to approximate relative stochastic risk (chiefly cancer induction), not immediate damage.

Practical examples and interpretation

Typical annual background exposures vary by location but are commonly on the order of a few millisieverts (mSv). Medical exposures span a wide range: a simple X-ray gives a small fraction of a millisievert, while complex CT procedures can deliver several to tens of millisieverts. Acute doses measured in whole sieverts can produce short-term radiation sickness; higher values over a short period are associated with severe health effects.

History, use and limitations

The unit is named for Swedish medical physicist Rolf Sievert, who pioneered dosimetry and studies of biological effects of radiation. In practice the sievert is central to radiation protection, regulation and reporting because it links physical measurements to estimated health risk. However, it is not a direct physical measurement; its meaning depends on the choice of weighting factors and assumptions about risk models. For information on biological response, see organism's response.

Regulators and health professionals use sievert-based quantities to set dose limits for workers and the public, to plan medical imaging, and to guide emergency response to radiological incidents.