What is the SI unit commonly used to express dose equivalent limits?

• A. Rad
• B. Rem
• C. Gray
• D. Sievert

Answer: (D)

The main idea is that dose limits are expressed in a unit that reflects the actual biological risk, not just the energy deposited. The appropriate SI unit for expressing dose equivalent is the sievert. Dose equivalent combines the amount of energy deposited (absorbed dose) with a factor that accounts for how damaging different types of radiation are to tissue. This weighting factor, or radiation weighting factor, multiplies the absorbed dose to yield a measure of biological effect: H = D × WR, and the resulting unit is the sievert.

That’s why gray, the unit for absorbed dose, isn’t used for dose equivalent limits. Rad and rem are older, non-SI units: rad is for absorbed dose, rem is the older unit used for dose equivalent. Using the sievert allows regulatory limits to consistently reflect risk across different radiation types. For example, alpha radiation has a higher WR than gamma or x-rays, so the same energy deposition results in a much larger dose equivalent (in sieverts) with alpha, illustrating why the sievert is the relevant unit for dose-equivalent limits.