Which units are used to express the annual dose limits?

• A. rem and Sv
• B. Gy and Sv
• C. rad and Gy
• D. J/kg

Answer: (A)

Annual dose limits are expressed in units of dose equivalent, which reflect the biological risk of radiation. The sievert (Sv) is the SI unit for dose equivalent, and the rem is a traditional unit where 1 rem equals 0.01 Sv. That’s why limits are given in rem or Sv, sometimes in a smaller unit like mSv or mrem.

Absorbed energy deposited per mass is measured in grays (Gy) or, historically, rads; these do not account for radiation type or biological effect. J/kg is another way to express absorbed dose and is numerically the same as Gy, but for regulatory dose limits we use Sv/rem to reflect potential harm, not just energy deposited.