The cumulative, effective dose of x-rays on living material is measured in

• A. Grays
• B. Rems
• C. Sieverts
• D. Coulombs per kilogram

Answer: (C)

The concept here is that risk from radiation is described by the effective dose, which combines how much energy is deposited in tissues with how sensitive those tissues are to radiation. Absorbed dose, in Grays, tells you energy deposited per unit mass but doesn’t reflect biological harm. Equivalent dose, measured in Sieverts (for X-rays the radiation weighting factor is 1, so the equivalent dose in Sv equals the absorbed dose in Gy). The effective dose then sums these tissue-specific equivalent doses using tissue weighting factors, giving a single Sievert value that estimates overall risk to the body. That’s why the cumulative, effective dose is expressed in Sieverts. Rems are an older unit of dose equivalent, and coulombs per kilogram is a measure of exposure in air, not tissue dose; Grays measure energy deposition but not the biological effect like the Sievert does.