The gray (Gy) and the rad are both units of absorbed dose — the energy deposited per unit mass of tissue (1 Gy = 1 J/kg). The gray is the SI unit, while the rad (radiation absorbed dose) is the older CGS unit. The conversion is exact: 1 Gy = 100 rad, so 1 mGy = 100 mrad and 1 rad = 0.01 Gy.

The sievert (Sv) and the rem (roentgen equivalent man) are both units of equivalent dose, which weights absorbed dose by the biological effectiveness of the radiation type. The sievert is the SI unit; the rem is its older CGS counterpart. The conversion is exact: 1 Sv = 100 rem, so 1 mSv = 100 mrem and 1 rem = 0.01 Sv.

Absorbed dose (measured in gray or rad) quantifies the physical energy deposited in tissue, regardless of radiation type. Equivalent dose (measured in sievert or rem) adjusts absorbed dose by a radiation weighting factor (w_R) that reflects the relative biological harm of the radiation.

For X-rays and gamma rays w_R = 1, so 1 Gy = 1 Sv. For alpha particles w_R = 20, so 1 Gy of alpha radiation = 20 Sv. Gray and sievert are not interchangeable — converting between them requires knowing the radiation type.

The global average effective dose from natural background radiation is approximately 2.4 mSv per year (240 mrem/yr), according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Sources include cosmic rays (~0.39 mSv), terrestrial gamma rays (~0.46 mSv), inhaled radon (~1.26 mSv), and ingested radionuclides (~0.29 mSv). Medical exposures add a further 0.6–1.2 mSv on average in developed countries.

A standard posteroanterior (PA) chest X-ray delivers an effective dose of approximately 0.02 mSv (2 mrem). A chest CT scan delivers roughly 5–7 mSv (500–700 mrem), about 250–350 times more than a plain X-ray. These doses are well within the ranges studied for population health effects and far below acute radiation thresholds.