'Gy' is the international unit of 'absorbed dose'. Absorbed dose is defined as the deposited energy from incident radiation per unit mass of target material, such as air or body tissue. 1Gy corresponds to 1 Joule of energy deposition in 1 kilogram of material. In terms of the older unit, known as 'rad', 1Gy = 100 rad. For environmental radiation monitoring in Japan, ordinarily, air is assumed to be the target material.

'Sv' is the international unit of radiological effect level, a 'dose equivalent.' The biological effect of different types of radiation can depend upon the radiation type and its energy. 'Dose equivalent' is used to relate the differing effects from various radiation types to the human body. The adjustment is made in terms of a 'Quality Factor' for the type of radiation and a 'Weighting Factor' for the affected human tissue. For example, a person exposed all over with gamma rays, corresponding to 1Gy absorbed dose, will receive a dose equivalent of 1Sv. In terms of the older unit, known as 'rem', 1Sv is approximately equal to 100rem.