Kinds of radiation doses

Radiation dose is a measure of the amount of exposure to radiation. There are three kinds of doses defined for radiological protection:

• Absorbed dose
• Equivalent dose
• Effective dose

Absorbed dose is the amount of energy deposited by radiation in a mass, regardless if it is deposited in animals, plants or people. An absorbed dose is expressed in grays (Gy).  This is a large quantity, so milli-grays (mGy) is more commonly used (1 Gy = 1,000 mGy).

Different types of radiation can cause different amounts of damage. Each type of radiation is assigned a factor to show how much damage it does. For example, gamma rays have a factor of 1, but alpha particles have a factor of 20. To find the equivalent dose, the absorbed dose is multiplied by the factor for the type of radiation. Equivalent dose is expressed in sieverts (Sv). This is a large quantity, so the millisievert (mSv) is more common (1 Sv = 1000 mSv). The millisievert is used to describe the biological effects of radiation. An equivalent dose is calculated for individual organs.

Different parts of body are more sensitive to radiation than others. Each part of the body is also assigned a weighting factor to show how sensitive it is. To find the effective dose, the equivalent dose for one organ is multiplied by the weighting factor for the organ. The effective dose is expressed in sieverts and millisieverts. The effective dose can be called the whole-body dose, if the dose for all body parts are added together.