We have now taken into account the varying biological effects of different types of radiation on a particular tissue (or body part) type, T.  However, it should  come as no surprise to find out that the same radiation exposure to different parts of the body can have very different results. That is, if the entire body were irradiated with a uniform beam of a single type of radiation, some parts of the body would react more dramatically than others.  To take this effect into account, the International Commission on Radiological Protection, the ICRP, came up with a list of tissue weighting factors, denoted W_T, for a number of organs and tissues that most significantly contribute to an overall effective biological damage to the body. (This list was published in ICRP’s Publication 60 in 1990.) See Table 11.3 below for values of the tissue weighting factor W_T. 

Table 11.3: WT Values (ICRP, 1990) Tissue or Body Part WT Gonads 0.20 Bone Marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Esophagus 0.05 Thyroid 0.05 Skin 0.01 Bone Surface 0.01 Remainder (adrenals, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus, and uterus) 0.05 Total: 1.00

The ICRP then went on to define the effective human-equivalent dose, denoted H_E, for the  determination of the whole-body biological damage due to various forms of radiation exposure in different parts of the body.  this effective equivalent dose is given as follows:

where W_T is the ICRP’s tissue weighting factor for the type of tissue or body part T, and H_T is the human-equivalent dose for tissue T defined in Eq. (11.3).  The units of H_E are sieverts,  Sv, the same as those of H_T.