Equivalent Dose

Introduction

Radiation dose is a measure of the impact of radiation on the body. It is a complex concept because of the several different ways radiation can act on your body and the fact that different parts of the body have different levels of sensitivity.

To determine the level of harm caused by a radiation dose it is necessary to understand:

  1. How much energy the radiation deposits in the affected tissue (absorbed dose)
    Absorbed dose (Gy = J/kg) is the energy (in Joules) absorbed in an amount of matter (in kg). Often seen as mGy = 1/1,000 Gy.
  2. How much harm that deposited energy causes to those affected tissues (equivalent dose)
    Equivalent dose (Sv) is a measure of the ability to damage living tissue. It is obtained by multiplying the Absorbed dose (Gy) by a radiation weighting factor (WR) which accounts for the different effectiveness of different radiations. Often used as mSv = 1/1,000 Sv (milli-sievert) or µSv = 1/1,000,000 Sv (micro-sievert).
  3. The level of harm that is caused to the body by the harm done to those affected tissues (effective dose).
    Effective dose (Sv) Is a measure of the ability to harm an individual. Obtained as the equivalent dose (Sv) multiplied by a tissue weighting factor WT representing the susceptibility and importance of the organ affected and summed over the whole body.

In the response phase to a nuclear accident the main interest of decision makers is the likely effective dose (a surrogate for harm) to individuals which will be expressed in μSv (micro-sieverts) (hopefully), mSv (milli-sieverts) (possibly), or Sv (sieverts) (we really hope not!).

The limit on effective dose for any person other than an employee or trainee is 1 mSv in any calendar year, a CT scan of the head gives 1.4 mSv, 2.7 mSv/year is the average annual background dose in the UK, 6.9 mSv is the average annual radon dose to people in Cornwall, 20 mSv/yr is the annual dose limit in the UK nuclear industry, 100 mSv is the level at which changes in blood cells can be readily observed, acute radiation effects start at about 1000 mSv and 5000 mSv would be expected to kill half of those exposed.

Equivalent Dose