Radioactivity is invisible, inaudible and cannot be felt. In order to protect ourselves from this ‘hidden’ phenomenon, we have to be able to detect it, understand how it affects us, and evaluate what constitutes a harmful dose.
Experts distinguish between three types of radioactive and more generally radiations dose:
- Activity, or the number of disintegrations occurring per second,
- Amount of energy deposited locally by a radiations ,
- Biological dose, which takes into account the effect of radiation on living tissue.
Activity is a measure of the number of disintegrations occurring in a radioactive source every second – which is to say how many rays are emitted in that time. The units of activity are known as becquerels (Bq), where 1 Bq is the activity of a substance that has one nucleus decaying per second. Obviously, becquerels are tiny units, and as a result kilo- (kBq), mega- (MBq) and giga- (GBq) becquerels are more frequently used. Even though radioactivity is a good measure of the intensity of a source, this decay count reveals nothing about the energy each disintegration releases, or how harmful the emitted radiation can be.
In calculating the amount of energy coming from a radioactive source and absorbed by living tissue, a number of factors need to be considered. The most important of these are the proximity to the source and the obstacles the radiations have to pass through to reach their target.
If one kilogram of matter absorbs one joule of radiated energy, it is said to have absorbed a dose of 1 gray (1 Gy). In contrast to the Bq, this is a particularly large unit, and so the use of milligrays (mGy) is more common. It is measurements in milligrays that are used in medicine to determine safe and effective strengths for radiation therapy.
The biological doses are expressed in sieverts (Sv) or millisieverts (mSv). The main biological dose, the effective dose, is a way of expressing the real physiological significance of an exposure for the entire body. To calculate the effective biological dose in sieverts from the absorbed doses (in grays), one has to apply a series of factors that account for the radiation type, the duration of the exposure, and the parts of the body that were exposed.
In radiotherapy, elevated doses of radiation are concentrated locally over short periods of time, which is more harmful to the targeted cells. To be able to determine the precise length of time needed, a concept known as the ‘equivalent dose’ has been introduced. This is a dose which multiplies the absorbed dose (in grays) in an organ or tissue by a ‘weighting factor’ specific to the type of radiation involved: a value which differs depending on the part of body being discussed.
The ‘effective dose’ is more useful in discussing exposure on the entire body and evaluating the corresponding risks. Examples of ‘effective dose’ are the doses resulting from a year exposure to natural radioactivity, radioactivity encountered in the workplace, radiation emitted by medical testing and finally accidental exposure to radioactivity (such as the remnants of the Chernobyl disaster, which amounts to around 0.019 mSv per person per year in Western Europe).