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From activity to dose
- The radioactive contamination of materials is usually expressed as activity of a radionuclide per mass (unit Bq/kg) or as activity per volume (unit Bq/l or Bq/m³).
- The potential hazard to human health depends not only on the type of radionuclide and its activity, but also on whether the radionuclide acts upon the human body externally or whether it finds its way into it.
- In order to be able to make statements about possible hazards to human health, the measured activities (per mass or volume) of the radioactive substances have to be converted into doses (organ dose or effective dose, unit Sv).
The radioactive contamination of materials - regardless of whether that is air, water, soil, building materials or food - is usually expressed as activity of a radionuclide per mass (unit Bq/kg) or as activity per volume (unit Bq/l or Bq/m³). In particular cases, the activity can also be related to the surface area, for example in the case of the deposition of radionuclides on the ground after the Chernobyl reactor accident (unit Bq/m² or Bq/km²).
How is the activity of a radionuclide measured?
The activity of a radionuclide is purely a measurement parameter. It indicates the number of atomic nuclei decaying per second. The activity of a radionuclide in a sample can be measured very sensitively using special measurement instruments in the laboratory, either directly (in the case of gamma emitters) or following radiochemical processing of the sample (in the case of alpha and beta emitters). However, the activity does not reveal how dangerous the radionuclide can be for humans.
Which factors determine the harmfulness of a radionuclide?
The potential hazard to human health depends not only on the type of radionuclide and its activity, but also on whether the radionuclide acts upon the human body externally or whether it finds its way into it. An alpha emitter, for example, is completely harmless outside the body, as it is already fully shielded by a few centimetres of air, regardless of its activity level. However, if this alpha emitter is whirled up and if a larger amount of it (higher activity) gets into the human body via inhaled air, this may cause damage to health.
Dose values make radiation exposure levels comparable
In order to be able to make statements about possible hazards to human health, the measured activities (per mass or volume) of the radioactive substances have to be converted into doses (organ dose or effective dose, unit Sv). Only these calculated doses can be considered as a measure of the possible health damage to an organ (organ dose) or to the entire human body (effective dose). By using these doses (not the activities!), the hazard levels of various radionuclides can be compared among each other or with other exposures to radiation such as radiation exposure from X-rays or cosmic radiation.
Dose from radioactive substances outside the human body
In the case of radioactive substances which act upon the human body externally (external radiation exposure), the dose level depends on the type of radionuclide and its activity and also on its distribution within the environment (e.g. in the soil, in building materials) as well as on the place and duration of residence of the individual.
Dose from radioactive substances within the human body
- the type of radionuclide,
- the incorporated activity,
- the uptake pathway (via inhaled air or food) and
- the chemical form of the radionuclide.
State of 2017.10.23