When we know radionuclides are harmful for human health, if any amount enters in our body it can be harmful.
There is an unfortunate dichotomy built in ionizing radiation protection. The basis of radiation protection is that the risk from radiation extends to zero with no threshold. Nevertheless, radiation protection regulations establish 'safe' levels of radiation dose. People like to see a hard line definition of safe, yet radiation protection has two hard lines; acceptable and zero.
The reality is that the linear-no-threshold radiation policy was adopted to be conservative and easy to calculate. It is in no way a reflection of actual radiation harm. There are safe levels. The regulatory levels are set well below levels known to cause harm. The levels for internal dose are set well below what is usually considered well below. The reason for the extra conservatism is the concern one has for being personally contaminated. Personal contamination is inescapable, thus causing undue concern. Concern does not make it worse, just personal.
We have internal radionuclides as a part of life. We cannot exist without potassium and we cannot have potassium without K-40. Our food contains other natural radionuclides. The normal internal radionuclides are very far below SAFE. The acceptable limits set by international agencies are far below SAFE.
To complement the good answer of Joseph, keep in mind that EVERYTHING is radioactive, and for completely natural reasons.
1kg of beef : 100 Bq
1m3 of air inside house : 100 Bq
1 human body : 5-10 kBq
How could you prevent anybody from ingesting a single Bq?
There are two kinds of radionuclides present in the environment. One which are naturally occurring e.g. Uranium, Thorium and K-40; second ones are man made e.g. Cesium-137, Strontium-90, I-131 etc. Incidentally, every natural matter contains a defined amount of natural radioactivity ( search for Banana Dose on Google). Setting a limit, in case of radionuclides is based on principle of ALARA (As low as reasonably Achievable). For example, limits for Uranium in Drinking water has been set as 30 ppb (15 ppb by WHO). So if Uranium is present in amounts higher than this said limit, the regulatory agencies should get active and search for cause of increase. They are also expected to take preventive measures by which if the levels are higher; what measures can be taken to reduce it (by RO treatment etc.). The limits are set to raise an alarm for preventive measures well in advance. Infact, the limits are sometimes set 100-1000 times below the Lowest Observable Adverse Effect Level (LOAEL).
For radiation protection purposes, the International Commission on Radiological Protection (ICRP) has classified all health effects into stochastic effects and tissue reactions (formerly called deterministic or nonstochastic effects). On one hand, the effective dose limit aims to keep the risk of stochastic effects below an acceptable level, where a clear distinction between ‘safe’ and ‘dangerous’ cannot be derived due to he probabilistic nature of stochastic effects and the properties of the linear nonthreshold model. On the other hand, the equivalent dose limits aims to prevent the occurrence of the tissue reactions, where no effects are assumed to occur below a threshold. These dose limits are considered both for internal and external exposures, but “committed” dose is used for internal exposure. This is because in internal exposure situations, tissues or organs receive radiation emitted during the decay of incorporated radionuclide as a function of time following the intake, and temporal dose distribution should be therefore considered. A committed equivalent dose is the time integral of the equivalent dose rate to be delivered to a particular tissue or organ. A committed effective dose (to the whole body) is the sum of the committed tissue or organ equivalent doses each multiplied by tissue weighting factors. The integration time amounts to a period of 50 years for adults, and the period from intake to the age of 70 years for others.
Provided below is the example levels for food safety regulations implemented following the Fukushima nuclear accident.
From 17 March 2011 to 31 March 2012, the “provisional regulation values” were used for radioactivity in food and drink, which were defined not to exceed a committed effective dose of 5 mSv/year for for radiocesium, and a committed equivalent dose to the thyroid of 50 mSv/year for radioiondine (for more details, please see the following paper).
https://www.researchgate.net/publication/51718211_Food_safety_regulations_What_we_learned_from_the_Fukushima_nuclear_accident
The regulation value used from 1 April 2012 was defined not to exceed a committed effective dose of 1 mSv/year (for more details, please see the following paper).
https://www.researchgate.net/publication/230585762_Safety_regulations_of_food_and_water_implemented_in_the_first_year_following_the_Fukushima_nuclear_accident
Article Food safety regulations: What we learned from the Fukushima ...
Article Safety regulations of food and water implemented in the firs...
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