In addition to the cosmic and terrestrial sources, all people have some radioactive isotopes inside their bodies from birth. These isotopes are especially potassium-40, carbon-14, and the isotopes of uranium and thorium. The variation in radiation dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources. The average annual radiation dose to a person from internal radioactive materials other than radon is about 0.3 mSv/year, which:
- 0.2 mSv/year comes from potassium-40,
- 0.12 mSv/year comes from the uranium and thorium series,
- 12 μSv/year comes from carbon-40.
The most important isotope with regard to dose as potassium-40. The dominant component of inhalation exposure is the short-lived decay products of radon. But this issue is so important that it was treated separately in the previous section (Radon – Health Effects).
Excluding internal contamination by external radioactive material (radon, uranium, etc.), potassium-40 and carbon-40 are the largest components of internal radiation exposure from biologically functional components of the human body.
Potassium-40
Potassium is a naturally-occurring chemical element with atomic number 19, which means there are 19 protons and 19 electrons in the atomic structure. The chemical symbol for potassium is K (from Neo-Latin kalium).
Natural potassium consists primarily of isotope K-39 (93.26%). Therefore the atomic mass of the potassium element is close to the atomic mass of the K-39 isotope (39.098 u). Natural potassium also consists of two other isotopes: K-41 (6.73%) and K-40 (0.012%). Potassium-40 is an unstable (radioactive) naturally occurring isotope of potassium and has a very long half-life of 1.251×109 years. Therefore, this isotope belongs to primordial nuclides because its half-life is comparable to the age of the Earth.
Traces of K-40 are found in all potassium, the most common radioisotope in the human body. K-40 is a radioactive isotope of potassium with a long half-life of 1.251×109 years and undergoes both types of beta decay. From this point of view, the human body can also be considered a source of antimatter.
- About 89.28% of the time (10.72% is by electron capture), it decays to calcium-40 with the emission of a beta particle (β−, an electron) with a maximum energy of 1.33 MeV and an antineutrino, which is an antiparticle to the neutrino.
- Very rarely (0.001% of the time) will it decay to Ar-40 by emitting a positron (β+) and a neutrino.
Potassium-40 inside Body – Radiation Dose
The potassium concentration in the human body is strictly based on the homeostatic principle. Potassium is more or less distributed in the body (especially in soft tissues) following intake of foods. A 70-kg man contains about 126 g of potassium (0.18%), most of that is located in muscles. The daily consumption of potassium is approximately 2.5 grams. Hence the concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv.
Banana Equivalent Dose – BED
Banana equivalent dose, BED, is an informal dose quantity of ionizing radiation exposure. Banana equivalent dose is intended as a general educational example to compare a dose of radioactivity to the dose one is exposed to by eating one average-sized banana. One BED is often correlated to 10-7 Sievert (0.1 µSv).
Bananas contain significantly high potassium concentrations, which are vital for the functioning of all living cells. The transfer of potassium ions through nerve cell membranes is necessary for normal nerve transmission. But natural potassium also contains a radioactive isotope potassium-40 (0.012%). Potassium-40 is a radioactive isotope of potassium that has a very long half-life of 1.251×109 years and undergoes both types of beta decay.
One BED is often correlated to 10-7 Sievert (0.1 µSv). The radiation exposure from consuming a banana is approximately 1% of the average daily exposure to radiation, which is 100 banana equivalent doses (BED). A chest CT scan delivers 58,000 BED (5.8 mSv). A lethal dose, which kills a human with a 50% risk within 30 days (LD50/30) of radiation, is approximately 50,000,000 BED (5000 mSv). However, this dose is not cumulative in practice, as the principal radioactive component is excreted to maintain metabolic equilibrium. Moreover, there is also a problem with the collective dose.
The BED is only meant to inform the public about the existence of very low levels of natural radioactivity within a natural food. It is not a formally adopted dose measurement.
Internal Dose from Uranium and Thorium
As was written, all people also have some radioactive isotopes inside their bodies from birth. These isotopes are potassium-40, carbon-14, and isotopes from the uranium and thorium series. The variation in radiation dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources. The average annual radiation dose to a person from internal radioactive materials other than radon is about 0.3 mSv/year, which:
- 0.2 mSv/year comes from potassium-40,
- 0.12 mSv/year comes from the uranium and thorium series,
- 12 μSv/year comes from carbon-14.
UNSCEAR has based on a large number of investigations, presented values about the annual intake by humans of the different isotopes. We can mention the following:
- Ra-226 (22 Bq/year),
- Pb-210 (30 Bq/year),
- Po-210 (58 Bq/year) and
- Ra-228 (15 Bq/year).
Note that the dominant component of natural background exposure, which comes from the short-lived decay products of radon, is not involved here. This issue is so important that it was treated separately in the previous section (Terrestrial Radiation).
As a result, the UNSCEAR 2000 report estimates an annual effective dose of 0.12 mSv, which comes from internal exposure to isotopes of the uranium and thorium series. The main contributor to this dose is Po-210. Note that polonium-210, the decay product of lead-210, emits a 5.3 MeV alpha particle, which provides most of the equivalent dose. The radiation weighting factor for alpha radiation is equal to 20. An absorbed dose of 1 mGy by alpha particles will lead to an equivalent dose of 20 mSv.
Internal Doses from carbon-14 and tritium are described in the following article: Cosmogenic Radionuclides
Internal Radiation – Is it dangerous?
We must emphasize that eating bananas, working as an airline flight crew, or living in locations, increases your annual dose rate. But it does not mean that it must be dangerous. In each case, the intensity of radiation also matters. It is very similar to heat from a fire (less energetic radiation). If you are too close, the intensity of heat radiation is high, and you can get burned. If you are at the right distance, you can withstand it without any problems, and it is comfortable. If you are too far from a heat source, the insufficiency of heat can also hurt you. In a certain sense, this analogy can be applied to radiation also from radiation sources.
In the case of internal radiation, we are usually talking about so-called “low doses.” A low dose means additional small doses comparable to the normal background radiation (10 µSv = average daily dose received from natural background). The doses are very low; therefore, cancer induction’s probability could be almost negligible. Secondly, and this is crucial, the truth about low-dose radiation health effects still needs to be found. It is unknown whether these low doses of radiation are detrimental or beneficial (and where is the threshold). Government and regulatory bodies assume an LNT model instead of a threshold or hormesis not because it is the more scientifically convincing but because it is, the more conservative estimate. The problem with this model is that it neglects many defense biological processes that may be crucial at low doses. The research during the last two decades is very interesting and shows that small doses of radiation given at a low dose rate stimulate the defense mechanisms. Therefore the LNT model is not universally accepted, with some proposing an adaptive dose-response relationship where low doses are protective, and high doses are detrimental. Many studies have contradicted the LNT model, and many have shown adaptive response to low dose radiation resulting in reduced mutations and cancers. This phenomenon is known as radiation hormesis.
