Nick Connor

All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view. The only sources of radon in the environment are radioactive decays of primordial radionuclides such as thorium, uranium.

• 

  Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.

• Radon-220. The radon-220 isotope, commonly referred to as thoron,  is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.

It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.

See also: Radon – Health Effects

Source of Radon-222

Radon-222 is a gas produced by the decay of radium-226. Both are a part of the natural uranium series. Since uranium is found in soil throughout the world in varying concentrations, also dose from gaseous radon is varying throughout the world. Radon-222 is the most important and most stable isotope of radon. It has a half-life of only 3.8 days, making radon one of the rarest elements since it decays away so quickly. An important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses. The fact radon is gas plays a crucial role in spreading of all its daughter nuclei. Simply radon is a transport medium from bedrock to atmosphere (or inside buildings) for its short-lived decay products (Pb-210 and Po-210), that posses much more health risks.

Radon is usually the largest natural source of radiation contributing to the exposure of members of the public, sometimes accounting for half the total exposure from all sources. The health risk due to exposure to radon and thoron comes principally from the inhalation of the short-lived decay products (Pb-210 and Po-210) and the resulting alpha particle irradiation of the bronchi and the lungs.

See also: Radon – Health Effects

See also:

Radon

Radon is a colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. Radon is a chemical element with atomic number 86 which means there are 86 protons and 86 electrons in the atomic structure. The chemical symbol for Radon is Rn. All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view.

• 

  Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.

• Radon-220. The radon-220 isotope, commonly referred to as thoron,  is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.

It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.

See also: Radon – Health Effects

Radon – Properties

Atomic Mass of Radon

Atomic mass of Radon is 222 u. 

Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance.

The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10^-24 grams. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.

Density of Radon

Density of Radon is 9.73g/cm³.

Typical densities of various substances at atmospheric pressure.

Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:

ρ = m/V

In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m³). The Standard English unit is pounds mass per cubic foot (lbm/ft³).

See also: What is Density

See also: Densest Materials of the Earth

Radon – Melting Point and Boiling Point

Melting point of Radon is -71°C.

Boiling point of Radon is -61.8°C.

Note that, these points are associated with the standard atmospheric pressure.

Radon – Specific Heat, Latent Heat of Fusion, Latent Heat of Vaporization

Specific heat of Radon is 0.09 J/g K.

Latent Heat of Fusion of Radon is 2.89 kJ/mol.

Latent Heat of Vaporization of Radon is 16.4 kJ/mol.

See also:

Radon

Radon-222 is a radioisotope of radon. Radon is a colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view.

• 

  Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.

• Radon-220. The radon-220 isotope, commonly referred to as thoron,  is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.

It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.

See also: Radon – Health Effects

Radon-222

Radon-222 is a gas produced by the decay of radium-226. Both are a part of the natural uranium series. Since uranium is found in soil throughout the world in varying concentrations, also dose from gaseous radon is varying throughout the world. Radon-222 is the most important and most stable isotope of radon. It has a half-life of only 3.8 days, making radon one of the rarest elements since it decays away so quickly. An important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses. The fact radon is gas plays a crucial role in spreading of all its daughter nuclei. Simply radon is a transport medium from bedrock to atmosphere (or inside buildings) for its short-lived decay products (Pb-210 and Po-210), that posses much more health risks.

Radon is usually the largest natural source of radiation contributing to the exposure of members of the public, sometimes accounting for half the total exposure (about 2 mSv/year) from all sources. The health risk due to exposure to radon and thoron comes principally from the inhalation of the short-lived decay products (Pb-210 and Po-210) and the resulting alpha particle irradiation of the bronchi and the lungs.

As long as these isotopes are outside the body, only the gamma radiation will be able to give a dose. But radon is a gas and diffuses out of the ground to mix with air. The half-life of radon-222 is long compared with the residence time of air in the lungs, so that relatively little radon decay during respiration. Moreover, radon is a noble gas and its inertness prevents its long-term retention within body. But when radon disintegrates, the daughter metallic isotopes (Pb-210 and Po-210) are not inert and they be attached to other molecules like water and to aerosol particles in  air. When these particles are inhaled, some of lead-210 is retained by the body. Ingestion of lead-210 is also a possible way. Since lead-210 is a weak beta emitter, it does not cause major doses. Lead-210 is thus a transport medium from indoor air to the body. The radiation from radon and its decay products is a mixture of alpha particles and beta particles as well as gamma radiation. When the isotopes come inside the body, all types of radiation contribute.

But it is the polonium-210, the decay product of lead-210, that emits a 5.3 MeV alpha particle, which provides most of equivalent dose. Alpha particles, that belongs to high-LET radiation, are fairly massive and carry a double positive charge, so they tend to travel only a short distance and do not penetrate very far into tissue if at all. However alpha particles will deposit their energy over a smaller volume (possibly only a few cells if they enter a body) and cause more damage to those few cells (more than 80 % of the absorbed energy from radon is due to the alpha particles). Therefore, 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. In summary, radon and lead can be viewed as different sorts of carriers for polonium-210.

The amount of isotopes ingested with the food is negligible, and all concern is about the breathing and the deposition of radon daughters in the bronchi and in the lungs. Among non-smokers, radon is the largest cause of lung cancer and, overall, the second-leading cause. The average annual radiation dose to a person from radon is about 2 mSv/year and it may vary over many orders of magnitude from place to place. According to a 2003 report EPA’s Assessment of Risks from Radon in Homes, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon.

It must be emphasized, cigarettes contain also polonium-210, originating from the decay products of radon, which stick to tobacco leaves. Polonium-210 emits a 5.3 MeV alpha particle, which provides most of equivalent dose. Heavy smoking results in a dose of 160 mSv/year to localized spots at the bifurcations of segmental bronchi in the lungs from the decay of polonium-210. This dose is not readily comparable to the radiation protection limits, since the latter deal with whole body doses, while the dose from smoking is delivered to a very small portion of the body.

See also:

Radon

Radon-220 is a radioisotope of radon. Radon-220 is commonly referred to as thoron, since it is a natural decay product of the most stable thorium isotope (thorium-232). Radon-220 is a member of thorium series.

All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view.

• 

  Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.

• Radon-220. The radon-220 isotope, commonly referred to as thoron,  is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.

It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.

See also: Radon – Health Effects

See also:

Radon

Terrestrial radiation refers to sources of radiation that are in the soil, water, vegetation and in the earth’s mantle. The major isotopes of concern for terrestrial radiation are potassium, uranium and the decay products of uranium, such as thorium, radium, and radon.  When the Earth was formed, a number of radioactive elements were formed. After the four billion years, all the shorter-lived isotopes have decayed. But some of these isotopes have very long half-lives, billions of years, and are still present. These radionuclides are known as primordial radionuclides and contributes also to the annual dose to an individual. Because most of the natural radioactive isotopes are heavy, more than one disintegration is necessary before a stable atom is reached. This sequence of unstable atomic nuclei and their modes of decays, which leads to a stable nucleus, is known as the radioactive series. Radioactive decays of members of these series generate heat.

Liquid Earth’s Core

All three naturally-occurring isotopes of uranium (²³⁸U, ²³⁵U and ²³⁴U) and naturally-occurring isotope of thorium have very long half-life (e.g. 4.47×10⁹ years for ²³⁸U). Because of this very long half-life uranium and thorium are weakly radioactive and contributes to low levels of natural background radiation in the environment. These isotopes are alpha radioactive (emitting alpha particle), but they can also rarely undergo a spontaneous fission.

All naturally-occurring isotopes belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.54×10⁹ years). Uranium has the second highest atomic mass of these primordial nuclides, lighter only than plutonium. Moreover the decay heat of uranium and thorium and their decay products (e.g. radon, radium etc.) contributes to heating of Earth’s core. Together with potassium-40 in the Earth’s mantle is thought that these elements are the main source of heat that keeps the Earth’s core liquid.

See also:

Terrestrial Radiation

Terrestrial radiation refers to sources of radiation that are in the soil, water, and vegetation. The major isotopes of concern for terrestrial radiation are potassium, uranium and the decay products of uranium, such as thorium, radium, and radon. Note that, terrestrial radiation includes an external exposure caused by these radionuclides. An internal dose caused by these redionuclides is discussed in: Internal Source of Radiation.

These radionuclides are in trace amounts all around us. When the Earth was formed, a number of radioactive elements were formed. After the four billion years, all the shorter-lived isotopes have decayed. But some of these isotopes have very long half-lives, billions of years, and are still present. These radionuclides are known as primordial radionuclides and contributes to the annual dose to an individual. Because most of the natural radioactive isotopes are heavy, more than one disintegration is necessary before a stable atom is reached. This sequence of unstable atomic nuclei and their modes of decays, which leads to a stable nucleus, is known as the radioactive series.

Dose from Terrestrial Radiation

Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources also varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels. The average dose rate that originates from terrestrial nuclides (except radon exposure) is about 0.057 µGy/hr. The maximum values have been measured on monazite sand in Guarapari, Brazil (up to 50 µGy/hr and in Kerala, India (about 2 µGy/hr), and on rocks with a high radium concentration in Ramsar, Iran (from 1 to 10 µGy/hr).

The major isotopes of concern for terrestrial radiation are uranium and the decay products of uranium, such as thorium, radium, and radon. Radon is usually the largest natural source of radiation contributing to the exposure of members of the public, sometimes accounting for half the total exposure from all sources. It is so important, that is is usually treated separately. The average annual radiation dose to a person from radon and its decay products is about 2 mSv/year and it may vary over many orders of magnitude from place to place.

Radiation from Thorium and its Decay Products

Thorium cascade significantly influences radioactivity(disintegrations per second) of natural samples and natural materials. All the descendants are present, at least transiently, in any natural thorium-containing sample, whether metal, compound, or mineral.  For example, pure thorium-232 is weakly radioactive (proportional to its long half-life), but a thorium ore is about 10 times more radioactive than the pure thorium-232 metal because of its daughter isotopes (e.g. radon, radium etc.) it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain they also generate radon, a heavy, inert, naturally occurring radioactive gas.

See also:

Terrestrial Radiation

Terrestrial radiation refers to sources of radiation that are in the soil, water, and vegetation. The major isotopes of concern for terrestrial radiation are potassium, uranium and the decay products of uranium, such as thorium, radium, and radon. Note that, terrestrial radiation includes an external exposure caused by these radionuclides. An internal dose caused by these redionuclides is discussed in: Internal Source of Radiation.

These radionuclides are in trace amounts all around us. When the Earth was formed, a number of radioactive elements were formed. After the four billion years, all the shorter-lived isotopes have decayed. But some of these isotopes have very long half-lives, billions of years, and are still present. These radionuclides are known as primordial radionuclides and contributes to the annual dose to an individual. Because most of the natural radioactive isotopes are heavy, more than one disintegration is necessary before a stable atom is reached. This sequence of unstable atomic nuclei and their modes of decays, which leads to a stable nucleus, is known as the radioactive series.

Dose from Terrestrial Radiation

Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources also varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels. The average dose rate that originates from terrestrial nuclides (except radon exposure) is about 0.057 µGy/hr. The maximum values have been measured on monazite sand in Guarapari, Brazil (up to 50 µGy/hr and in Kerala, India (about 2 µGy/hr), and on rocks with a high radium concentration in Ramsar, Iran (from 1 to 10 µGy/hr).

The major isotopes of concern for terrestrial radiation are uranium and the decay products of uranium, such as thorium, radium, and radon. Radon is usually the largest natural source of radiation contributing to the exposure of members of the public, sometimes accounting for half the total exposure from all sources. It is so important, that is is usually treated separately. The average annual radiation dose to a person from radon and its decay products is about 2 mSv/year and it may vary over many orders of magnitude from place to place.

Radiation from Uranium and its Decay Products

Uranium cascade significantly influences radioactivity (disintegrations per second) of natural samples and natural materials. All the descendants are present, at least transiently, in any natural uranium-containing sample, whether metal, compound, or mineral. For example, pure uranium-238 is weakly radioactive (proportional to its long half-life), but a uranium ore is about 13 times more radioactive than the pure uranium-238 metal because of its daughter isotopes (e.g. radon, radium etc.) it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain they also generate radon, a heavy, inert, naturally occurring radioactive gas.

See also:

Terrestrial Radiation

Primordial radionuclides are radionuclides found on the Earth that have existed in their current form since before Earth was formed. Primordial radionuclides are residues from the Big Bang, from cosmogenic sources, and from ancient supernova explosions which occurred before the formation of the solar system. Bismuth, thorium, uranium and plutonium are primordial radionuclides because they have half-lives long enough to still be found on the Earth. Potassium-40 also belongs to primordial nuclides.

When the Earth was formed, a number of radioactive elements were formed. After the four billion years, all the shorter-lived isotopes have decayed. But some of these isotopes have very long half-lives, billions of years, and are still present. These radionuclides are known as primordial radionuclides and contributes to the annual dose to an individual. Because most of the natural radioactive isotopes are heavy, more than one disintegration is necessary before a stable atom is reached. This sequence of unstable atomic nuclei and their modes of decays, which leads to a stable nucleus, is known as the radioactive series.

Liquid Earth’s Core

All three naturally-occurring isotopes of uranium (²³⁸U, ²³⁵U and ²³⁴U) and naturally-occurring isotope of thorium have very long half-life (e.g. 4.47×10⁹ years for ²³⁸U). Because of this very long half-life uranium and thorium are weakly radioactive and contributes to low levels of natural background radiation in the environment. These isotopes are alpha radioactive (emitting alpha particle), but they can also rarely undergo a spontaneous fission.

All naturally-occurring isotopes belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.54×10⁹ years). Uranium has the second highest atomic mass of these primordial nuclides, lighter only than plutonium. Moreover the decay heat of uranium and thorium and their decay products (e.g. radon, radium etc.) contributes to heating of Earth’s core. Together with potassium-40 in the Earth’s mantle is thought that these elements are the main source of heat that keeps the Earth’s core liquid.

 

See also:

Terrestrial Radiation

In addition to the cosmic and terrestrial sources, all people also have some radioactive isotopes inside their bodies from birth. These isotopes are especially potassium-40, carbon-14 and also 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 of 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 is 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 et.c), potassium-40 and carbon-40 are 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 potassium element is close to the atomic mass of 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. It has a very long half-life of 1.251×10⁹ years. Therefore, this isotope belong to primordial nuclides, because its half-life is comparable to the age of the Earth.

Traces of K-40 are found in all potassium, and it is the most common radioisotope in the human body. K-40 is a radioactive isotope of potassium which has a very long half-life of 1.251×10⁹ years and undergoes both types of beta decay. From this point of view, also a human body can be considered as a source of antimatter.

• About 89.28% of the time (10.72% is by electron capture), it decays to calcium-40 with 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) it will 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 in 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 gram. 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 is 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 which 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, the dose that kills a human with a 50% risk within 30 days (LD50/30) of radiation, is approximately 50,000,000 BED (5000 mSv). However, in practice, this dose is not cumulative, 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 and 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 especially 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 of 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 have, 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 by 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 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, eating bananas, working as airline flight crew or living in locations with, increases your annual dose rate. But it does not mean, that it must be dangerous. In each case, intensity of radiation also matters. It is very similar as for 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 there without any problems and moreover it is comfortable. If you are too far from heat source, the insufficiency of heat can also hurt you. This analogy, in a certain sense, can be applied to radiation also from radiation sources.

In case of internal radiation, we are talking usually about so called “low doses”. Low dose here means additional small doses comparable to the normal background radiation (10 µSv = average daily dose received from natural background). The doses are very very low and therefore the probability of cancer induction could be almost negligible. Secondly, and this is crucial, the truth about low-dose radiation health effects still needs to be found. It is not exactly known, whether these low doses of radiation are detrimental or beneficial (and where is the threshold). Government and regulatory bodies assume a LNT model instead of a threshold or hormesis not because it is the more scientifically convincing, but because it is the more conservative estimate. Problem of this model is that it neglects a number of defence biological processes that may be crucial at low doses. The research during the last two decades is very interesting and show 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 of these have shown adaptive response to low dose radiation resulting in reduced mutations and cancers. This phenomenon is known as radiation hormesis.

See also:

Sources

Radon is a colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. All isotopes of radon are radioactive, but the two radon isotopes radon-222 and radon-220 are very important from radiation protection point of view.

• 

  Radon-222. The radon-222 isotope is a natural decay product of the most stable uranium isotope (uranium-238), thus it is a member of uranium series.

• Radon-220. The radon-220 isotope, commonly referred to as thoron,  is a natural decay product of the most stable thorium isotope (thorium-232), thus it is a member of thorium series.

It is important to note that radon is a noble gas, whereas all its decay products are metals. The main mechanism for the entry of radon into the atmosphere is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. When radon disintegrates, the daughter metallic isotopes are ions that will be attached to other molecules like water and to aerosol particles in the air. Therefore all discussions of radon concentrations in the environment refer to radon-222. While the average rate of production of radon-220 (thoron) is about the same as that of radon-222, the amount of radon-220 in the environment is much less than that of radon-222 because of significantly shorter half-life (it has less time to diffuse) of radon-222 (55 seconds, versus 3.8 days respectively). Simply radon-220 has lower chance to escape from bedrock.

Radon-222

Radon-222 is a gas produced by the decay of radium-226. Both are a part of the natural uranium series. Since uranium is found in soil throughout the world in varying concentrations, also dose from gaseous radon is varying throughout the world. Radon-222 is the most important and most stable isotope of radon. It has a half-life of only 3.8 days, making radon one of the rarest elements since it decays away so quickly. An important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses. The fact radon is gas plays a crucial role in spreading of all its daughter nuclei. Simply radon is a transport medium from bedrock to atmosphere (or inside buildings) for its short-lived decay products (Pb-210 and Po-210), that posses much more health risks.

Health Effects of Radon

Radon is usually the largest natural source of radiation contributing to the exposure of members of the public, sometimes accounting for half the total exposure from all sources. The health risk due to exposure to radon and thoron comes principally from the inhalation of the short-lived decay products (Pb-210 and Po-210) and the resulting alpha particle irradiation of the bronchi and the lungs.

As long as these isotopes are outside the body, only the gamma radiation will be able to give a dose. But radon is a gas and diffuses out of the ground to mix with air. The half-life of radon-222 is long compared with the residence time of air in the lungs, so that relatively little radon decay during respiration. Moreover, radon is a noble gas and its inertness prevents its long-term retention within body. But when radon disintegrates, the daughter metallic isotopes (Pb-210 and Po-210) are not inert and they be attached to other molecules like water and to aerosol particles in  air. When these particles are inhaled, some of lead-210 is retained by the body. Ingestion of lead-210 is also a possible way. Since lead-210 is a weak beta emitter, it does not cause major doses. Lead-210 is thus a transport medium from indoor air to the body. The radiation from radon and its decay products is a mixture of alpha particles and beta particles as well as gamma radiation. When the isotopes come inside the body, all types of radiation contribute.

But it is the polonium-210, the decay product of lead-210, that emits a 5.3 MeV alpha particle, which provides most of equivalent dose. Alpha particles, that belongs to high-LET radiation, are fairly massive and carry a double positive charge, so they tend to travel only a short distance and do not penetrate very far into tissue if at all. However alpha particles will deposit their energy over a smaller volume (possibly only a few cells if they enter a body) and cause more damage to those few cells (more than 80 % of the absorbed energy from radon is due to the alpha particles). Therefore, 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. In summary, radon and lead can be viewed as different sorts of carriers for polonium-210.

The amount of isotopes ingested with the food is negligible, and all concern is about the breathing and the deposition of radon daughters in the bronchi and in the lungs. Among non-smokers, radon is the largest cause of lung cancer and, overall, the second-leading cause. The average annual radiation dose to a person from radon is about 2 mSv/year and it may vary over many orders of magnitude from place to place. According to a 2003 report EPA’s Assessment of Risks from Radon in Homes, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon.

It must be emphasized, cigarettes contain also polonium-210, originating from the decay products of radon, which stick to tobacco leaves. Polonium-210 emits a 5.3 MeV alpha particle, which provides most of equivalent dose. Heavy smoking results in a dose of 160 mSv/year to localized spots at the bifurcations of segmental bronchi in the lungs from the decay of polonium-210. This dose is not readily comparable to the radiation protection limits, since the latter deal with whole body doses, while the dose from smoking is delivered to a very small portion of the body.

Radon inside Houses

It must be emphasized, the concentrations of radon-222 and radon-220 in the soil and in the building materials vary over many orders of magnitude from place to place and show significant time variation at any given site. Locations with higher radon background are well mapped in each country. In the open air, it ranges from 1 to 100 Bq/m3, even less (0.1 Bq/m3) above the ocean. In caves or aerated mines, or ill-aerated houses, its concentration climbs to 20–2,000 Bq/m3. In the outdoor atmosphere, there is also some advection caused by wind and changes in barometric pressure.

Problems with radon are in houses, where it can accumulate especially, due to its high density, in low areas such as basements and crawl spaces. Radon can also occur in ground water – for example, in some spring waters and hot springs. Several possibilities exist for the release of radon into houses. The fact radon is a noble gas plays a crucial role in spreading of all its daughter nuclei. Simply radon is a transport medium from bedrock to atmosphere (or inside buildings) for its short-lived decay products (Pb-210 and Po-210), that posses much more health risks. The main sources are the rock or soil on which the house is built, as well as the water supply. The main mechanism for the entry of radon into buildings is diffusion through the soil. As a gas, radon diffuses through rocks and the soil. The radon gas can penetrate into the house through cracks (due to a chimney effect) in the floor and walls of the basement. Heating of the air creates a suction of air from the lower part of the house, towards the higher part of the house. Without any radon membrane,  this actually means that air from the ground beneath the house is sucked into the house through numerous floor cracks and openings.

Furthermore, the building materials (e.g., some granites) is also a source for radon. Another source for radon is the water supply. Water from wells, in particular in regions with radium rich granite, may contain high radon concentrations. This is material with higher uranium/radium concentrations from which radon is continuously generated. Such materials, e.g., slag, fly ash, etc., could be used in some locations. For building materials that are used for the construction of houses, the critical limits for the specific radium concentrations must be determined.

The greatest risk of radon exposure arises in buildings that are airtight, insufficiently ventilated, and have foundation leaks that allow air from the soil into basements and dwelling rooms. The inside radon level vary considerable with weather, time of the year and even time of the day – and of course with the airing system. For example, sleeping with an open window can reduce the radon content considerable.

Most countries have adopted a radon concentration of 200–400 Bq/m³ for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (150 Bq/m³), then no action is necessary. Very high radon concentrations (>1000 Bq/m³) have been found in houses built on soils with a high uranium content and/or high permeability of the ground.

Mitigation of Radon

Mitigation of radon in the air is accomplished through ventilation, either collected below a concrete floor slab or a membrane on the ground, or by increasing the air changes per hour in the building. A radon resisting membranes are usually produced from low density polyethylene (LDPE) and are extended across the whole of the building, including the floor and walls. Another way for mitigation of radon is a treatment system using aeration or activated charcoal to remove radon from domestic water supplies.

See also:

Sources