Radon as natural factor for the exposure of the population
The major contribution to the internal exposure of the population on Earth has the natural radioactive element Radon (Rn-222) with atomic number 86 in Mendeleev periodic table of elements. The average annual effective dose, received by a person as a result of radon inhalation and its short-living progenies in air, is estimated to be 1.2 mSv/y.
The world average annual effective dose from internal and external exposure, due to the natural radiation background, is equal to 2.4 mSv/y. Consequently 50 % from the natural exposure of the Earth population is caused by Radon (Rn-222). From radiation protection point of view, Radon is the dominant factor in human exposure from the natural radioactive sources.
Radon (Rn-222) is a radioactive inert (noble) gas, heavier than air. It is a result of alpha-decay of the atomic nucleus of Radium (Ra-226), which is a progeny in the Uranium-Radium decay chain, with parent radionuclide Uranium-238. After the radioactive decay of the gaseous Radon-222 (3.8 days half life) a whole chain of new products in solid aggregate state is formed, namely Polonium-218, Polonium-214, Bismuth-214 and Lead-214 (short-living Radon progenies with half life under 30 minutes). These Radon progenies stick to the existing microscopic dust particles in the air (aerosols) and when inhaled, they get caught in the human respiratory system.
The internal exposure is a result of short-living progenies inhalation, not the inhalation of radon itself, which is an inert (noble) gas. After many scientific investigations it was proved, that 98% of the human internal exposure is due to inhaling of radon progenies, which are caught into and act at the bronchial epithelium in the lungs, and only 2% of the exposure is due to Radon-222 itself.
The natural Uranium is spread in the entire crust of the Earth and its quantity is estimated approx. to be 3.10-4 % (average about 3 g per ton of soil). The Radon respectively, as member of the Uranium-Radium decay chain, is distributed everywhere on Earth – in soils, rocks, minerals, waters, air.
The Radon emanation from Radium-226 in soil varies from 1% up to 80%, depending on its characteristics. Part of the Radon, existing in the soil, diffuses through the air pores, reaches soil’s surface and is released into the air near the ground. The process of Radon release from the soil and its entering into the air is called “exhalation”. The activity of Radon exhaled from 1 m2 per second is called “Radon exhalation rate”. After calculations and measurements performed it was estimated, that for the most soil types the Radon exhalation rate is approx. 60 Bq/(m2.h) (Radium-226 concentration in the soil about 25 Bq/kg).
The exhaled Radon passes from the soil surface to the air. The Radon distribution through the air into the environment occurs by diffusion and convection and depends on some natural and climatic factors – atmosphere pressure, air temperature, rainfalls, wind, topography and other characteristics. The Radon exhalation rate from seas and oceans is about 100 times less, than from soil. Respectively the Radon concentration in the air above seas and oceans is considerably less than 10 Bq/m3.
The Radon concentration in the air decreases with altitude. In addition, the Radon concentration in air changes with seasons – the highest values are registered during summer months. During twenty-four-hour period radon concentration varies as well (the maximum is during the night, minimum – during the day).
Typical average Radon concentration in open air near the ground is 10 Bq/m3. As a rule, the Radon concentration in buildings is higher and depends on the construction materials. The typical Radon concentration in buildings is 40 Bq/m3.
For making calculations from “radon concentration” to “effective dose”, “conversion (transition) dose coefficients” are used. The International Commission on Radiation Protection (ICRP) recommends corresponding conversion dose coefficients for Radon. For the population, living in buildings (dwellings) 7000 hours per year, the conversion dose coefficient for Radon recommended by ICRP is: 0.017 mSv/y for 1 Bq/m3.
Using this conversion dose coefficient, it could be calculated, that for one year, a person living in building with average annual Radon concentration of 60 Bq/m3, would take annual effective dose of 1 mSv (60 x 0.017 = 1.02). If the Radon concentration is 40 Bq/m3 – typical average Radon concentration in buildings – the annual effective dose would be 0.68 mSv (40 x 0.017 = 0.68).
ICRP and the International Atomic Energy Agency (IAEA) recommend in their publications, that the competent authorities in every state should implement in their legislation “referent Radon levels” for buildings (intervention levels), which are between 200 Bq/m3 and 600 Bq/m3. If the Radon concentration in buildings reaches the referent levels, the competent authorities in the country should intervene undertaking corrective measures. The most used measure for the decrease of Radon concentration in buildings is ventilation improvement, isolation of Radon incoming paths, “sealing” of floors and walls with proper covering materials.
ICRP estimates that the referent level of 600 Bq/m3 conforms to 10 mSv annual effective dose for the population. In Bulgaria “Regulation on Basic Norms of Radiation Protection” (2004) the following referent levels for average annual Radon concentration in buildings are determined: 250 Bq/m3 for new buildings and 600 Bq/m3 for already built constructions.
In the last few years in Europe and North America large thorough scientific studies on the role and impact of Radon on people were performed. In accordance with them, the World Health Organisation (WHO) and the Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) suggested lower Radon referent levels than the ones suggested by IAEA and ICRP. This question is still under discussion and a new Radon referent levels will be implemented in the European legislation until 2010.
There are two more Radon isotopes in the nature:
– Thoron (Radon-220) – gaseous progeny from the natural radioactive family of Thorium with ancestor Thorium-232;
– Actinon (Radon-219) – gaseous progeny from the natural radioactive family of Actinium with ancestor Uranium-235.
From the radiation protection point of view, the main attention is paid to Radon-222 – the heaviest Radon isotope, which is progeny in the Uranium-Radium decay chain. When people talk about “Radon”, they have in mind “Radon-222”.
The contribution of Thoron (Radon-220) and Actinion (Radon-219) in human exposure is less than the contribution of Radon (Radon-222).