Key words: Radon, radon sources, radon meassurements, emanation coefficient, radon concentration, radon map, anti-radon measures

RADON IN THE INTERIOR OF BUILDINGS

Radon is a ubiquitous natural radioactive gas. Radon is formed by the decay of uranium, which is present in various quantities in all Earth's crust materials. Further, the radon is converted to the solids atoms ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi and ²¹⁴Pa with a half-life of 3.825 days. The entire chain is terminated with non-radioactive lead ²⁰⁶Pb.

Physical properties of radon:

• Boiling point -62 °C
• Melting point -71 °C
• Evporation heat 16,40 kJ/mol
• Melting heat  2,89 kJ/mol
• Evaporation entropy  77,02 J/deg.mol
• Melting entropy 14,35 J/deg.mol
• Critical temperature +104,3 °C
• Critical pressure  6 322,7 kPa
• Critical density 1,2.10³ kg/m

Radon is an inert gas. Its daughter products are harmful to health. They are inhaled along with carrier solid and liquid aerosols into the lungs where they settle down. Alpha radiation irradiated pulmonary epithelium, there is a potential risk of developing lung cancer. This irradiation is considered one of the causes of lung cancer. However, this is a long-term issue. The disease erupt after several decades of staying in a house with elevated levels of radon decay products. In general, the higher the concentration and the longer the exposure, the higher the risk.

The unit for volume activity of radioactive substances is 1 Bq/m³, which gives one average decay per second in 1 m³ of substance. Similarly, specific activity is defined for 1 kg of substance [1 Bq/kg]. The value of 1 Bq/m³ corresponds to 3.6 atomic decay of radon 222 per hour in one m³.

Decree no. 422/2016 on Radiation Protection and Security of a Radioactive Source sets a reference level for natural irradiation inside a building with a living room. The reference level for the volume activity of radon is set at 300 Bq/m³. This value refers to the average value of the usual air change during use.

In outdoor air, the equivalent volume activity of radon (EOAR) is 7 to 12 Bq/m³. In the geological bed of rocks and soils, radon concentrations are three orders of magnitude (kBq/m3) higher.

Radon concentrations in ground air of rocks and soils range from units to hundreds of kBq/m3. Exceptionally, values above 1 000 kBq/m³ are mostly found on tectonic fault lines, fractures and mylonite zones.

RADON SOURCES

In the wild radon emission surface from bedrock, where it immediately mixes with ambient air. This leads to its strong dilution and minimal effects on the human organism. On the contrary, it is in the indoor environment of buildings.

RADON IN THE ROCKS

If more radon measurements are performed in a particular rock type, then it is possible to roughly estimate the extent of radon volume activity in soil gas. The highest values of radon volume activity are typical of the magmatic rocks of the Bohemian Massif: durbacity and syenity, granites and granodiorites. Silurian sedimentary rocks originating in the Palaeozoons also have high radon volume activities but do not occupy large areas and therefore do not bring a high risk. Rock-shaped rocks such as the paramary, the orator of migamates, which have a medium radon index, occupy a large part of the Bohemian Massif. In the case of alluvial and tertiary sediments such as sandstone or sand, claystone, clays, the values of radon volume activity are generally lower.

The radon index also depends on the tectonic breakage of the rocks. The breakage of crushed surface areas in the rock increases the values of radon volume activity. There is a place for radon migration. Increased radon values can also occur on rock contacts with significantly different permeability and degree of weathering. In the case of determining the radon index category for a building site, it is appropriate to use all available geological information and background material because higher values of radon volumetric activity due to tectonics or rock contact may occur even in small areas.

Prognostic map distinguishes the areas according to the risks of radon ingress into buildings. In areas with high radon index is a higher incidence of homes with higher concentrations, while in areas with low radon index is excessively burdened little houses. In areas with a high radon index, the occurrence of higher concentration homes is more frequent. On the contrary, in the low radon index, overloaded houses by radon is minimum.The map was prepared by the Czech Geological Survey (authors I. Barnet, J. Mikšová, J. Procházka)

OCCURRENCE OF RADON IN INDOOR BUILDING ENVIRONMENTS

The current construction is characterized by the high airtightness of the building envelope. These constructions have well-sealed all structures such as roof, perimeter cladding, windows and ceilings. The higher the airtightness of the building envelope is, the higher indoor radon concentration is. Regular ventilation (natural or forced) reduces radon concentrations in high airtightness buildings. Low concentrations of radon are typical for buildings with leaky fillings of holes, which ensures a constant exchange of air.

The average value of radon in buildings in the Czech Republic is 118 Bq/m³. The Czech Republic is among the countries with the highest concentration of radon in appartmnes in the world.

Radon penetrates into the interiors of buildings through the foundation structure - leaks in floors or walls of the basement, floors without adequate insulation, shafts, ducts or wells. Inalienable possibility of penetration of radon in the indoor environment is diffusion through the contact surface substructure and subsoil. Built-in materials or water can also be a source of radon. The basic radon input paths are cracks in the concrete floor, contact masonry with the floor, cracks in masonry under the terrain, joints in the wooden floor, cracks in masonry, gap around the inlet pipe and cavities in the masonry. The building condition of the building has a significant impact on the amount of radon in the buildings (quality and condition of the insulation, sealing of the penetrations, etc.).

Using radon-rich water can release this gas into objects.  It is not significant compared to the penetration of radon directly from the subsoil into the building. Water supplied from the public water supply is regularly monitored for the content of radioactive substances.

RADON IN BUILDING MATERIALS

The source of higher volumetric activities of radon in the ambient air may be an increased concentration of radium 226 in building materials. Natural materials are crushed, milling and heat-treated, which can lead to a greater release of radon from the building material into the interior of the building. In the past, various types of waste (cinder or slag) used in building materials have proven problematic. At present, all building materials must have a radon certification.

RADON MEASUREMENT

Radon cannot be perceived by human senses. The measurement is the only way to to reliably determine radon concentration in a building. A person certified by the State Office for Nuclear Safety can only perform the measuring activity.

The concentration of radon itself (also known as radon volume activity and referred to as OAR) or concentration of radon conversion products (also called equivalent radon volumetric activity and referred to as EOAR) can be measured.

The relation EOAR = 0.4.OAR is valid. Guidance values are expressed in concentrations of conversion products, and it is necessary to verify that they are reported in EOAR.

Measurements take place over a longer period of time because the radon concentration is not constant over time and it is changing over the course of the year and within one day. It is recommended to carry out measurements:

• Measurement for one year by trace detectors, if there is no rush
• Measurements for at least one week, when there is a rush and it is necessary to establish an indicative value

Emanation coefficient is the proportion of radon released and the total amount generated in building materials.

ANTIRADON MEASURES

If the concentration of radon conversion products in the house exceeds the reference level specified in Decree No. 422/2016 Coll., On Radiation Protection and the Safety of Radionuclide Source, appropriate building modifications should be made, depending on the amount of exceedance. The necessary background for the projection of these adjustments is so-called radon diagnostics, which is a whole set of measurements designed to identify sources and radon entry paths into the house. A person certified by the State Office for Nuclear Safety may only conduct radon diagnostics.
 
Primarily, simple, fast and easy-to-implement measures are chosen which have the least possible impact on building structures and which do not significantly reduce the operation of the building. At the same time, it is usually a relatively inexpensive measure, which can gradually be performed by the owner of the house itself. Basic intervention at the source is done by selecting a suitable place of construction, choosing the suitable building material and choosing to prevent the penetration of radon into buildings.

As a protection of new and modernized structures against the effects of radon can be used gas-tight foil under the baseplate with the dimension of the radon risk area and the use of certified building materials. Careful implementation of insulation work and appropriate material selection is assumed. It is necessary to avoid undue interference with horizontal insulation and the use of unknown building materials.

Manufacturers of building materials are required to prove the safety of building materials in terms of the content of radioactive materials. At present, all natural radionuclides (not only radium) are monitored. The new assessment criterion is the mass activity index calculated from radia, thorium and potassium activities. The index is determined exclusively by the laboratory due to the relatively high content of natural radioactive elements anywhere in the soil.