Some 50% of the radiation dose received by Finns comes from the radon in indoor air. The average radon concentration of Finnish dwellings is about 120 becquerels/m³ (Bq/m³). The most effective way to reduce the radiation dose would be to lower the radon concentration of indoor air. According to Regulation 1044/2018 of the Ministry of Social Affairs and Health, the reference value for indoor air radon concentration in dwellings and other living areas is 300 Bq/m³. New dwellings must be designed and built in such a way that the radon concentration does not exceed 200 Bq/m³.

The radon concentration is defined as the annual mean of the radon concentration, measured or estimated based on a measurement over a continuous period of one year. The measurement must be continuous and last for at least two months. The measurement must be made between the beginning of September and the end of May.

What is radon?

Radon is a non-odourless, tasteless and invisible radioactive gas generated as a result of the fission of uranium. It continues to degrade into solid degradation products. Degradation products are transported from indoor air to the lungs via inhalation and attach on the inside of the lungs. The radon gas is removed with exhalation. The lung radiation dose resulting from this has been found to increase the risk of lung cancer. To a highly varying extent, uranium is present in the bedrock and mineral soils.

In the Espoo area, the Geotechnical Engineering Unit has collected concentration values for the total radiation in the bedrock and for the radon concentrations in indoor air. Based on research data, the radon risk in zoned areas or in individual buildings can be assessed.

Building Control requires that the occurrence of radon on the plot is investigated, or preparations for it are made, in the design and construction of construction projects.

Room air radon measurements can be ordered from the Radiation and Nuclear Safety Authority (STUK).

Radon in Espoo

According to a study by the Radiation and Nuclear Safety Authority, Espoo falls under category 3, the second lowest-risk, in a four-tier risk classification used in Finland. Thick clay zones are the most safe environments, because clay material is so tight that radon cannot move in it. Houses built on clayey soil often have fewer problems with radon. Any radon in these houses may come from building materials and filling stone materials. The revised Ministry of Social Affairs and Health housing health guideline provides that, as a rule, radon-safe construction should be a requirement in all areas and building land throughout the country.

Enrichment of radioactive minerals into the bedrock has been reported in Espoo. Enrichment has taken place here and there randomly and often in a pocket-like fashion, so that reliable location-specific forecast data cannot be provided. Even on the same plot, variation can be detected in the bedrock in terms of radiating pockets. These constitute potential radon sources that increase the radon risk if radon protection is not taken into consideration.

Without special anomalies, natural soil contains 10–100 times more radon than what is permitted in housing. That is why poor construction may result in increased radon concentrations in room air, even in locations where radon levels are normal.

Rock fracture affects the radon risk. The more fractured the bedrock is, the more there are channels available for radon. It would be recommended to limit blasting to the necessary minimum and to use as small charges as possible.

Radon in construction

In recent decades, the use of ground slabs, breeze blocks and slope solutions has become more common in the foundations of buildings. These construction methods increase the routes available for radon from the ground into the dwelling. The difference between the outdoor and indoor temperatures creates a vacuum that ‘absorbs’ radon-rich air from the ground into warm indoor spaces. In winter, temperature differences are greater, which means that the flow of radon into the dwelling is also higher. The lack of replacement air valves in mechanical venting may increase the vacuum and the radon concentrations of room air. Radon may also carry into dwellings with building materials. However, no high indoor air radon concentrations caused by building materials have been observed in Finland. In addition, radon may be released with the use of domestic water. The radon concentration in borehole water may be so high that when showering and washing clothes or dishes, for example, the radon concentration in room air increases.

Measures when constructing

The main route for radon to access dwellings is through the foundations and through fractures and cracks carried by air flows. In radon areas, the base floor structure should be such that radon flow routes are cut and the foundation is ventilated. Current knowledge has it that a well-designed ventilated crawlspace is the most effective and safe solution.

The Building Information Foundation RTS sr has published an RT file (RT 81-11099). The file provides an illustrative description of how to protect from radon.

With the slab solution, it is important to combine the careful sealing of the base floor and base-floor ventilation with ventilation pipes installed in back sand. With the right structural solutions, great results can be achieved even in difficult radon risk areas. Any floor resting on the ground is risky without any precautions. Sealing the joints of the slab is very important. The joint between the foundation and the slab, the joint around the fireplace foundation, and the perimeter of the sewer, water supply, electricity supply and other pipes must also be sealed well. The ventilation piping to remove radon is installed in back sand before the slab is cast.

The purpose of ventilation of a base floor resting on the ground is not strong ventilation but evening out the pressure difference between the dwelling and the base floor. Due to ventilation, there is almost always a vacuum in the room, which also sucks replacement air through the base floor into the dwelling. An exhaust pipe is routed from the ventilation piping of the base floor through the building to the roof, where radon has a direct path to outside air. When the exhaust pipe passes through a warm room to the roof, ventilation takes place by means of gravity. If necessary, a small exhaust fan can be installed on the end of the exhaust pipe.

Even without conducting detailed studies, it is appropriate to recommend that radon protection be applied to new buildings, since at this stage the construction costs are low. Radon reconstruction is difficult and costly to perform later.