Measuring radon

Radon measurements in dwellings are used to determine the average radon concentration that people are exposed to. The reference level for dwellings is set at the annual average radon concentration.

Radon measurement in dwellings is carried out between the beginning of September (1 September) and the end of May (31 May), usually with an alpha track detector. The duration of the measurement must be at least two months, but a more accurate estimate of the annual average can be obtained when measuring for three months or more. If the duration of the measurement is less than two months, the long-term average cannot be estimated reliably enough. The annual average is estimated by multiplying the result obtained during the measurement period by a factor of 0.9.

In an ordinary dwelling, two detectors are used, one of which is placed in the living room and the other in the bedroom. These rooms are typically the most frequently occupied, so the results obtained best reflect the exposure of the occupants. If you spend more time in another space, you should naturally place the detector there. If the dwelling has two floors, measurements are recommended to be conducted on both floors. A small apartment (<100 m²) can be measured with one detector if there is only one floor and the foundations and ventilation are the same in each room.

The alpha track detector for a radon measurement is ordered from service providers using radon measurement methods approved by STUK, who will send the detector to your home by post. Instructions on how to place the detectors at home are also provided. At the end of the measurement period, the detectors are returned to the service provider by post.

• Radon measurement methods and their suppliers approved by STUK (Stuk.fi)
• Ordering radon measurement from STUK (Stuk webstore)

The radon concentration in air at the workplace is determined primarily using an alpha track detector (integrating measurement) that is kept at the workplace for a minimum of two months, preferably three months, between the beginning of September and the end of May. If work at the workplace generally takes place outside the measurement period, e.g. in summer, the radon measurement should also be made outside the measurement period. The alpha track detector indicates the long-term average radon concentration. After this, if necessary, time-associated variations in the radon concentration can be analyzed by a so-called continuous radon measurement. Radon concentration during working hours

A key point in radon control at workplaces is that a reliable measurement method is used when determining the radon concentration. Radon measurements can be ordered from STUK or from a company that uses a measurement method that is approved by STUK.

Rapid radon measurements are a good tool for radon remediation, as they can be used to assess whether a radon remediation has been successful in just a few days. However, radon concentrations in buildings are inherently variable. Therefore, a rapid measurement is not a substitute for a two-month alpha detector measurement.

Radon measurements are regulated by a Decree of the Ministry of Social Affairs and Health on ionising radiation (1044/2018). A radon measurement must be continuous and last at least for two months. The measurement must take place between the beginning of September and the end of May. In addition, radon measurements at workplaces must be carried out using a measurement method approved by STUK and the measuring device must be properly calibrated.

Radon levels vary somewhat from day to day and from week to week, depending on weather conditions, among other things. The magnitude of the variation is building-specific. In some houses radon levels vary only slightly, but in others radon levels can vary between, for example, 100 and 1000 becquerels per cubic metre. The longer the measurement, the less random variation will affect the result. Therefore, the radon measurement should last at least two months.

Electronic radon monitors are available on the market that display the average radon concentration for a week or a day. Short-term measurements can be used as a tool for radon remediation when selecting remedial measures and assessing the effects of remediation. When interpreting measurement results, it is important to remember that weather conditions and the ventilation performance of a building can have a significant impact on the results.

A short-term measurement can be used to assess whether further remedial action is needed before a long-term check measurement of radon concentration for at least two months. Many continuous monitors are good at averaging radon concentrations over, for example, 48 hours, but the 48-hour average may in some cases be very different from the longer-term average. Therefore, the result of a radon remediation should always be verified with a measurement of at least two months.

More information

• Reference levels and regulations concerning radon in dwellings (Stuk.fi)
• Radon at workplace (Stuk.fi)
• Radon measurements approved by STUK (Stuk.fi)

Decree

• Decree of the Ministry Affairs and Health on Ionizing Radiation (Finlex.fi)

STUK does not recommend radon measurements of the soil pore air, because the measurement results are difficult to interpret. Soil air radon concentration is subject to both temporal and local variation. During construction, excavation work is carried out on the site and filling soil and gravel and possibly road surface coatings are brought to the site, so the situation before and after the construction may be very different. There is no reference value for radon concentration in the air in the voids, so radon-safe construction cannot be ignored on the basis of the measurement result.

Soil radon measurements are covered by ISO 11665-11. The standard specifies requirements concerning, for example, the sufficient number of sampling points, duration of sampling, sampling depths and sealing of sampling tubes. In other words, radon measurements of the soil pore air are measurements that require a high level of professional expertise. Typically, there should be several sampling points on the plot. At the time of sampling, the soil type, sample temperature, soil moisture and soil air permeability are also determined. If carried out correctly, such a survey is very expensive and therefore not cost effective. The cost of radon prevention in a new building is lower than the cost of a radon survey on the site. In addition, earthwork (e.g. filling gravel and materials) typically carried out during construction changes the situation so that the results are always indicative.

The soil consists of the granules and their interstitial spaces. The interstitial space contains pore air, water (moisture), and, for example, mycelium and soil microbes. When radon is formed in a soil granule, it can get stuck in the granule, be released into the pore air or dissolve in water. In deep soil, the situation is constant: radon enters the pore air as quickly as it leaves it through radioactive decay. Radon concentration in deep soil air can be up to 100,000 Bq/m³.

Radon is transported in the soil by diffusion. Due to the low radon concentration in the outdoor air, the pore air is diluted when it comes to the surface. Wind can also dilute the pore air in the surface layers of the earth.

Land on the plot is rarely completely homogeneous, which means that it does not usually consist of just one type of soil with consistent moisture throughout. Instead, the plot may have different types of soil, rock and stones, and the plot may be sloping. The following properties in the soil, among others, affect radon flows from the soil into the dwelling:

• Soil radium-226 content. Radium-226 is an isotope of the uranium series that produces radon. Different types of soil and rock minerals may have different concentrations of radium. In addition, geochemical processes have caused radium to dissolve in certain layers and become enriched in another layer.
• Air permeability of the soil. The higher the air permeability, the easier it is for the pore air to flow through the base floor of the building.
• Porosity of the soil. The higher the porosity in the soil, the more pore air there is in the soil volume.
• Moisture of the soil. Radon is best released from soil when its moisture is approximately 5% by volume. Moisture helps release radon from the granule into the pore air. On the other hand, in a very humid land, radon cannot be transported as a gas.
• Formations of the soil. On high gravel ridges and slopes with steep edges, a so-called chimney effect occurs. In summer, air in the voids, which is colder than the outdoor air, flows downwards, and the radon concentration of the pore air at the top of the ridge or slope may be very low even at a very deep level. In contrast, at the foot of a slope or ridge, there may be high radon concentrations even in the topsoil. In winter, the situation is reversed.
• Thoron concentration in the soil. The soil usually contains a similar amount of thorium series elements, and so the thoron concentrations in the pore air are in the same range as the radon concentrations.

All measurement results are subject to uncertainty due to sampling and the measurement method itself. For indoor radon measurements, the main uncertainty is due to the natural variability of indoor radon concentrations.

The natural variation depends on factors such as outdoor air temperature, weather conditions such as wind direction and speed, and the humidity of the soil. Indoor radon levels are also affected by changes in the building, such as the use of ventilation equipment, ventilation rates and the difference between outdoor and indoor air temperatures. Consequently, radon measurements taken on the same calendar days in successive years may give different results. The variation is typically in the order of ±30%.

The variation in radon levels in different living areas of the dwelling is also in the order of ±30%. The greatest concentrations are generally found in the spaces below ground level and the smallest in the upper floors. For example, the most significant route of radon into the dwelling may be an unsealed lead-through in the sub-slab, with the greatest radon concentrations occurring in the vicinity of this lead-through. Measurement should therefore be carried out in such a way that the occupant's exposure to radon can be determined. For this reason, the recommended measurement locations are the living room and the bedroom, which are the most frequently occupied areas. The radon concentration in these rooms is therefore the most representative of the occupant's exposure to radon.

The indoor air temperature in a residence is usually between 20°C and 24°C. The radon detector is calibrated for this temperature range. If the average indoor air temperature in the measured dwelling is 18°C, the measurement is about 10 percent too small. If the average indoor air temperature in a residence is 26°C, the result is again about 10 percent too big.

Uncertainty of measurement method

The uncertainty associated with the measurement method is typically smaller than the natural variation in radon concentration. The number of alpha traces calculated from the radon film inside the radon detector has been used to assess the uncertainty in the measurement method. The fewer the number of traces on the film, the greater the uncertainty associated with the number.

Another factor that adds to the uncertainty is the radon exposure the film may have received during the mailing. STUK has investigated this by mailing radon measuring detectors all over Finland. The radon tracks on the films that have been generated during the mailing have been counted.

The conversion of the number of tracks into radon concentration is done by using the measurement time and the calibration function. The mathematical adjustment of the calibration function involves an uncertainty which is also accounted for in the result.

The calibration has been performed in the radon chambers of the STUK Measurement Standard Laboratory. The radon concentration in the radon chambers is subject to a small uncertainty, which has also been taken into account in the measurement result.

Combining all these uncertainties listed above with the uncertainty in the measurement method gives an overall uncertainty of the measurement result, which does not take into account the uncertainty in the sampling. The expanded uncertainty reported in the measurement report means that the overall uncertainty is reported at some confidence level. The uncertainty reported with a coverage factor k = 2 corresponds to a confidence level of about 95 percent. The actual radon concentration over the measurement period is therefore within the uncertainty limits given in the measurement report with a probability of 95 percent.

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