An interlaboratory comparison for European radon calibration facilities was conducted to evaluate the establishment of a harmonized quality level for the activity concentration of radon in air and to demonstrate the performance of the facilities when calibrating measurement instruments for radon. Fifteen calibration facilities from 13 different European countries participated. They represented different levels in the metrological hierarchy: national metrology institutes and designated institutes, national authorities for radiation protection and participants from universities. The interlaboratory comparison was conducted by the German Federal Office for Radiation Protection (BfS) and took place from 2018 to 2020. Participants were requested to measure radon in atmospheres of their own facilities according to their own procedures and requirements for metrological traceability. A measurement device with suitable properties was used to determine the comparison values. The results of the comparison showed that the radon activity concentrations that were determined by European calibration facilities complying with metrological traceability requirements were consistent with each other and had common mean values. The deviations from these values were normally distributed. The range of variation of the common mean value was a measure of the degree of agreement between the participants. For exposures above 1000 Bq/m³, the variation was about 4% for a level of confidence of approximately 95% $\left(k=2\right)$. For lower exposure levels, the variation increased to about 6%.

This paper compares the results of measurements taken in the underground workings of active and tourist mines. In these facilities, the aerosol size distributions of ambient aerosols at key workplaces and the distributions of radioactive aerosols containing radon decay products were determined. Based on these studies, dose conversions used for dose assessment and unattached fractions were determined. In addition, radon activity concentrations and potential alpha energy concentrations of short-lived progeny were also measured in the mines to determine the equilibrium factor. The dose conversions varied between 2–7 mSv/(mJ × h × m⁻³). The unattached fraction measured in active coal mines ranged from 0.01–0.23, in tourist mines from 0.09–0.44, and in the tourist cave it was 0.43. The results showed significant discrepancies between the effective doses determined from current recommendations and legal regulations and those determined from direct measurements of parameters affecting exposure.

An international comparison of continuous monitors measuring radon activity concentration was performed to validate the traceability of the European radon calibration facilities. It was carried out by comparing the secondary standards used by these previous facilities, ranging from 100 Bq·m⁻³ to 300 Bq·m⁻³. Secondary standards were individually compared to a secondary reference device previously calibrated in a reference radon atmosphere traceable to a primary standard. The intercomparison was organized by the National Institute for Nuclear, Chemical, and Biological Protection (SUJCHBO) in the period from October 2019 to April 2020 within the European Metrology Program for Innovation and Research (EMPIR), JRP-Contract 16ENV10 MetroRADON. Eight European laboratories participated in this study. The results of the experiment are presented and discussed.

Since 2020 a large consortium has been engaged in the project EMPIR 19ENV01 traceRadon to develop the missing traceability chains to improve the sensor networks in climate observation and radiation protection. This paper presents results in the areas of: Novel 226Ra standard sources with continuous controlled 222Rn emanation rate, radon chambers aimed to create a reference radon atmosphere and a reference field for radon flux monitoring. The major challenge lies in the low activity concentrations of radon in outdoor air from 1 Bq∙m-3 to 100 Bq∙m-3, where below 100 Bq∙m-3 there is currently no metrological traceability at all. Thus, measured values of different instruments operated at different locations cannot be compared with respect to their results. Whin this paper, new infrastructure is presented, capable of filling this gap in traceability. The achieved results make new calibration services, far beyond the state of art, possible.