Year-to-Year Variability of Indoor Radon Concentrations in Finnish Homes

Background: Health risks from radon exposure depend on long-term radon concentrations. This study quantified interannual variability in residential radon concentrations in Finland and evaluated its implications for exposure assessment and measurement recommendations. Methods: A random sample of 277 dwellings from the national radon registry, representative of Finnish regions, building types, and construction years, was monitored using year-long Makrofol-based track-etch detectors over four consecutive years (2021–2025). Year-to-year variability was characterized using the coefficient of variation (CV). Simulations incorporating the observed CVs, previously established seasonal correction factors, and measurement uncertainty were conducted to determine thresholds for follow-up measurements in relation to reference levels. Results: The coefficient of variation (CV) for normalized annual mean concentrations was 0.16, with 90% of dwellings exhibiting CV ≤ 0.30. Although substantial variation occurred at the individual dwelling level, no statistically significant differences in overall radon levels were observed between years. Simulations showed that applying a 100 Bq/m3 threshold for recommending follow-up measurements results in an approximately 3% false-negative rate relative to the 200 Bq/m3 reference level. Lowering the threshold to 70 Bq/m3 reduces this probability to approximately 1%. Conclusions: These findings are consistent with international studies and provide an empirical basis for recommendations on follow-up radon measurements and for quantifying uncertainties in radon exposure assessment in Finnish dwellings.