Radon concentration variability and microclimate controls in two Slovene show caves

The study presents 8-years long dataset (2017–2025) of high-resolution radon concentration measurements in Županova Jama and 2-years long dataset (2022–2024) in Postojnska Jama. Radon concentrations from two show caves were compared with outside and cave air temperature as well as atmospheric pressure to assess diurnal and seasonal variability. Županova Jama showed consistently higher radon levels with a 2023 mean of 4030 Bq/m³, compared to 2278 Bq/m³ in Postojnska Jama. Diurnal variations occurred in both caves, with night values typically higher in Županova Jama, while Postojnska Jama displayed day and night concentrations more balanced depending on ventilation regimes. In Postojnska Jama, summer radon maxima followed outside air temperature peaks with a 12–18 h lag. Seasonally, both caves showed higher radon concentrations in summer and lower in winter. Long-term monitoring near Županova Jama revealed an increase in mean outside air temperature (+ 0.86 °C from 2018 to 2024) and a parallel rise in radon, particularly in 2023–2024. The influence of atmospheric pressure was found to be seasonally dependent at both sites, with higher radon concentrations during periods of stable summer pressure and lower values during higher winter oscillations. Exposure limits indicate safe annual times of 512 h for Postojnska Jama and 284–289 h for Županova Jama, values not hazardous for visitors but important for cave guides. The findings underline the importance of continuous monitoring and cave-specific ventilation management to mitigate long-term health risks.