Cave radon exposure, dose, dynamics and mitigation

Abstract

Many caves around the world have very high concentrations of naturally occurring222Rn that may vary dramatically with seasonal and diurnal patterns. For most caves with a variable seasonal or diurnal pattern,222Rn concentration is driven by bi-directional convective ventilation, which responds to external temperature contrast with cave temperature. Cavers and cave workers exposed to high222Rn have an increased risk of contracting lung cancer. The International Commission on Radiological Protection (ICRP) has re-evaluated its estimates of lung cancer risk from inhalation of radon progeny (ICRP 115) and for cave workers the risk may now (ICRP 137) be 4–6 times higher than previously recognized. Cave Guides working underground in caves with annual average222Rn activity > 1,000 Bq m-3and default ICRP assumptions (2,000 workplace hours per year, equilibrium factor F = 0.4, dose conversion factor DCF = 14 μSv (kBq h m-3)-1 could now receive a dose of > 20 mSv y-1. Using multiple gas tracers (δ13C-CO2, Rn and N2O), linked weather, source gas flux chambers, and convective air flow measurements a previous study unequivocally identified the external soil above Chifley Cave as the source of cave222Rn. If the source of222Rn is external to the cave, a strategy to lower cave222Rn by passively decreasing summer pattern convective ventilation, which draws222Rn into caves, is possible without harming the cave environment. A small net annual average temperature difference (warmer cave air) due to geothermal heat flux produces a large net annual volumetric air flow bias (2–5:1) favoring a winter ventilation pattern that flushes Rn from caves with ambient air. Rapid anthropogenic climate change over decades may heat the average annual external temperature relative to the cave temperature that is stabilized by the thermal inertia of the large rock mass. Relative external temperature increases due to climate change (Jenolan Caves, 2008–2018, 0.17°C) reduces the winter pattern air flow bias and increases Rn concentration in caves.