Post-seismic indoor radon exposure in ground-floor gyms: amplified health risk during physical exercise

Earthquakes trigger geophysical changes that enhance the release and upward migration of radon-222, potentially elevating indoor radon concentrations in ground-contact buildings well above pre-seismic baseline levels during disaster recovery phases. Although radon carcinogenesis, earthquake-related radon anomalies, and exercise physiology have each been extensively studied in isolation, their convergence in high-occupancy ground-floor gymnasiums has not previously been examined. This paper advances and evaluates the following hypothesis: post-seismic indoor radon accumulation in ground-floor gymnasiums, when combined with exercise-induced increases in pulmonary ventilation, produces internal radiation doses that substantially exceed those predicted by standard concentration-based radon reference levels, representing a disproportionate and under-recognized lung cancer risk for physically active populations during earthquake recovery. To evaluate this hypothesis, we synthesize evidence across earthquake geophysics, exercise physiology, and radon dosimetry. Pre-earthquake indoor radon concentrations of 50-300 Bq/m³ reflect documented baseline levels; post-earthquake concentrations of 500-1,000 Bq/m³ represent plausible elevations supported by documented post-seismic soil-gas and groundwater anomalies and earthquake-induced building envelope damage. Three user profiles are modeled: a casual visitor (control; dose conversion factor, DCF = 6.9 nSv/(Bq·h·m^-3); 130-260 h/y), a recreational gym user (DCF = 11 nSv/(Bq·h·m^-3); 130-260 h/y), and an elite endurance athlete (DCF = 14 nSv/(Bq·h·m^-3); 800-1,200 h/y). Vigorous exercise increases minute ventilation 5-15-fold and shifts breathing to predominantly oral, amplifying radon progeny inhalation dose approximately 2-fold relative to rest at the same concentration. Model-based illustrations indicate that elite athletes training at post-earthquake concentrations of 500-1,000 Bq/m³ may accumulate estimated annual effective doses of 5.6-16.8 mSv/y, substantially exceeding average annual natural background radiation of 1.0-1.8 mSv/y. Session duration and exercise intensity are identified as immediately modifiable risk factors, with 30-min session reductions capable of halving annual dose for recreational users. These findings suggest that standard concentration-based radon guidelines substantially underestimate lung cancer risk for physically active populations in post-earthquake environments because they do not account for exercise-induced physiological amplification of dose. Proactive post-earthquake radon screening in ground-floor gymnasiums, activity-specific temporary exposure guidance, and integration of radon-resistant measures into seismic building codes represent cost-effective and actionable mitigation strategies.