Indoor distribution, diurnal variation, and health risk assessment of Radon-222 and Thoron-220 in selected Northwestern Nigerian cities

Abstract

Indoor exposure to naturally occurring radionuclides such as radon-222 and thoron-220 remains a significant public health and environmental concern, particularly in regions with uranium-bearing geology and limited ventilation awareness. This study was motivated by the need to establish baseline data and evaluate potential radiation hazards in Northwestern Nigeria, where limited information exists on indoor radon and thoron distribution. The study assessed indoor radon-222 and thoron-220 concentrations, diurnal variation, and associated health risks in selected residential, commercial, and institutional buildings across the region. Measurements were obtained using the RAD7 continuous radon monitor under controlled conditions during different observation periods. The mean indoor radon concentrations ranged between 25.4 ± 7.8 and 102.3 ± 18.2 Bq m⁻³, while thoron ranged from 15.6 ± 5.4 to 85.7 ± 10.9 Bq m⁻³. Concentrations were generally higher at night and early morning, corresponding to periods of reduced ventilation. The calculated annual effective doses ranged from 0.64 to 2.58 mSv y⁻¹, below the ICRP recommended limit of 10 mSv y⁻¹, while the estimated lifetime excess cancer risk (ELCR) ranged between 0.21 × 10⁻³ and 0.92 × 10⁻³. Spatial variations were attributed mainly to geological differences, building materials, and occupancy patterns. These findings provide a scientific basis for improving building design, enhancing ventilation practices, and developing local radiation-safety guidelines to minimize long-term exposure risks.