Radiometric Investigation of Indoor Environments: Assessment of Radon and Gamma Radiation Exposure in Residences of a Tropical Metropolitan City in Brazil

Humans are continuously exposed to environmental radiation, with about 80% of annual ionizing dose originating from natural sources. Radon gas and its decay products account for nearly 40% of total exposure, while cosmic radiation and naturally occurring radionuclides in soil, water, and food each contribute about 15%. The remaining 20% is mostly related to artificial sources, particularly medical procedures. Radon is the most significant natural source due to its established association with lung cancer. The World Health Organization (WHO) recommends concentrations below 100 Bq/m³, with values above 300 Bq/m³ considered concerning. This study presents a radiometric assessment of indoor environments in Belo Horizonte, Brazil — the country’s sixth-largest capital, with a population of approximately 2.7 million and an area of about 330 km². Radon concentrations were measured using long-term CR-39 detectors, and ambient gamma dose equivalent H*(10) was estimated using thermoluminescent dosimeters (LiF:Mg,Cu,P). Detectors remained in indoor locations for approximately three months during both dry/cold and rainy/warm seasons. A total of 510 environments and 816 sampling points were included. Radon concentrations showed seasonal variation, with higher levels during the dry season. In this period, nearly one-third of residences exceeded recommended values. H*(10) ranged from 0.00 to 3.08 mSv/year, consistent with expected background levels. Risk categories were communicated to residents together with mitigation guidance, primarily improved ventilation. The study provides updated radiometric data for a tropical urban setting and proposes a cost-effective methodology for preliminary radon and gamma assessments, supporting broader national monitoring and public health initiatives.