Review and statistical analysis of activity values reported for coastal sands worldwide

The activity of natural radionuclides is unevenly distributed across the Earth's crust, with certain areas exhibiting significantly higher levels than others, known as High Background Radiation Areas (HBRAs). This study presents a statistical analysis of reported activity values for coastal sands globally. Through this statistical analysis, costal sands were classified into four categories based on their activity levels, providing a standardized framework to compare the natural radioactivity of these sands. This classification is a valuable tool for identifying populations exposed to different radiation levels, which is essential for the study of stochastic effects. The study proposes thresholds to define HBRAs as regions with activity values exceeding 203 Bq/kg for ²³⁸U, 517 Bq/kg for ²³²Th, or 960 Bq/kg for ⁴⁰K. Regions with lower values are classified as NonHBRAs. Further subdivision of these categories resulted in four distinct regions: NonHBRA^-, NonHBRA⁺, HBRA^-, and HBRA⁺. The activity values for these subdivisions are 92 Bq/kg and 2,903 Bq/kg for ²³⁸U, 94 Bq/kg and 7,230 Bq/kg for ²³²Th, and 901 Bq/kg and 2,298 Bq/kg for ⁴⁰K. By calculating the external dose rates from the reported activity data, a threshold of 357 nGy/h was identified as the dose boundary separating NonHBRAs from HBRAs. The values for the subdivisions resulted 101 nGy/h and 3,867 nGy/h. The study also explores the content of these natural radionuclides in relation to their bearing minerals and discusses correlations between the reported activity values and the characteristics of the sands. Additionally, the activity of the anthropogenic radionuclide ¹³⁷Cs (reported values ranging from the detection limit to 63 Bq/kg) is examined.