Article publication date
February 2026
ARPANSA review date
August 2026
Summary
This study provides the first terrestrial gamma-radiation map of Sydney city and establishes baseline natural background radiation levels across the region. The study, conducted in 2025, measured terrestrial and cosmic gamma radiation levels across the land and sea. The study primarily used a low-cost portable gamma detector. Soil samples were also collected from 26 locations (mostly public parks) in Sydney. The soil samples were used to estimate (using high purity Germanium detectors) the activity concentration (Bq/kg) of naturally occurring radionuclides (Uranium-238 (238U), Thorium-232 (232Th) and Potassium-40 (40K). Finally, annual effective doses(mSv/year) from outdoor natural background radiation were estimated. Average terrestrial and cosmic gamma doses were found to be 0.24 mSv/year and 0.17 mSv/year, respectively. The mean soil activities reported were: 29 Bq∕kg (232Th), 19 Bq∕kg (238U), and 179 Bq∕kg (40K). Geological composition, such as lithology and radionuclide content, of the soil/rock in the region was identified as the primary factor influencing the spatial distribution of background radiation and reported doses across the city. This study confirms that the radionuclides in soil/rock and background radiation doses in Sydney are consistent and within the range of typical global values.
Article published in
Journal of Environmental Radioactivity
Link to study
Sydney’s first terrestrial gamma-radiation map
ARPANSA commentary
This study offers a valuable means of communicating that ionising radiation is a natural part of our everyday environment, occurring as background radiation and presenting no significant risk to the public. The activity concentrations of the radionuclides in this study are well correlated with local geology; 238U and 40K concentrations in particular, which are slightly lower than the previously reported Australian or global average values (UNSCEAR, 2024 p. 77). The lower concentration of naturally occurring radionuclides explains the lower reported year-long terrestrial outdoor dose rate (0.24 mSv/y) when compared to the global average (0.3 mSv/y) (UNSCEAR 2024). The reported year-long outdoor cosmic dose (0.17 mSv/y) is nearly half the global average (0.33 mSv/y) and that reported for Australia in a previous study (0.35 mSv/y) (Tate et al., 2021). This discrepancy is likely due to the sensitivity of the detector to cosmic radiation. The total average annual dose, combining terrestrial and cosmic components, has been estimated to be 3 mSv globally (UNSCEAR, 2024). Of this, total external terrestrial radiation dose is 0.4 mSv and total inhalation dose is 1.8 mSv. The later dose is mainly constituted by radon (222Rn) and decay products originating from 238U decay. Notably, the current study did not report on internal radiation doses resulting from the inhalation and ingestion of radionuclides (e.g., radon), which generally constitute a major contribution to the total public dose from natural radiation sources.
The Australian Radiation Protection and Nuclear Safety Agency’s Environmental Radiation Monitoring System maintains a network of automated ambient gamma detectors in Lucas Heights (NSW), Yallambie (VIC), Darwin (NT) and Joondalup (WA) to continuously monitor background radiation and evaluate exposure trends. Over the past four decades, Geoscience Australia and the State and Northern Territory Geological Surveys have collected gamma-ray spectrometric data (through airborne surveys using low flying aeroplanes/helicopters) across the majority of Australia. Collectively, the Australian public is provided relevant information about the levels of naturally occurring radionuclides and expected radiation exposure in our environment. In Australia, the Code for Radiation Protection in Existing Exposure Situations sets out the requirements for the protection of the public and the environment in existing exposure situations.
