Hidden Radon in Tanzania’s Hot Springs Revealed in First National Baseline Study

Tanzania’s geothermal springs have long drawn bathers, tourists, and communities seeking the therapeutic comfort of naturally heated water, yet until now nobody had measured how much radioactive radon gas dissolves in those waters. A new study published in Environmental Geochemistry and Health delivers the country’s first baseline dataset on radon-222 concentrations in thermal springs, and its findings carry immediate implications for the millions of visitors and residents who come into contact with these waters every year. Radon is an invisible, odorless noble gas produced by the radioactive decay of radium-226 within uranium-bearing rocks, and when it dissolves in groundwater it can deliver radiation doses to humans through two distinct pathways: ingestion, when water is swallowed, and inhalation, when the gas escapes into the air and its short-lived decay products are breathed in. Because thermal springs sit at the intersection of deep geology and everyday human activity, they represent a natural laboratory for understanding how the Earth’s crust quietly shapes public health.

The research team, led by Aloyce Isaya Amasi of the Nelson Mandela African Institution of Science and Technology together with George Herbert Zinga and Shovi Furaeli Sawe of the Tanzania Atomic Energy Commission, sampled five widely used hot springs distributed across five administrative regions: Chemka/Kikuletwa in Kilimanjaro, Lake Manyara in Manyara, Misughaa in Singida, Songwe in Songwe, and Kilambo in Mbeya. These sites span a remarkable geological range, from the volcanically active Eastern Rift to the central cratonic regions of the country, and each spring serves a different mix of bathers, recreational swimmers, and tourists. Water samples were collected directly at the emergence points and analyzed using an AlphaGUARD professional radon monitor equipped with an AquaKIT arrangement, a reference-grade instrument that measures radon activity concentrations in water through a closed-loop degassing and detection cycle. The choice of instrumentation matters: AlphaGUARD systems have been validated against international reference standards, giving the Tanzanian measurements a level of technical credibility that allows direct comparison with datasets from Europe, Asia, and elsewhere in Africa.

The results revealed striking spatial variability that surprised even the researchers. Radon activity concentrations ranged from a low of 0.56 plus or minus 0.39 becquerels per liter at Songwe to a high of 103.34 plus or minus 44.91 becquerels per liter at Misughaa, a difference of nearly two orders of magnitude across the country. Intermediate values were recorded at Chemka, which averaged 4.96 plus or minus 0.76 becquerels per liter, Kilambo at 9.82 plus or minus 7.58 becquerels per liter, and Lake Manyara at 13.84 plus or minus 2.34 becquerels per liter. Statistical analysis confirmed that the differences among sites were highly significant, with a probability value below 0.001, meaning the variation is almost certainly geological in origin rather than a product of measurement noise or sampling chance. Within individual spring systems, particularly Misughaa and Kilambo, concentrations also fluctuated considerably from one sub-location to another, hinting at complex subsurface plumbing beneath each emergence point.

That internal variability is where the study becomes scientifically rich. The authors attribute the heterogeneity to localized hydrogeological controls, including fracture-mediated groundwater flow, groundwater-rock interaction, and radon degassing that occurs before the water reaches the surface. Radon-222 has a half-life of only 3.8 days, so its concentration in any given water sample is a snapshot of recent contact with radium-bearing rock. Water that travels quickly through fractured crystalline basement picks up radon efficiently, because fractures expose fresh rock surfaces and shorten the transit time during which the gas can decay or escape. Conversely, water that lingers in open pools or shallow reservoirs loses radon to the atmosphere through partitioning at the air-water interface, a process whose efficiency depends strongly on temperature and salinity. The exceptionally high values at Misughaa, where individual samples reached up to 179 becquerels per liter, suggest that this spring taps a radium-rich source zone with rapid fracture-dominated ascent, while the low Songwe values indicate either depleted source rock or extensive pre-emergence degassing.

Against international safety benchmarks, the picture is largely reassuring but not uniformly so. The parametric value of 100 becquerels per liter established by Council Directive 2013/51/Euratom applies to water intended for human consumption, and every site mean fell below that threshold except Misughaa, whose site mean marginally exceeded the guideline and whose individual samples climbed well past it. It is important to note that thermal spring water is not typically consumed as drinking water in the way municipal supplies are; the relevant exposure scenario involves incidental ingestion during bathing and, more significantly, inhalation of radon released from warm water into the often-enclosed spaces of bathhouses and spa facilities. Nevertheless, the Misughaa exceedance matters because it identifies a specific location where sustained human contact with elevated radon levels is a realistic and recurring condition, and where targeted monitoring rather than blanket national regulation would be the proportionate response.

To translate concentrations into health consequences, the team calculated annual effective doses using accepted radiological protection models grounded in the recommendations of the International Commission on Radiological Protection and the exposure factor frameworks of the United States Environmental Protection Agency and the United Nations Scientific Committee on the Effects of Atomic Radiation. Effective dose combines the amount of radiation absorbed with sensitivity weightings for different organs and tissues, yielding a single figure in millisieverts that can be compared against international public exposure limits. The calculations showed that estimated annual effective doses from incidental ingestion remained below internationally recommended public exposure limits at all sites, although the higher end of the dose distribution occurred at Misughaa under frequent exposure scenarios, meaning that people who bathe regularly at that spring accumulate doses meaningfully larger than occasional visitors. The dose estimates also depend on assumptions about ingestion volume, exposure frequency, and the fraction of radon that transfers from water to air, and the authors were careful to frame their results as scenario-based estimates rather than individualized dosimetry.

The Tanzanian findings slot into a growing global literature on radon in thermal waters, and the comparisons are instructive. Studies in western Turkey, northern Pakistan, southern Poland, Slovenia, Hungary, South Africa, and India have documented thermal spring radon concentrations spanning similar ranges, with fracture-controlled geology repeatedly emerging as the dominant predictor of elevated values. The South African study of the Montagu thermal spa resort, for instance, measured radon levels and associated effective doses at a tourist destination structurally analogous to Tanzania’s most visited springs, while work in Slovenia and Hungary emphasized that inhalation in enclosed bathing halls can rival or exceed ingestion as a dose pathway. What distinguishes the new study is not the magnitude of its measurements but the fact that, until now, Tanzania’s substantial geothermal resource base had generated no published radon data whatsoever, leaving regulators at the Tanzania Atomic Energy Commission without an empirical foundation for environmental radiation protection decisions in this sector.

The practical implications extend beyond radiation safety into the economics of geothermal tourism. Tanzania has been actively developing its geothermal resources for both energy production and wellness tourism, and hot springs such as Chemka/Kikuletwa have become destinations for international travelers seeking natural bathing experiences. The new baseline dataset allows the country to integrate radiological monitoring into its emerging geothermal regulatory framework from the outset, rather than retrofitting safeguards after problems arise. The authors argue that their findings support both environmental radiation protection and sustainable geothermal resource management, and the data availability statement indicates that the essential measurements are published within the article while raw datasets can be requested from the corresponding author for academic and non-commercial validation purposes. For a country whose radiation protection infrastructure is still maturing, having defensible, instrument-verified numbers is the prerequisite for every subsequent policy step.

What comes next is likely to be a broader national survey. The five springs sampled here represent a first tranche, and Tanzania hosts dozens of additional thermal manifestations along the Western and Eastern Rift branches whose radon signatures remain unmeasured. The strong link between fracture density and radon concentration observed at Misughaa suggests that future work could combine radon mapping with structural geology and hydrogeological modeling to predict which springs warrant priority monitoring. Repeated seasonal sampling would also clarify whether radon levels fluctuate with recharge patterns, water table position, or seismic activity, a relationship documented in earlier studies of thermal springs in seismically active regions. For now, the message for bathers is measured rather than alarming: the overwhelming majority of Tanzanian thermal waters deliver radon doses well within internationally accepted limits, but the Misughaa spring demonstrates that local geology can push specific sites past guideline values, and that the only way to know which waters are safe for frequent use is to keep measuring them. In the quiet chemistry of hot spring water, Tanzania has found both a public health signal and a new window into the fractured plumbing of the East African Rift.

Subject of Research: Radon-222 concentrations in Tanzanian thermal spring waters and the associated radiological doses to users

Article Title: Assessment of radon concentrations in selected Tanzanian thermal waters and associated radiological risks to users: implications for environmental radiation protection

Article References: Amasi, A. I., Zinga, G. H., & Sawe, S. F. (2026). Assessment of radon concentrations in selected Tanzanian thermal waters and associated radiological risks to users: implications for environmental radiation protection. Environmental Geochemistry and Health, 48(14), Article 584. https://doi.org/10.1007/s10653-026-03468-2

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03468-2

Keywords: radon-222, thermal springs, Tanzania, geothermal water, natural radioactivity, annual effective dose, radiation protection, hydrogeology, groundwater, hot springs, radon monitoring, East African Rift