A laboratory study found that warmer conditions were associated with a steady rise in estimated thoron release from thorium-rich beach sand, while relative humidity (RH) followed a less predictable pattern. Across tests covering 35°C to 50°C and 40% to 60% RH, the reported surface-exhalation estimates ranged from 459 to 1931 Bq m−2 h−1. Over the full 15°C increase, made in 5°C steps, the average increase in the estimate was 34%.
At every tested temperature, raising RH from 40% to 45% increased the estimated rate. Moving to 50% produced a dip, and levels above 50% brought a further decline. The result was therefore not a simple upward or downward relationship between humidity and estimated thoron release.
Inside the chamber
Researchers examined beach sand rich in thorium-232 inside a 0.5 m3 radon chamber. An air heater, moisture generator and automated controller set the temperature and humidity conditions.
For each temperature-and-humidity combination, a closed-loop RAD7 monitor sampled air about 4 cm above the chamber floor and returned it 10 cm above the floor. When the near-floor thoron concentration was at steady state, a mass-balance equation was used to estimate surface exhalation—the calculated rate at which thoron left the sand surface.
At RH values of 50% or lower, the authors used the standard calculation. For 55% and 60% RH, they switched to a modified calculation that used the 50% RH concentration as one input and derived another from the RAD7 reading.
The turn in the humidity curve
At 50°C, the pattern is especially clear. Across 40%, 45%, 50%, 55% and 60% RH, the reported rates were 1813 ± 428, 1931 ± 450, 1765 ± 417, 753 ± 179 and 697 ± 177 Bq m−2 h−1, respectively. The largest point estimate came at 45% RH, followed by lower estimates at the two higher humidity settings.
The figures carry an important qualification: Table 4 uses the ± notation, but the document does not explain how those uncertainty terms were calculated. The table also shows a dash for the 35°C, 40% RH surface-exhalation entry, leaving that cell without a reported rate.
The high-humidity calculation also produced intermediate values that fed into the final estimates. At 50°C, the reported interface concentration was 863 ± 204 Bq m−3 at both 55% and 60% RH. The concentration at the stated diffusion height was 172 ± 40 at 55% RH and 202 ± 41 at 60% RH.
An explanation the study proposes
The authors attribute the upward temperature pattern to an increase in thoron’s diffusion coefficient as temperature rises. Their proposed explanation for humidity is that initial moisture fills sand pores and increases thoron’s availability for diffusion, while water hinders diffusion at 50% RH and water vapour above 50% restricts upward movement and reduces the concentration gradient.
Those explanations are part of the study’s interpretation of chamber measurements and transport calculations. The direct result is narrower: under the tested sand, temperature and humidity settings, the calculated exhalation rate changed in the pattern reported.
A narrow result
Because the experiment used thorium-rich beach sand in a 0.5 m3 chamber, the findings describe that test setup. They are not direct evidence of human exposure, dose or health effects, and they do not establish that natural soils would show the same pattern.
For now, the clearest takeaway is a laboratory pattern in estimated thoron release: higher temperatures tracked with higher estimates, while humidity brought an increase up to 45% RH and declines thereafter.
Paper data and sources
Original title: Study of thoron exhalation from soil under different temperature and humidity conditions.
Authors: Chitra Natarajan, Kothai Parthasarathy
Journal/Repository: Radiation protection dosimetry
Status: Corrected publication
First online: 2026-08-17
DOI: 10.1093/rpd/ncag090
Original paper · Full text