Integrating Radon/Thoron and Gamma Radiation Exposure for a Realistic Estimation of Dose Arising from Building Materials

Featured Application

The dose prediction and air ventilation models for conventional homes can be utilized by construction professionals and regulators to evaluate and manage radiation risks from radon, thoron, and gamma rays emitted from building materials. These models assist in establishing safety standards and guidelines for indoor environments, promoting safer construction practices and enhancing public health.

Abstract

Long-term exposure to radon, thoron, and gamma radiation from building materials poses a significant health risk to occupants. Current methods for estimating radiation doses often fail to consider the combined impact of these sources. Based on commonly used building materials available on the Irish market, this paper advocates for the development of a comprehensive dose estimation model that accounts for radon, thoron, and gamma radiation. To achieve this, several models and various scenarios (e.g., ventilation conditions and building characteristics) are integrated to convert radon and thoron gas doses into a common unit recognized in the existing literature. This approach enables the comparison of combined dose values with accepted radiation thresholds for building materials, typically set at 1 mSv, alongside data on material compositions. Previous studies suggested gamma radiation doses in Irish materials are unlikely to exceed 1 mSv annually. Our findings confirm this, showing gamma doses <0.4 mSv for all materials. However, combined radon–thoron doses exceeded thresholds in altered granites (e.g., Galway granite: 3.90 mSv), with thoron contributing ≤93% of total exposure due to uranium/thorium-rich minerals (e.g., monazite, zircon). Ventilation proved critical—high airflow (10 m³/h) reduced thoron doses by 90–95%, while current gamma-focused safety indices (I-index ≤ 1) inadequately addressed combined risks. These results highlight the previously underestimated importance of thoron and the necessity of multi-parameter models for regulatory compliance. The study establishes a novel framework to evaluate holistic radiation risks, urging revised standards that prioritize ventilation strategies and material mineralogy to protect public health in residential and commercial built environments.

1. Introduction

Radon (Rn-222), a naturally occurring radioactive gas from the decay of uranium in rocks, soils, and water, can accumulate to high concentrations indoors. Prolonged exposure can pose significant health risks, specifically lung cancer [1]. Thoron (Rn-220), like radon, is a geogenically produced radioactive gas but is a decay product of thorium [1]. While both gases pose considerable health risks due to their radioactive nature, thoron is often less discussed than radon. Recent assessments indicate that the contribution of thoron to radiation dose was previously misunderstood and, therefore, underestimated [2]. This underestimation stems from a historical focus on radon and gamma radiation in regulatory frameworks, neglecting the synergistic effects of thoron (UNSCEAR, 2022 [3]). One reason for this omission is the shorter half-life of thoron (55.6 s), compared to that of radon (3.8 days), which limits its ability to diffuse far from its source. Consequently, thoron concentrations typically decrease rapidly with distance from emitting surfaces, making it more challenging to detect and measure accurately compared to radon [4].

Despite these challenges, recent assessments indicate that the decay products of thoron, which are the actual contributors to the dose, have long enough half-lives to reach indoor environments even from the soil beneath building foundations [5]. However, it is understood that building materials and natural stones used in interiors are the primary sources of thoron [6]. Radon, including exhalation from soil, building materials, and water, remains the main source of radiation exposure for the general public, contributing almost 60% to the annual average dose in Ireland, as highlighted by the Environmental Protection Agency (EPA) and the Health Information and Quality Authority (HIQA) in their recent ‘Ionizing Radiation—National Dose Report—June 2024’ [7]. The report, which assesses the radiation exposure received from various sources, estimates over 8% contribution from thoron. Notably, Ireland’s thoron contribution exceeds the global average of 5% reported by the World Health Organization (WHO, 2021 [8]), while approximately 6% of the radiation dose comes from gamma radiation originating from the ground.

More homes are at risk from radon than previously thought, and about half a million (10%) of the total Irish population may be affected by high indoor radon levels. Furthermore, the average radiation dose received is 4.2 mSv [7], with more than two-thirds arising from radon and thoron. This remains among the highest in Europe [9], highlighting the importance of addressing both radon and thoron exposure, the major contributors to the dose, in indoor environments. This study introduces a novel methodology to bridge critical gaps in radiation risk assessment by integrating gamma radiation with radon and thoron exhalation dynamics under varied ventilation scenarios—an approach not yet standardized in existing guidelines [10]. Both the methodology and findings are directly applicable to radiation protection in other countries.

Rock and aggregate samples were collected from materials of both Irish origin (quarries and stone markets) and non-Irish origin (imported), covering a wide range of rock types. As there is no legal requirement on radon and thoron levels in building materials, models were developed for different housing types to convert radon and thoron activity to effective dose (mSv), which is the standard metric in scientific and regulatory contexts to assess and verify building and construction limits. A 1 mSv annual dose level is suggested as the threshold for building materials used indoors [11]. Existing studies, like [12], focus primarily on radon or gamma radiation in isolation, whereas our model quantifies combined thresholds and ventilation efficacy. Finally, mathematical models were examined to determine the optimal amount of rock surface coverage or appropriate ventilation rates needed to maintain the total effective dose (from both radon/thoron and gamma radiation) below the recommended levels.

The model developed in this study presents a novel methodology to quantify the effective dose from natural radioactive sources in building materials, which is beneficial for further strengthening guidelines and regulations to protect public health. This work addresses the lack of harmonized frameworks for multi-source radiation risk assessment, as identified by the International Commission on Radiological Protection (ICRP, 2007 [13]). Future research could expand these models to include radon and thoron emanating from the soil and rock [14] beneath building foundations, as well as radon from water sources. Such expansions would contribute to a comprehensive view of the doses from all natural radioactive sources, further enhancing the applicability of the radiation dose assessments conducted in this research.

2. Materials and Methods

2.1. Sample Collection

There are limited studies relating to gamma radiation in building materials in Ireland [15], and the previous research does not discuss the radon/thoron concentrations, exhalation rates of these gases, or the associated doses. Therefore, in the present study, we aimed to include as many samples as possible to provide a reasonable coverage of the most popular, available, and abundant materials in Ireland. These materials, which are also widely available in the EU and some of which are available worldwide, include the following: (1) natural stone available in the Irish market, both locally sourced and imported rock (38 samples); (2) products from active quarries with potential for elevated radon/thoron activity (20 samples); (3) aggregate samples, which are the main components of concrete production (theses samples were tested but are not included in this paper); (4) construction and demolition waste intended for recycling (8 samples); and (5) rock core samples provided by Geological Survey Ireland (12 samples). It is noteworthy that estimating radon and thoron exhalation rates could be of interest in other applications, such as geothermal energy potential characterization, as rocks with high radon/thoron potential may indicate a suitable environment for geothermal drilling. Supplementary Table S1 contains a full list of materials tested, their images and selected properties.

2.2. Gamma Radiation Testing

To quantify the levels of uranium (U-238 mg/kg), potassium (K-40%), and thorium (Th-232 mg/kg), and the dose rates in the selected materials, we utilized the Georadis (Brno, Czech Republic) GT40s equipped with a NaI (Tl 76 × 76 mm, 345 cm³, with bi-alkali PMT gamma detector in assay mode. In our experimental setup (Figure 1), a lead collimator was placed over each sample to ensure that the gamma radiation detected was primarily from a defined area on the sample’s surface. Specifically, the lead collimator restricts the gamma radiation detection to a one-dimensional surface, to isolate a circular surface area with a diameter of 2.5 cm. This configuration allowed for an approximate measurement of gamma radiation emanating from the targeted section of the sample.

2.3. Radon and Thoron Exhalation Rate Testing

Seventy-eight representative rock samples underwent radon and thoron testing using the RTM1688-2 (SARAD, Dresden, Germany) Monitor Setup, which is calibrated according to DIN ISO/IEC EN 17025:2018 standards [16]. The samples were placed in an airtight vacuum desiccator (Figure 1) to prevent gas leakage, verified by a gas leak detector spray. A small fan was used to ensure uniform air circulation within the chamber. We allowed between 36 and 48 h for radon and thoron equilibrium to be established between the samples and the surrounding air, with measurements taken at 1-h intervals to accurately capture gas emissions during this period. Considering Equations (1) and (2), the exhalation rates of ²²²Rn (E222Rn, Bq m⁻² h⁻¹) and ²²⁰Rn (E220Rn, Bq m⁻² h⁻¹) were calculated by extrapolating the slope of the growth curve (m) (Bq m⁻³ h⁻¹) and the equilibrium Rn-220 concentration (Cm) (Bq m⁻³), respectively [17]. Figure 2 shows an example of ²²²Rn/²²⁰Rn concentrations as a function of time and the derived exhalation rates.

$${\mathrm{E}}_{222\mathrm{Rn}}={\displaystyle \frac{(\mathrm{m}+{\mathsf{\lambda }}_{222}\times {\mathrm{C}}_0)\times \mathrm{V}}{\mathrm{S}}}$$

(1)

$${\mathrm{E}}_{220\mathrm{Rn}}={\displaystyle \frac{{\mathsf{\lambda }}_{220}\times {\mathrm{V}}_0}{\mathrm{S}}}{\displaystyle \frac{{\mathrm{C}}_{\mathrm{m}}}{{{\displaystyle \mathrm{e}}}^{-{\mathsf{\lambda }}_{220}\times ({\mathrm{V}}_1/\mathrm{Q})}}}$$

(2)

where λ₂₂₂ and λ₂₂₀ are ²²²Rn and ²²⁰Rn decay constants (h⁻¹), C₀ is the initial radon concentration (Bq m⁻³), V is the free total volume of the analytical system (m³), S is the surface of the sample, V₀ and V₁ (m³) are the free volume of the accumulation chamber and the volume between the outflow of the accumulation chamber and the inflow of the radon monitor, respectively. Q (15 L h⁻¹) is the flow rate in the system.

2.4. Dose Conversion and Ventilation Models

2.4.1. Dose from Gamma Radiation

The gamma dose index, or I-index, is a simplified radiological assessment tool used to estimate the potential external gamma radiation exposure from building materials [18]. It is calculated using the concentrations of the three radionuclides as follows (Equation (3)):

(mSv) = C_U/300 (Bqkg⁻¹) + C_Th/200 (Bqkg⁻¹) + C_K/3000 (Bqkg⁻¹)

(3)

where C_U is the concentration of uranium (U-238) in Bqkg⁻¹, and 12.35 Bqkg⁻¹ corresponds to 1 mg/kg of uranium. Similarly, C_Th is the concentration of thorium (Th-232) in Bqkg⁻¹, with 4.06 Bqkg⁻¹ equating to 1 mg/kg of thorium. For potassium (K-40), C_K represents the concentration in Bqkg⁻¹, and 313 Bqkg⁻¹ corresponds to 0.1% of potassium [19].

2.4.2. Dose from Radon and Thoron

Calculating the dose from radon and thoron in indoor environments is a complex process due to the various factors influencing the concentration and distribution of these gases [5]. Radon and thoron levels depend on several variables, including the type of building materials used, the ventilation rate, the design of the building, and the time occupants spend indoors. To address these complexities, three common housing scenarios are considered: a one-bedroom house, a two-bedroom house, and a three-bedroom house. A model of each housing scenario was created to estimate the effective (net) volumes for different building type scenarios. This technique helped to visualize and calculate the surface areas of potential radon sources in homes, determine the total volume of furniture inside the house, estimate the net air volumes for different housing types, and analyze data from numerous real-world cases. By studying dozens of actual instances, we approximated the range of surface areas of natural stones—recognized sources of radon and thoron in indoor environments—that could be present in each housing type (

Table 1. Annual gamma, radon, and thoron doses (mSv/year) for Irish building materials. Altered granites show the highest combined doses.

Model	1-Bedroom House (See Figure 2)	2-Bedroom House	3-Bedroom House
V effective (Veff) m3	121.16	230	380
Stone Surface Area (s) range m2 (10 to 30% of the total Surface Area of the house)	42–80	72–123	118–180

F_Rn-222 and F_Rn-220 are radon and thoron conversion factors equal to 2.7 × 10⁻⁶ and 7.0 × 10⁻⁶ mSv per Bqm⁻³, respectively. C_Rn-222 and C_Rn-222 represent the average radon and thoron exhalation per surface Area (Bqm⁻²).

). Additionally, to calculate the effective volume of indoor spaces, we subtracted the volume occupied by furniture and appliances from the total volume of the house. This allowed an estimation of the effective space in which radon and thoron can accumulate, providing a more accurate calculation of the potential exposure doses for the occupants.

Equation (4) for calculation of Dose [20,21]:

Dose (mSv) = (F_Rn-222 C_Rn-222 (Bqm⁻²) + F_Rn-220 C_Rn-220 (Bqm⁻²)) S(m²)/V_eff (m³)

(4)

2.4.3. Total Effective Dose and Suggested Dose Limit

The I-index is used to assess whether building material is safe for use based only on its gamma radiation levels. According to the European Union guidelines and other international standards,

I ≤ 1: The material is considered safe for use in building construction without restrictions.

I > 1: The material may pose a higher radiological risk, and further evaluation may be necessary to determine whether it is suitable for use, particularly in bulk quantities or in buildings where occupants might spend prolonged periods [22].

At the time of writing, there is no recognized index for the dose of radon and thoron emitted from building materials. It is possible that for some materials, the I index is below 1 mSv but adding the dose from radon and thoron might increase the total dose over the 1 mSv limit.

2.4.4. Modeling the Effect of Air Ventilation

Models were developed to simulate the effectiveness of ventilation rates in reducing exposure to radon and thoron gas for samples where the estimated annual dose from radon and thoron (in the worst-case scenario from Section 2.4.2) exceeded 0.3 mSv. Three ventilation scenarios were considered: low (0.5 m³/h), medium (2 m³/h), and high (10 m³/h) ventilation rates. Mathematical expressions were then derived to calculate the dose after ventilation, considering variables, such as ventilation time, house condition, ventilation rate, and the amount of stone surface area used. The formulae developed here are useful for adjusting ventilation rates or the amount of radon/thoron-emitting surface area in various housing conditions to keep the total annual effective dose, including the effect of gamma radiation, below the recommended limit of 1 mSv [23].

$$\begin{array}{l}\mathrm{Dose} \mathrm{after} \mathrm{Ventilation} \left(\mathrm{mSv}\right)=\\ \mathrm{T}\times ({\displaystyle \frac{\mathrm{S}{\times \mathrm{C}}_{\mathrm{R}\mathrm{n}220}\times {\mathrm{E}\mathrm{q}}_{\mathrm{R}\mathrm{n}220}\times {\mathrm{D}\mathrm{C}\mathrm{F}}_{\mathrm{R}\mathrm{n}220}}{\mathrm{Q}\times {\mathrm{V}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}}+{\displaystyle \frac{\mathrm{S}{\times \mathrm{C}}_{\mathrm{R}\mathrm{n}222}\times {\mathrm{E}\mathrm{q}}_{\mathrm{R}\mathrm{n}222}\times {\mathrm{D}\mathrm{C}\mathrm{F}}_{\mathrm{R}\mathrm{n}222}}{\mathrm{Q}\times {\mathrm{V}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}})\end{array}$$

(5)

In Equation (5), Q is the ventilation rate per hour, T is the system running time (assuming 8 h per day), Eq_Rn220 and Eq_Rn222 are Equilibrium Factors = 0.4 for radon and 0.01 thoron.

Also, dose conversion factors (DCFs) for radon are equal to 9 × 10⁻⁶ and 40 × 10⁻⁶ mSv per Bqm⁻³ per h, respectively.

The theoretical framework for the model integrates radon/thoron exhalation dynamics with gamma dose calculations, assuming steady-state conditions for gas diffusion (Fick’s law) and equilibrium between parent isotopes (U-238, Th-232) and their decay products. Key limitations include the exclusion of seasonal humidity effects on material exhalation rates and the assumption of homogeneous material composition, which may oversimplify real-world heterogeneity. Ventilation rates were modeled as constant, though actual airflow varies diurnally. Additionally, gamma dose estimates rely on mass activity concentrations without granular spatial resolution. Recent work by Kumar et al. (2014) [24] highlights similar challenges in reconciling lab-measured exhalation rates with field observations. This emphasizes the need for probabilistic modeling in future studies to account for environmental variability.

2.4.5. Elemental Composition and Mineral Mapping

Elemental composition and mineral mapping were conducted using a Tescan TIGER MIRA3 Field Emission Gun Scanning Electron Microscope (FEG-SEM) (Tescan, Brno, Czech Republic) equipped with backscattered electron (BSE) imaging and energy-dispersive X-ray spectroscopy (EDS). Samples were prepared by mounting polished thin sections in epoxy resin and coating them with a conductive layer of carbon to optimize imaging and analysis.

BSE imaging was used to investigate the microstructure and mineralogical matrix, with emphasis on identifying accessory minerals associated with uranium and thorium, such as uraninite (UO₂), thorianite (ThO₂), monazite [(Ce, La, Nd, Th)PO₄], and zircon (ZrSiO₄). Energy-dispersive X-ray spectroscopy was utilized to generate high-resolution elemental maps, providing spatial distributions of uranium and thorium across the sample surfaces. Regions of interest were identified based on preliminary gamma spectrometry and radon/thoron exhalation results. Analytical parameters, including accelerating voltage (15–20 kV), beam current, and working distance, were optimized to ensure the precise quantification of elemental concentrations and the accurate identification of mineral phases. This methodology provided detailed spatial and compositional data on uranium- and thorium-bearing minerals, facilitating the understanding of their contribution to radon and thoron emissions from the tested building materials.

3. Results and Discussion

3.1. Radon/Thoron Exhalation Rates vs. Radionuclide Activity

For each sample, radon and thoron exhalation rates were calculated based on the equilibrium thoron activity and radon growth model as shown in Figure 3. Supplementary Table S2 shows the detailed measurements of gamma radiation and radon thoron exhalation rates for each sample.

Table 2. Thoron exhalation rates (Bq/m²/h) and mineral properties of Irish rock samples.

		Radionuclide Activity (Min, Max, and Average)
Rock Type	#	U mg/kg			Th mg/kg			K %			I Index			Exhalation Rates of 220Rn (E220Rn, Bq m−2 h−1)			Exhalation Rates of 222Rn (E222Rn, Bq m−2 h−1)
Breccia	1	1.64			2.78			0.63			0.19			71			0.170
Brick	2	1.76	1.90	1.83	2.52	3.11	2.81	0.75	0.78	0.76	0.21	0.21	0.21	301	514	407	0.008	0.031	0.019
Concrete	3	1.76	1.86	1.80	2.90	3.13	2.98	0.75	0.87	0.80	0.21	0.22	0.21	106	567	277	0.001	0.007	0.005
Granite	25	1.34	3.87	1.80	2.09	7.94	2.99	0.56	1.21	0.73	0.17	0.40	0.21	10	45,889	6396	0.070	3.534	0.514
Gypsum	6	1.69	1.93	1.80	2.25	2.37	2.31	0.52	0.66	0.60	0.17	0.17	0.17	21	21	21	0.034	0.034	0.034
Limestone	14	1.48	1.87	1.60	1.68	2.95	2.36	0.51	0.68	0.60	0.16	0.19	0.17	15	15,213	1352	0.018	0.983	0.210
Phosphate	1	1.78			2.20			0.67			0.18			9			0.048
Porcelain Tile	3	1.45	1.93	1.63	2.23	2.92	2.53	0.65	0.92	0.75	0.17	0.23	0.19	73	1394	631	0.010	0.098	0.064
Quartzite	8	1.60	2.11	1.82	2.14	2.78	2.40	0.60	0.94	0.73	0.17	0.22	0.20	103	6091	1637	0.030	0.232	0.088
Rhyolite	1	1.83			2.34			0.62			0.18			2207			0.065
Sandstone	11	1.41	1.93	1.65	2.09	2.92	2.44	0.55	0.79	0.64	0.17	0.21	0.18	6	943	267	0.030	0.137	0.071
Volcanic	2	1.58	1.88	1.73	2.41	2.51	2.46	0.60	0.67	0.64	0.18	0.18	0.18	15	175	95	0.105	0.123	0.114
Greywacke	1	1.38			2.57			0.66			0.17			115			0.021

shows the statistics of the radionuclide activity of the samples grouped based on their geology. Along with Figure 4, the table provides a comprehensive comparison of radionuclide activity and radon/thoron exhalation rates across various building materials, with an emphasis on the range and mean values to highlight differences in radiological properties. Granite displays the widest range in thorium (Th) concentrations (2.09 to 7.94 mg/kg) and the highest variability in thoron (Rn-220) exhalation rates, ranging from 10 to 45,889 Bq m⁻² h⁻¹, with a mean of 6396 Bq m⁻² h⁻¹. This significant variation is largely due to the differing mineralogical composition of granite, which can include minerals, like thorite and monazite, that are rich in thorium and contribute to higher thoron emissions.

Some of the granites with high thoron exhalation rates were found to be highly altered (e.g., Donegal and Galway areas). Higher modal % of Th-bearing accessory minerals and the secondary porosity and fractured nature of altered granite could facilitate the escape of thoron gas, enhancing its potential to contribute to indoor radiation exposure [25]. Quartzite also exhibits notable radiological properties, with thoron exhalation rates ranging from 103 to 6091 Bq m⁻² h⁻¹ and a mean of 1637 Bq m⁻² h⁻¹. The variability in thoron exhalation from quartzite can be attributed to the presence of thorium-bearing minerals, such as monazite and zircon, which contribute to higher emissions. This suggests that quartzite, like granite, can potentially contribute to indoor radiation levels, particularly when used extensively in building construction [17,26].

In contrast, materials, like gypsum and rock-types, such as limestone, show much lower and more consistent thoron exhalation rates. For instance, gypsum has a uniform thoron exhalation rate of 21 Bq m⁻² h⁻¹, with a mean thorium concentration of 2.31 mg/kg, indicating a lower radiological risk. The consistency in the thoron exhalation rates of gypsum can be linked to its homogeneous mineral composition. Limestone exhibits a broader range of thoron exhalation (15 to 15,213 Bq m⁻² h⁻¹), but with a much lower mean of 1352 Bq m⁻² h⁻¹, which is still significantly lower than that of granite. This is likely due to the predominance of CaCO₃ with minimal thorium content, leading to reduced thoron emissions [2].

The Pearson correlation coefficients in

Table 3. Similarity test results between radon/thoron exhalation rates of materials with mineralogical drivers (U, Th, and K).

Pearson Correlation Coefficient):	E220Rn	E222Rn	Average 222Rn Surface Emission	Average 220Rn Surface Emission	I index	U mg/kg	Th mg/kg	K %	Dose Rate nSv/hr
E220Rn	1.00	0.60	0.73	0.96	0.46	0.35	0.35	0.49	0.49
E222Rn	0.60	1.00	0.89	0.61	0.67	0.74	0.50	0.50	0.69
Average Rn-222 Surface Emission	0.73	0.89	1.00	0.71	0.77	0.71	0.64	0.64	0.79
Average Rn-220 Surface Emission	0.96	0.61	0.71	1.00	0.48	0.36	0.36	0.51	0.50
I index	0.46	0.67	0.77	0.48	1.00	0.78	0.89	0.88	0.99
U mg/kg	0.35	0.74	0.71	0.36	0.78	1.00	0.50	0.57	0.78
Th mg/kg	0.35	0.50	0.64	0.36	0.89	0.50	1.00	0.68	0.86
K %	0.49	0.50	0.64	0.51	0.88	0.57	0.68	1.00	0.88
Dose rate nSv/h	0.49	0.69	0.79	0.50	0.99	0.78	0.86	0.88	1.00

provide important insights into the relationships between various radon and thoron exhalation rates, surface emissions, and radionuclide activities. The strong positive correlations between thoron exhalation rates (E220Rn) and its surface emission (r = 0.96), as well as between radon exhalation rates (E222Rn) and its surface emission (r = 0.89), underscore a direct and robust link between the amount of these gases emitted and their measurable surface presence. This indicates that increases in the exhalation rates of thoron and radon correspond to proportional increases in their respective surface emissions.

In contrast, the moderate correlation between E220Rn and E222Rn (r = 0.60) suggests a more nuanced relationship. While there is some association between thoron and radon exhalation rates, the correlation is less pronounced compared to the correlation between each gas’s exhalation rate and its surface emission. This implies that factors influencing the emission of radon may not directly impact thoron, and vice versa. Similarly, the moderate to strong correlation between Average Rn-222 Surface Emission and Average Rn-220 Surface Emission (r = 0.71) reflects a partial association between the emissions of these gases, though this relationship is weaker compared to their exhalation rates.

Moreover, the exceptionally strong correlation between the I Index and the Dose Rate (r = 0.99) underscores a significant relationship between this index—likely representing a composite measure of radon and thoron activities—and the radiation dose rate. This indicates that fluctuations in the I Index are closely tied to variations in the detected radiation dose. In contrast, the weaker correlations between radon and thoron exhalation rates with uranium (U mg/kg) and thorium (Th mg/kg) activities (generally below 0.5) suggest a less direct relationship when considering each element individually. This weaker association implies that other factors might play a more substantial role in influencing surface emissions and exhalation rates.

Furthermore, the correlations between radon and thoron exhalation rates with uranium and thorium suggest some degree of interference between these elements (i.e., Th is a significant input in radon prediction, and similarly, U is significant when predicting thoron exhalation rates. This interference indicates that interactions among these radionuclides could be affecting the observed exhalation rates. To better understand and model these interactions, advanced modeling techniques could be applied. By incorporating the effects of thorium and uranium interference, it may be possible to enhance the accuracy of predictions for radon and thoron exhalation rates. Such advanced models [27] could account for the combined influences of multiple radionuclides, leading to more refined assessments of radiation levels and improved monitoring methods.

3.2. Implications for Radiometric Surveys and Radionuclide Scanning

The observed correlations have significant implications for radiation monitoring and assessment technologies. Airborne or drone-borne vehicles equipped with gamma detectors [28,29,30] are effective for covering large areas, such as quarries or mines. Given the correlations between radon and thoron exhalation rates or surface emissions with radioelement activities, drones can be utilized to map radiation levels and identify high-activity zones efficiently. This capability is crucial for environmental monitoring and safety in areas with potential radon and thoron exposure.

Handheld gamma detectors are well-suited for localized monitoring at distribution centers, markets, and residential or workplace settings. The strong correlation between the I Index and the dose rate indicates that handheld devices can provide accurate measurements of radiation levels corresponding to radon and thoron activities. Regular use of these devices can help pinpoint high-radiation areas and ensure that safety measures are in place to mitigate potential health risks associated with elevated radiation exposure. Potential drawbacks of using handheld gamma detectors include potential instrument calibration issues, and the high sensitivity of some instruments, which necessitates adequate lead shielding to prevent overestimating gamma dose rates and uranium, thorium, and potassium concentrations.

Cross-Validation of Data from Handheld Gamma Detector (GT40) and Drone-Based Surveys with D230A Detector

As part of a supplementary research project [30], drone-based surveys were conducted to map gamma radiation variations across four quarries in County Donegal, Ireland: one sandstone, two quartzite, and one granite quarry. A D230A detector mounted on a drone was utilized for these surveys. Before the drone surveys, a GT40 handheld gamma detector (in assay measurement mode) was employed to collect dose rate, uranium, thorium, and potassium activity data at specific locations.

Table 4. Cross-validation of dose rate (DR) uranium (U), thorium (Th), and potassium (K) collected using a GT40 hand-held gamma detector with those measured during the drone-borne surveys using a D230A gamma detector.

D230A	GT-40
	Dose Rate (DR) nSv/h	U [mg/kg]	Th [mg/kg]	K [%]
DR	0.73
U		0.53
Th			0.79
K				0.50

presents the results of a cross-validation analysis of radioelement data (see Supplementary Table S3) collected using both handheld and drone-borne surveys, employing the Pearson correlation test (p < 0.05). Overall, there is a good agreement between the datasets collected using both methods. Dose rates exhibit a strong correlation (R² = 0.73), while the highest agreement (R² = 0.79) was observed for thorium values. This enhanced agreement for thorium is likely due to its higher activity at the source, minimizing the impact of the background radiation effect. It is noteworthy that regression methods were applied to the drone-based gamma data to interpolate values between flight lines and generate smooth surface maps. Therefore, it is expected that the interpolation of data may have slightly affected (decreased) the correlation coefficients between the data.

3.3. Converted Doses

Supplementary Figure S1 provides insights into various stone materials sourced from different regions, focusing on their exposure to gamma radiation and radon/thoron exhalation. These factors contribute significantly to the total effective radiological dose. The dataset includes a wide range of stone types from Ireland, other EU countries, and non-EU sources.

The total annual effective dose and the I index, which indicates radiation hazard levels, show considerable variation among these materials. Notably, in the worst-case scenario, the highest total annual effective dose recorded for Irish-origin materials comes from granite sourced in the Galway region, with a dose of 3.90 mSv and an I index of 0.29 mSv, highlighting its significant radiological importance.

The accompanying figure illustrates the contributions of gamma radiation and radon/thoron exhalation rates for samples that had the highest dose contributions.

In the tested samples, which originate from Ireland, gamma radiation contributes between 0.17 mSv and 0.40 mSv to the total effective dose, accounting for about 7% to 33% of the overall exposure. In contrast, radon and thoron exhalation play a much more substantial role, contributing between 0.01 mSv and 3.61 mSv—approximately 7% to 93% of the total effective dose (Figure 5). These data underscore that for the measured samples, radon and thoron are the primary sources of radiation exposure in Irish samples, while gamma radiation acts as a secondary contributor.

For the samples tested, which originated within the EU (but excluding Ireland), gamma radiation contribution ranges from 0.18 mSv to 0.20 mSv, making up about 15% to 32% of the total dose. The total annual effective dose for these samples varies from 0.18 mSv to 0.68 mSv, with radon and thoron doses ranging from 0.00 mSv to 0.46 mSv, contributing roughly 0% to 85% of the total dose. This re-emphasizes that while gamma radiation constitutes a modest fraction of the total dose in EU samples, radon and thoron remain the more significant contributors.

For the tested samples, which originated from outside the EU, gamma radiation contributes between 0.16 mSv and 0.19 mSv, accounting for about 9% to 44% of the total dose. The total annual effective dose ranges from 0.17 mSv to 2.22 mSv, with radon and thoron contributions varying from 0.00 mSv to 2.03 mSv—representing approximately 56% to 91% of the total dose. This variation highlights the diverse impact of gamma radiation across non-EU samples, where radon and thoron generally emerge as the predominant sources of overall radiation exposure.

3.4. Effect of Ventilation

Table 5. Details of dose ranges, surface activity, and air ventilation rates for the investigated samples.

Dose mSv. (Worst Case Scenario with No Ventilation)				Surface Activity Bq m−2		Air Ventilation Rate
						Low		Medium		High
Sample	I index	Radon	Thoron	Thoron	Radon	0.5 m3/h		2 m3/h		10 m3/h
37	0.291	1.627	3.612	782	95.28	1.327	2.528	0.332	0.632	0.066	0.126
12	0.188	0.916	2.034	440	45.09	0.685	1.305	0.171	0.326	0.034	0.065
29	0.189	0.701	1.556	337	23.81	0.446	0.850	0.112	0.212	0.022	0.042
QG1	0.404	0.562	1.248	270	129.42	1.162	2.213	0.290	0.553	0.058	0.111
GSIG	0.178	0.511	1.134	97	46.59	0.418	0.797	0.105	0.199	0.021	0.040
Q3A	0.201	0.234	0.519	112	6.42	0.138	0.262	0.034	0.066	0.007	0.013
5	0.199	0.205	0.455	99	11.67	0.165	0.314	0.041	0.078	0.008	0.016
23	0.188	0.205	0.205	98	12.79	0.173	0.329	0.043	0.082	0.009	0.016
QG2	0.351	0.203	0.450	96	3.50	0.104	0.197	0.026	0.049	0.005	0.010
DW8	0.192	0.200	0.445	92	10.74	0.153	0.291	0.038	0.073	0.008	0.015
34	0.177	0.192	0.426	84	13.39	0.165	0.315	0.041	0.079	0.008	0.016
36	0.205	0.174	0.387	81	10.89	0.145	0.276	0.036	0.069	0.007	0.014
35	0.177	0.168	0.374	61	18.45	0.184	0.350	0.046	0.088	0.009	0.018
Q4A	0.199	0.132	0.294	64	13.43	0.149	0.284	0.037	0.071	0.007	0.014

presents a worst-case scenario model for radon and thoron dose reduction in a one-bedroom house, focusing on samples with radon and thoron dose contributions higher than 0.3 mSv. The analysis considers two configurations: 10% and 30% of the surface area covered by stone, and the impact of varying air ventilation rates. At a ventilation rate of 0.5 m³/h, with 10% surface coverage, doses range from approximately 0.041 to 1.327 mSv. Increasing the ventilation to 2 m³/h reduces these doses to a range of 0.079 to 0.662 mSv, representing a reduction of about 50–80%. When the surface coverage is increased to 30%, the doses are generally higher due to the greater exposure, but the relative reduction remains similar. At the highest ventilation rate of 10 m³/h, the doses decrease to a range of 0.005 to 0.126 mSv, corresponding to a reduction of approximately 90–95%, regardless of surface coverage.

Notably, the reduction in thoron dose is more significant than the reduction in radon dose due to ventilation. This can be attributed to the shorter half-life of thoron (approximately 55.6 s) compared to radon (3.8 days). Thoron decays more rapidly, meaning that effective ventilation can remove thoron gas from indoor environments before it has a chance to decay and contribute significantly to radiation dose. As a result, higher ventilation rates are particularly effective in reducing thoron levels, leading to the observed substantial reductions in thoron dose across different ventilation scenarios. Based on the charts of Figure 6, a formula was developed to calculate the reduced dose from radon and thoron because of active air ventilation.

Reduced Dose after Ventilation = 0. 1192Q^−1.001 × e(^{0.7805. Dose Initial})

(6)

To mitigate indoor radiation exposure, practitioners first measure the annual radiation dose from radon, thoron, and gamma sources using detectors, such as the RAD7 (Durridge, Billerica, MA, USA). In Ireland, where the average annual radiation dose is 4.2 mSv, approximately 60% (2.5 mSv/year) comes from radon and thoron. If 0.4 mSv/year is also attributed to gamma radiation (total = 2.9 mSv/year), to meet the recommended safety threshold of 1 mSv/year, the radon/thoron contribution must be reduced to 0.6 mSv/year (because 1 mSv/year total − 0.4 mSv/year gamma = 0.6 mSv/year). Using an empirical model derived from experimental data, the required ventilation rate (Q) is calculated to be 1.4 m³/h. This airflow can be achieved with a small exhaust fan or mechanical ventilation system. Because of the extremely short half-life of thoron (55.6 s), even modest ventilation drastically reduces its dose contribution. However, the longer half-life of radon (3.8 days) means additional measures—such as sealing foundation cracks, installing radon barriers, or substituting high-exhalation building materials (e.g., granite) with low-emission alternatives (e.g., limestone)—are often necessary for long-term mitigation. Post-installation validation using RAD7 detectors confirms compliance with the 1 mSv/year threshold and accounts for real-world variables, like uneven airflow or material heterogeneity.

3.5. Effect of Mineralogy

While a detailed investigation into the effect of mineralogy on high radon or thoron exhalation rates is of interest, it falls outside the main scope of this paper. However, to illustrate how mineral composition can influence radon/thoron potential, a high-resolution mineral scan of a Galway granite sample (shown here to have high radon and thoron potential) was evaluated. Prior to analysis, minerals of interest were manually introduced to the microscope. Some minerals commonly associated with radon and thoron emission (Figure 7) include those containing uranium and thorium, such as the following:

• Uraninite (UO₂): A primary uranium ore with high radioactivity (gamma radiation source) and radon potential (alpha radiation source).
• Thorianite (ThO₂): A thorium-rich mineral with high radioactivity (gamma radiation source) that contributes to thoron emissions (alpha radiation source).
• Monazite (Ce,La,Nd,Th)PO₄: Contains significant thorium (gamma radiation source) and can be a source of thoron and, to a lesser extent, radon (alpha radiation sources).
• Zircon (ZrSiO₄): Often contains trace uranium and, to a lesser extent thorium (gamma radiation sources), contributing to radon and thoron (alpha radiation sources).

Figure 8 presents a composite of backscattered electron (BSE) images of the Galway granite sample and the corresponding mineral maps. BSE imaging reveals the mineralogical matrix and texture, while the elemental maps highlight the spatial distribution of uranium- and thorium-bearing minerals within the sample. The granite sample scan revealed a considerable number of rutile, zircon, monazite, and hematite minerals. These kinds of detailed imaging and mapping are beneficial for future research aimed at identifying the specific minerals contributing to elevated radon and thoron levels and the degree of their contribution. By analyzing these mineralogical features and the chemical composition of the samples, it is anticipated that a better understanding of how the distribution and concentration of uranium and thorium-bearing minerals influence overall radon and thoron emissions can be achieved. This approach provides mineralogical insights, which enable a refined understanding of radiation risks associated with various building materials. The approach demonstrated here highlights a novel method for material testing and certification for radon and thoron potential studies, ultimately facilitating the development of more effective radiation protection resource management strategies for building materials.

4. Conclusions

This study presents a comprehensive assessment of radon and thoron exhalation rates, alongside associated gamma radiation exposure, from various building materials commonly utilized in Ireland, and widely available in countries around the world. Results indicate substantial variability in the radiological properties of these materials, with granite and quartzite exhibiting notably high thoron exhalation rates due to their mineralogical composition. These findings highlight the potential for certain natural rock materials to contribute significantly to indoor radiation exposure, particularly in poorly ventilated environments. Building on our measurements and modeling, the following comparative contextualization places our key radiological metrics—radon (E₍₂₂₂₎), thoron (E₍₂₂₀₎) exhalation rates and gamma dose contributions—alongside published values for the same natural building materials:

Granite: In this study, the mean thoron exhalation rate of tested granites was 6396 ± 3534 Bq m⁻² h⁻¹, considerably higher than the 3500 ± 800 Bq m⁻² h⁻¹ reported by Frutos-Puerto et al. (2020) [2]. Its radon exhalation (0.51 ± 0.46 Bq m⁻² h⁻¹) also exceeds the literature value of 0.20 ± 0.05 Bq m⁻² h⁻¹. The annual gamma dose from granite in our work (0.17–0.40 mSv y⁻¹) aligns with, though slightly under the upper bound of, the 0.20–0.50 mSv y⁻¹ range found by Sas et al. (2017) [31].

Quartzite: Our measured mean thoron flux for quartzite (1637 ± 277 Bq m⁻² h⁻¹) is modestly above the 1200 ± 500 Bq m⁻² h⁻¹ documented by Lucchetti et al. (2019) [32], while its radon exhalation (0.75 ± 0.36 Bq m⁻² h⁻¹) similarly surpasses their 0.50 ± 0.20 Bq m⁻² h⁻¹ value. The gamma dose (0.18–0.20 mSv y⁻¹) matches closely with the 0.19 mSv y⁻¹ they reported.

Limestone: We observed thoron exhalation in tested limestones at 1352 ± 1213 Bq m⁻² h⁻¹, relative to the 1000 ± 400 Bq m⁻² h⁻¹ from Nunes et al. (2023) [33]. Its radon flux (0.35 ± 0.15 Bq m⁻² h⁻¹) is likewise above the 0.25 ± 0.10 Bq m⁻² h⁻¹ figure in that study. The resulting gamma dose (0.16–0.19 mSv y⁻¹) is in good agreement with the 0.15 mSv y⁻¹ noted by Lee et al. (2004) [34].

Correlation analyses suggest a potential link between the factors controlling uranium and thorium distribution. Some accessory minerals (e.g., zircon) contain both uranium and thorium, potentially influencing this correlation. Conversely, other accessory minerals (e.g., monazite) contain thorium but not uranium, leading to variability in the radon–thoron correlation across samples with contrasting accessory mineralogy. This variability is further influenced by the specific chemistry of accessory minerals, such as the range of thorium concentrations observed in zircon (from negligible to relatively high). These observations indicate a degree of interaction between these factors, emphasizing the need for advanced models that consider both uranium and thorium concentrations, as well as mineralogical composition when evaluating radiological risks associated with building materials. Furthermore, the gamma dose index (I-index) suggests that materials deemed safe based solely on gamma radiation may still pose health risks due to the combined effects of radon, thoron, and gamma radiation if not adequately managed.

The ventilation models proposed in this study offer insights to develop practical strategies for mitigating radon and thoron exposure in indoor settings. Adjusting ventilation rates or limiting the use of materials with high emission rates can help to maintain radiation doses below recommended safety thresholds. However, further research is needed to refine these models, particularly by incorporating additional sources of radon and thoron, such as soil and water, for a more comprehensive assessment of indoor radiation exposure. In conclusion, this research provides valuable data and methodological advancements that can inform future guidelines and regulations concerning building materials. Given the importance of the construction sector in Ireland, and across the EU, particularly with an emphasis on the extensive use of locally sourced building materials, it is important to consider the radiological implications of these materials to ensure the safety and well-being of building occupants. From a policy perspective, these results argue strongly for revising European construction standards. We recommend that the EU Construction Products Regulation be updated to require manufacturers to report radon and thoron exhalation rates alongside radionuclide concentrations, and to adopt a combined radon + thoron + gamma dose index capped at 1 mSv y⁻¹ under typical occupancy and ventilation. Building codes should also mandate minimum ventilation rates—such as a 0.5 h⁻¹ air-change rate in rooms built with high-emission materials—and encourage the use of lower-emission alternatives wherever possible.

Future research should expand upon these findings by investigating the long-term health effects of low-level radiation exposure and developing more robust models to predict indoor radon and thoron concentrations under diverse environmental conditions. A particular focus should be placed on elucidating the mineralogical factors influencing exhalation rates.