The study area in this study is Jakarta—a megacity of Indonesia and its surroundings located on Java Island (Figure 1). Jakarta city and the surrounding areas were chosen as the research locus because it has a reasonably high population with high levels of human activity indoors, both in the office and at home. Based on the Office of Statistical Agency (BPS) data for 2022, the population of Jakarta, West Java, and Banten provinces are 10,680,000, 49,405,000, and 12,252,000 people, respectively (18). Java Island is home to 154,000,000 people, 56% of the country’s total population of 275,000,000 people, making Java the most populated island in the world. Jakarta and its suburbs are home to almost a quarter of Java’s population (18). Jakarta is a tropical, humid city with annual temperatures between 24 and 34°C and a relative humidity of 75–85%. The average mean temperatures are 26°C in January and 28°C in October. The annual rainfall is more than 1,700 mm. Sea winds often modify temperatures. Jakarta, like any other large city, also has its share of air and noise pollution (29).

A total of 81 samples of building materials are used in the study area, among the types and brands, with details of 19 sand samples, 16 cement samples, 13 brick samples, 17 AAC samples of 10 cm thick, and 16 AAC samples of 7 cm thick. Sampling was carried out randomly at 17 regional areas, namely, Pondok Cabe (A), Pisangan (B), Cirendeu (C), Soekarno Hatta Airport (D), Serpong (E), Balaraja (F), Cikupa (G), Lebak Bulus (H), Cimanggu (I), Parung (J), Sawangan (K), Jatiasih (L), Jatiwarna (M), Bambu Apus (N), Kebayoran Lama (O), Salemba (P), and Kemayoran (Q). Details of building material samples are shown in Table 1. Samples were prepared in the laboratory using a similar protocol to Ndjana Nkoulou et al. study (16). First, they were dried using an oven set at 105°C for 24 h. After that, the samples were weighed, and their volumes were determined. Volume measurements were carried out directly on the material for brick and AAC samples. In contrast, volume measurements were carried out for sand and cement samples by measuring the volume of the sample holder.

²²²Rn surface exhalation rate is the flux of ²²²Rn per square meter per second or per hour from the source, such as rocks, soil, or building materials (30, 31). There are several methods for measuring ²²²Rn surface exhalation rate, namely the closed-chamber method (CCM) and the open-chamber method (OCM). The closed-chamber method (CCM) is commonly used to measure the exhalation rate in building materials by putting the sample inside an airtight container and measuring the ²²²Rn activity concentration in the air in the container (32, 33).

²²²Rn activity concentration was measured using a RAD7 ²²²Rn monitor (DURRIDGE Company, Inc., USA) with an α solid-state detector. The diagram illustrating the measurement of the ²²²Rn exhalation rate by RAD7 is presented in Figure 2. When the container containing the sample is connected to the detector, ²²²Rn gas accumulated due to the exhalation of ²²²Rn in the sample will be pumped to flow into the drying column (Drierite, Drierite Company, Inc., USA) and then into the inlet filter of the RAD7 tool. In RAD7, the sample air will decay so that α particles emitted from the polonium isotope will be detected. RAD7 uses α spectrometry techniques to convert α particles into electrical signals. This detector can also separate electrical pulses produced from ²¹⁸Po and ²¹⁴Po with energies of 6 and 7.69 MeV, respectively (34, 35). The sampling cycle was set for 1 h with 6 cycles. The spectrum obtained was then analyzed using Capture software provided by DURRIDGE. ²²²Rn activity concentration measurements were conducted in dry conditions (relative humidity less than 10%). After each measurement, the ²²²Rn inside the RAD7 is purged by pumping clean air. The RAD7 was calibrated for ²²²Rn measurement in the ²²²Rn chamber of IREM (Institute of Radiation Emergency Medicine), Hirosaki University, Japan, which annually conducts intercomparison with the Federal Office for Radiation Protection (BfS), Germany. The ²²²Rn surface exhalation rate is determined using Equation 1 (35).

where E_Rn is ²²²Rn surface exhalation rate in Bq m⁻² s⁻¹, C_Rn is the average of the stabilized ²²²Rn activity concentration in Bq m⁻³ obtained by direct measurements using RAD7 device, V is the effective volume of the closed chamber as in the chamber volume of 1.42 × 10⁻² m³ decreased by sample volume, λ_Rn is ²²²Rn decay constant in 2.1 × 10⁻⁶ s⁻¹, and A_s is the surface area of the sample in m².

After the measurements of ²²²Rn surface exhalation rates, the samples were crushed, sieved, and dried in an oven for the second time to remove moisture contained in the materials. After that, the samples were weighed and put into a U8-type vial container, a cylindrical polypropylene container sized 48 mm × 55 mm. The samples in the vials were then tightly sealed to prevent ²²²Rn gas from escaping from inside to allow secular equilibrium to be achieved after 30 days. The measurements are conducted using a calibrated gamma spectrometry system with a p-type High Purity Germanium detector (HPGe), GEM60-83-XLB-C-SMP (ORTEC, USA), with a relative efficiency of 60% and a resolution of 1.86 at 1.33 MeV of ⁶⁰Co. The detector is mounted inside a 10-cm thick cylindrical lead shielding, lined with a layer of tin and copper to prevent ambient gamma interference on the measurements, and the X-rays of Compton scattering in the lead reach the detector. The pulse from the detector is analyzed by a multichannel analyzer that is directly connected to a PC with the associated reading software Gamma Vision (ORTEC, USA).

The calibration of the gamma spectroscopy system utilizes a soil matrix reference material from the intercomparison network of Analytical Laboratories for the Measurement of Environmental Radioactivity (ALMERA-IAEA), CRM IAEA soil 448, and CRM IAEA Soil 444. To maintain the reliability and validity of the spectrometry system, the laboratory keeps a routine internal calibration using a mixed radionuclide source. The laboratory also participates annually in the intercomparison network of ALMERA and the Indonesian metrology network for radionuclide measurement.

The ²²⁶Ra activity concentration is obtained by analyzing the photoelectric peaks of 295 keV; 351 keV of ²¹⁴Pb; and the photoelectric peak of 609 keV of ²¹⁴Bi. ²³²Th activity concentration is obtained by analyzing the photoelectric peaks of 238 keV of ²¹²Pb and 911 keV of ²²⁸Ac. ⁴⁰K activity concentration is determined by examining the single photoelectric peak of 1,460 keV (36). The measurement duration for each sample was 80,000 s. Equation 2 was used to determine the activity concentration A (Bq kg⁻¹) of each radionuclide (15, 37, 38).

where C_s is the count rate of the sample (count s⁻¹); C_bg is the count rate of the background (count s⁻¹); 𝜀 is the detector efficiency; 𝛾 is the gamma emission probability; W is the sample weight (kg); f_c is the correction factor; k is the coverage factor in k = 1.96 for a confidence interval of 95%; and U is the combined uncertainties of the measurements calculated obtained using Equation 3 (39).

Where u is the relative uncertainty from each sample count standard deviation, detector efficiency, measured sample weight, gamma emission probability, growth factor, attenuation factor, and summing factor. We also analyze the background radiation for each radionuclide to determine the detection limit (LD) using Equation 4 (40).

Where DT is the decision threshold based on the measured background, k is the coverage factor, and U is the uncertainty in measurement. The ⁴⁰K, ²²⁶Ra, and ²³²Th detection limits were 0.21, 0.08, and 0.08 Bq kg⁻¹, respectively.

Indoor ²²²Rn concentration originates from sources such as inside/outside air exchange, water supply, cracks in the house’s basements, and building materials used, especially when the house is built from bricks, sand, and rocks. The contribution of building materials to the indoor ²²²Rn concentration is often significant. It is possible to assess that contribution by knowing that ²²⁶Ra contains the building materials since ²²²Rn is a ²²⁶Ra decay product. Equation 5 expresses the indoor ²²²Rn from the ²²⁶Ra in building materials in a steady state condition (16, 41, 42).

where C 226 R a is the ²²⁶Ra concentration in the building materials (Bq kg⁻¹); ε the emanation fraction (0.2); λ the ²²²Rn decay constant (0.00756 h⁻¹); τ the air exchange rate (0.5 h⁻¹), ρ and r is the density (100 kg m⁻³) and half-thickness layer (7 cm) of the structural elements (wall and floor) of the room, respectively; and A and V are the room surface area (m²) and inner volume (m⁻³), respectively. Activity index I was defined to estimate the radiological risk presented by building materials.

The activity index I is calculated using ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations in the building materials. The Equation 6 expresses the different coefficients of calculation (43).

The assessment of the annual effective doses from external, inhalation, and ingestion received by an individual living in a typical house in the study area built using the studied building materials can be obtained using RESRAD-BUILD presented in Figure 3. This computer code is an open-access software developed by the Argonne National Laboratory (Argonne, USA) (41, 44). In detail, the dose resulting from exposure to external radiation emitted by walls and floor, dose received from inhalation of ²²²Rn decay products, and dose from ingestion of radionuclides deposited and resuspended in the house are obtained from RESRAD-BUILD computer software. The exposure scenario is for an average adult who is 1.60 m in height and 80 kg of weight. The flowing parameters were used along with radionuclide concentrations of building materials. The indoor occupancy factor is 0.8 (16, 44). A room model with the dimensions of 4 m × 3 m × 3 m is considered, the half layer walls’ thickness of 7 cm, 1 number of rooms per occupant, radionuclides deposition velocity of 0.01 m s⁻¹, radionuclides resuspension rate of 5 × 10⁻⁷ s⁻¹, adult individual breathing rate of 20 m³ day⁻¹ were used (16, 45, 46).

The ²²²Rn activity concentration values measured directly for the entire samples range from 13 ± 1 to 895 ± 9 Bq m⁻³ with an average value of 116 ± 7 Bq m⁻³. The lowest ²²²Rn activity concentration came from the brick sample (J3) from the Parung area. In contrast, the highest value came from the 10 cm thick AAC sample (I4) obtained from the Cimanggu area. The ²²²Rn activity concentration values obtained from the direct measurements were used to calculate the ²²²Rn surface exhalation rates by applying Equation 1. The distribution of the ²²²Rn surface exhalation rate for all the building materials studied is presented in Figure 4. The principal data obtained for each building material type, sand, cement, brick, 10 cm AAC, and 7 cm AAC was presented in Table 2. The ²²²Rn surface exhalation rate ranges from 4 × 10⁻² to 1.6 × 10⁻⁰ mBq m⁻² s⁻¹ with an average of 4 × 10⁻¹ mBq m⁻² s⁻¹.

The results obtained from gamma spectrometry analysis of the building materials to determine ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations were presented in Table 3. The ²²⁶Ra activity concentration values ranged from 4 ± 0.2 to 107 ± 4 Bq kg⁻¹; ²³²Th activity concentration ranged from 1 ± 0.1 to 85 ± 3 Bq kg⁻¹; and ⁴⁰K activity concentration ranged from 20 ± 1 to 940 ± 26 Bq kg⁻¹. The lowest ²²⁶Ra activity concentration came from the 10-cm thick AAC sample (A4) obtained from the Pondok Cabe area. In comparison, the highest value came from the 7-cm thick AAC sample (I5) obtained from the Cimanggu area, and the lowest ²³²Th activity concentration came from the 7-cm thick AAC sample (A5) obtained from the Pondok Cabe area. In comparison, the highest ²³²Th activity concentration came from a sand sample (N1) obtained from the Bambu Apus area, and the lowest ⁴⁰K activity concentration came from a 7-cm thick AAC sample (A5) obtained from the Pondok Cabe area. In contrast, the highest ⁴⁰K activity concentration came from a sand sample (H1) obtained from the Lebak Bulus area.

Using the typical room size in the study area and the average ²²⁶Ra content of the building materials, the contribution of the building materials to the indoor ²²²Rn activity concentration was calculated to be 28 Bq m⁻³. The activity index was calculated and ranged from 0.2 to 0.5, with an average of 0.3. The highest value of the activity index was obtained from 7-cm AAC, while the lowest value came from cement. Furthermore, the annual effective doses from different exposure pathways for various building materials calculated using the RESRAD-BUILD computer code are presented in Table 4. In a realistic situation, a typical house constructed with a mixture of building materials generated annual effective doses of 0.01 mSv by external directly from the source, 1.6 × 10⁻⁶ mSv by ingestion from deposition, and 0.1 mSv from ²²²Rn. The total annual effective derived from these exposures in the typical house is 0.11 mSv.

The average ²²²Rn surface exhalation rate values of the building materials studied were lower than the global average value estimated at 1.6 × 10¹ mBq m⁻² s⁻¹ (45). Surface exhalation rate measurements are essential to determine potential high-risk areas due to ²²²Rn inhalation (35). Among the all-studied building materials, the lowest ²²²Rn surface exhalation rate value comes from a brick sample (E3) from Serpong. In contrast, the highest value comes from a 10-cm thick AAC sample (I4) from Cimanggu. The ²²²Rn surface exhalation rate value varies from one sample to another. Factors that may influence the ²²²Rn exhalation rate are the level of ²²⁶Ra content in the material for ²²²Rn surface exhalation rate, grain size, porosity, humidity, ²²²Rn diffusion in the material’s pores, and changes in pressure and material texture (47). The ²²²Rn surface exhalation rate of building material with other countries was presented in Table 5. For comparison purposes, the ²²²Rn exhalation rate of bricks for selected countries was expressed in Bq m-2 h-1. It is noted that the results obtained in this remain lower than those of Turkey, Saudi Arabia, and India (48–50), but in the range of the results found in Ecuador (30). The ²²²Rn surface exhalation rate level does not pose a significant risk to the public regarding the ²²²Rn exhalation rate. Furthermore, the frequency distribution of ²²²Rn activity concentration was shown in Figure 5, and was tested for its normality by using the Kolmogorov–Smirnov test. The Kolmogorov–Smirnov test can be used for more than 50 samples. The conditions that must be met when carrying out the Kolmogorov–Smirnov test are if the significance value is more than 0.05, then the data used in the research has a normal distribution. However, on the contrary, if the significance value is less than 0.05, then the data used does not have a normal distribution (51).

The average activity concentration values of ²³²Th (21 Bq kg⁻¹) and ⁴⁰K (217 Bq kg⁻¹) in all building materials studied did not exceed the global average values of 45 Bq kg⁻¹ and 412 Bq kg⁻¹. In comparison, the average values of the activity concentration of ²²⁶Ra (34 Bq kg⁻¹) are close to the global average value of 32 Bq kg⁻¹ based on UNSCEAR reports (4). A comparison between the results of ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations in this study with the results of similar studies reported in various countries was shown in Table 6. In this study, the highest ²²⁶Ra activity concentration value was obtained at 42 Bq kg⁻¹, lower than Italy and Malaysia; the highest ²³²Th activity was 42 Bq kg⁻¹, lower than India, China, and Malaysia, while the highest ⁴⁰K activity was 390 Bq kg⁻¹ lower than Malaysia, China, Iran, Italy. The concentration values of natural radionuclides in building materials vary from one country to another. Variations in the activity concentration values of natural radionuclide activity may be caused by differences in mineral content in the soil and the geographical origin of the raw materials (17). The high activity concentration of ²²⁶Ra indicates the high uranium activity concentration in the soil and rocks in the raw material source area. The high activity concentration of ²²⁶Ra indicates the high uranium concentration in the soil and rocks in the raw material source area. A high activity concentration of ²²⁶Ra will have the potential for high ²²²Rn gas released by the material. High ²³²Th activity concentration activity will have the potential for high levels of thoron gas released by the material (24). On the results of research that has been carried out, the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in sand samples (D1, G1, H1, and L1) and 10 cm thick AAC (N3) have values higher than the global average according to UNSCEAR (4). Sand samples with natural radionuclide concentrations exceeding the global average were obtained from different regions but came from the same mining area, namely Rangkasbitung. According to Lebak Regency Regional Regulation Number 2 of 2014, there are 25 sand mineral mining areas in Lebak Regency, one of which is the Rangkasbitung area (51). AAC samples with natural radionuclide concentrations higher than the global average come from the Tangerang, Gunung Sindur, and Serang areas. The high activity concentration values of ²²⁶Ra, ²³²Th, and ⁴⁰K in these materials do not come from contamination but from natural radionuclides in the source materials.

This study used numerical and graphical methods to investigate the normality distribution of ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations in building material samples. All statistical analyses were performed using ORIGIN PRO 2023 software (student version). The frequency distribution of radionuclides in building materials was assessed using the Kolmogorov–Smirnov normality test with significance values of 0.46, 0.67, and 0.06 for ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively. As all the significance values were more than 0.05, all the radionuclide distributions had a normal distribution. The graphical representation of the data distributions of ²²⁶Ra, ²³²Th, and ⁴⁰K were presented in the form of histograms shown in Figures 6–8, respectively. It can be seen that ²²⁶Ra, ²³²Th, and ⁴⁰K are distributed normally or lognormally.

The indoor ²²²Rn concentration derived from building materials calculated in this study is lower than the global average value of 40 Bq m⁻³, considerably less than the WHO reference value of 100 Bq m⁻³, and the Indonesian national reference level of 300 Bq m⁻³ (52). However, it exceeds the values obtained by Ndjana et al., where the authors reported values of 10 and 7 Bq m⁻³ from building materials (16). Therefore, a large indoor ²²²Rn survey should be undertaken in future studies. As presented in the results section, the effective dose from ²²²Rn represents 92% of the total effective inhalation dose. The activity index introduced by the IAEA to assess the radiological risk of natural radionuclides contained in building materials, as calculated in this study, was low. The activity index average value of 0.3 was below the reference value of 1 and does not require any restriction from their use (43). Also, the total annual effective doses from different exposure pathways calculated presented values ranging from 6.4 × 10⁻² to 1.1 × 10⁻¹ mSv depending on the building materials type. For the typical house built using a mixture of building materials, the total annual effective dose of 0.11 mSv was less than the global average value of 2.4 mSv, which resulted from all natural sources combined. This total annual effective dose is in the range of the results found in Ecuador by Tene et al. while studying the radiological exposure from building materials, and the range was from 0.019 up to 0.112 mSv (30).