RADON EXHALATION FROM BUILDING MATERIALS USED IN YEMEN

Abstract

The present article seeks to determine the annual effective doses of ²²²Rn exposure, effective radium content and radon exhalation rates in some building materials from the local market of Ibb province, Yemen. A total of 33 samples of building materials were collected from the target area. The radon exhalation rate and effective radium content in these samples were measured using solid-state nuclear track detector, which has become an important tool in every investigation of the radon levels in the surrounding environment. Surface exhalation rate has been found to vary from 178.90 to 1267.6 mBq m⁻² h⁻¹, whereas mass exhalation rate has been found to vary from 5.51 to 33.25 mBq kg⁻¹ h⁻¹. All the values of effective radium content in all samples under test were found to be quite lower than the permissible value of 370 Bq kg⁻¹ recommended by Organization for Economic Cooperation and Development. Annual effective doses have also been estimated.

INTRODUCTION

Radioactivity is widely spread in the earth’s environment and it is found in various geological formation, e.g. soils, rocks, plants, water and air^{(1, 2)}. Most building materials derived from soil and rocks contain the natural radionuclides such as uranium (²³⁸U) and thorium (²³²Th) and their daughter products and singly occurring potassium (⁴⁰K)⁽³⁾. Concentrations of radionuclides present in building materials vary depending upon the local geology of each region in the world^{(4, 5)}. The building industry also uses large amounts of waste from other industries⁽⁶⁾. These radioactive elements are sources of three radioactive decay series. Radon gas is one of the decay products of these series. It consists of three isotopes, namely: (1) ²²²Rn (called radon, belongs to ²³⁸U decay series); (2) ²²⁰Rn (called thoron, belongs to ²³²Th decay series); and (3) ²¹⁹Rn (called action, belongs to ²³⁵U decay series). ²²²Rn has 3.82 days half-life, whereas ²²⁰Rn (55.6 s) and ²¹⁹Rn (3.96 s) have much shorter half-lives than ²²²Rn. That is why ²²⁰Rn and ²¹⁹Rn are given less importance in environmental studies^{(7, 8)}. Radon (²²²Rn), as an emitter of α-particles with energy 5.48 MeV, is the most crucial and dangerous radioactive gaseous element in the science of environmental radioactivity^{(9, 10)}.

Most individuals spend more than 80% of their time indoors and the rest is spent on outdoor activities. The internal and external radiation exposure from building materials creates prolonged exposure situations⁽¹¹⁾. The external radiation exposure is caused by the gamma emitting radionuclides, which in the uranium series mainly belongs to the decay chain segment starting with ²²⁶Ra. The internal (inhalation) radiation exposure is largely due to ²²²Rn, and marginally to ²²⁰Rn, and their short lived decay products⁽¹²⁾. The study of alpha activity in building materials is very important because alpha radiation is 1000 times more carcinogenic than gamma radiation⁽⁴⁾. The noble gas ²²²Rn, although not so noble in its health hazard effect, is chemically inert and can move through the Earth and structural materials^{(9, 10)}. Once the radon atoms are formed by the decay of the parent ²²⁶Ra, they move either by diffusion or by transport mechanisms or by both^{(13, 14)}. Radon exhalation is a complex phenomenon depending upon a number of parameters such as effective radium content in soil, soil morphology, soil moisture, vegetation, temperature, atmospheric pressure, rainfall and soil grain size⁽¹⁵⁾. The amount of the produced radon from the grains that finally enters by recoil effect and diffusion process in the porous system of the material is defined as ‘effective radium content’⁽¹⁶⁾.

Recently, several interesting studies of people exposed to radon have confirmed that radon in homes and workplaces represent a serious health hazard^{(3, 17, 18)}. The exposure of people to high concentrations of indoor radon for long periods causes pathological effects and functional respiratory changes, which consequently lead to an increased risk of developing lung cancer^(19–21).

The aim of this study is to determine the annual effective doses of ²²²Rn exposure, effective radium content and radon exhalation rates in some building materials from local market of Ibb province in Yemen by solid-state nuclear track detectors (CR-39). Such studies can be useful for keeping reference records and developing data panels to ascertain changes in the environmental radioactivity due to nuclear, industrial and other human activities over time⁽²²⁾. Ibb province is located between Thamar and Taiz provinces. The capital city of Ibb province is ~193 km to the south of Sana’a, the capital city of Yemen. Figure 1 shows Ibb’s location⁽²³⁾. The province has a total area of 5383 km² and it is located at latitude 13°58′48″ and longitude 44°10′48″⁽²⁴⁾.

Figure 1.

Map showing the study area in Ibb⁽²³⁾.

MATERIALS AND METHODS

For the purposes of this study, a total of 33 samples of building materials were collected from different companies and stores in Ibb, comprising 15 slab samples and 18 porous powdery building material samples. The concentration and exhalation rate of radon have been determined using CR-39 detectors because of their ability to register tracks at different levels of sensitivity^(25–27). The CR-39 polymer sheets of TASTRAK were produced and provided by Track Analysis Systems Ltd. (TASL), Bristol, UK. The polymeric detector samples, for the present study, were cut to a size of 1.5 × 1.5 cm² and adhered to a plastic can of known dimension (7 cm in diameter and 11 cm in height)^(28–31). To avoid the track contribution from thoron in the can, the CR-39 detectors were kept at distance ~7 cm or more from the sample in accordance with the protocol^(32–36). This is because the half-life time of ²²⁰Rn (55.6 s) is ~0.00017 that of radon ²²²Rn (3.82 d). This shows that most of the thoron will decay inside the building material and radon diffusion length, so that, thoron would be still in the order of 2 or 3 cm^(37–39).

Two types of experiments for measurement of radon exhalation rates of building materials were used:

• The container was placed inversely on the surface of the tile samples of building materials. These samples were washed, cleaned, dried and used without crushing as used naturally. The contact between the chamber and the building material parts was sealed with silicon⁽⁴⁰⁾.

• The powder samples were dried in an oven at a temperature of 80°C for 24 h to remove all the moisture content^{(19, 27)}, crushed to fine grain size (100 μm)⁽²⁰⁾. Then each sample of 125 g weight was placed at the bottom of a cylindrical sealed can. The mouth of the cylindrical can was sealed with a cover and fitted with CR-39 plastic track detectors at the top inner surface so that they were facing the specimen^{(14, 41, 42)}. The cover was sealed by silicon and duct tape from the outside in order to avoid radon leakage⁽⁴³⁾. The sample was left for 1 month to allow radioactive equilibrium to reach between ²²⁶Ra, ²²²Rn and their decay products. Furthermore, in order to ensure the radioactive equilibrium of ²²⁶Ra, ²²²Rn and their decay products, it is important to ensure that no ²²²Rn is lost from the sample container^{(20, 44)}. During the exposure period of 3 months^{(10, 15)}, the sensitive side of the detector is exposed freely to the emergent radon from the sample in the can so that it could record the tracks of alpha-particles resulting from the decay of radon. At the end of the expose time, the detectors were removed and chemically etched for 9 h in 6.25 N NaOH at 70 ± 1°C⁽²¹⁾. After this chemical treatment, these CR-39 solid-state nuclear track detectors were washed in distilled water in order to stop the chemical reaction, dried in air and the tracks were counted using an optical microscope at a magnification of ×400.

The mass exhalation and surface exhalation rate of radon were obtained from the following expressions^{(10, 31, 45)}:  

$$E_A=\frac{C{\mathrm{\lambda }}_{\mathrm{Rn}}V}{A\left[T+\left(1/{\mathrm{\lambda }}_{\mathrm{Rn}}\left(e^{-{\mathrm{\lambda }}_{\mathrm{Rn}}T}-1\right)\right)\right]}$$

(1)

 

$$E_M=\frac{C{\mathrm{\lambda }}_{\mathrm{Rn}}V}{M\left[T+\left(1/{\mathrm{\lambda }}_{\mathrm{Rn}}\left(e^{-{\mathrm{\lambda }}_{\mathrm{Rn}}T}-1\right)\right)\right]}$$

(2)

where, E_A is the surface exhalation rate (Bq m⁻² h⁻¹), E_M is the mass exhalation rate (Bq kg⁻¹ h⁻¹), C is the integrated radon exposure (Bq m⁻³ h), A is the total surface area of the building material sample from which radon is exhaled (3.85×10⁻³m²), V is the effective volume of the emanation container or can (m³), M is the mass of the powder sample (0.125 kg), λ_Rn is the decay constant of radon (h⁻¹) and T is the exposure time in hour.

The risk of lung cancer from domestic exposure due to radon and its daughter nuclides can therefore be computed directly from the effective dose equivalents. The radiation hazards due to radon and its daughters are calculated from the radon exhalation rates of building material samples. The contribution of indoor radon concentration from building materials can be calculated from the following relation⁽⁴⁶⁾:  

$${\overline{C}}_{\mathrm{Rn}}=\frac{E_A\times S}{({\mathrm{\lambda }}_{\mathrm{Rn}}+{\mathrm{\lambda }}_{\mathit{V}})\times V_r}$$

(3)

where, ${\overline{C}}_{\mathrm{Rn}}$ is radon concentration in construction materials contributing to indoor radon, V_r is the room volume (m³) and λ_V is air exchange rate (h⁻¹). In these calculations, the maximum radon concentration from building materials was assessed by assuming the room to be a cavity with the ratio S/V_r = 2.0 m⁻¹, where S and V_r are the internal surface area and volume of the room, respectively, and the air exchange rate λ_V was taken to be 0.5 h^{−1(47, 48)}.

The annual effective doses of ²²²Rn exposure were estimated using the following equation⁽⁴⁹⁾:  

$$E_{\mathrm{Rn}}={\overline{C}}_{\mathrm{Rn}}\times F_e\times T_{\mathit{a}}\times D_{\mathrm{Rn}}$$

(4)

where, E_Rn is the annual effective dose of ²²²Rn (Sv y⁻¹), T_a is the annual work time = 7000 hy⁻¹, D_Rn the dose conversion factor for ²²²Rn decay products [Sv/Bq h m⁻³ (EERC)] and F_e is the equilibrium factor. The value of F_e was assumed to be 0.4⁽⁴⁹⁾.

The effective radium content was measured for the same samples using CR-39 detectors because of its capability to register tracks at different levels of registration sensitivity^{(10, 31, 50)}. After the sealing of the can, the activity concentration of radon begins to increase with time T as follows^{(10, 51)}:  

$$C_{\mathrm{Rn}}=C_{\mathrm{Ra}}\left(1-e^{-{\mathrm{\lambda }}_{\mathrm{Rn}}T}\right)$$

(5)

where, C_Ra is the effective radium content^{(10, 31, 34, 52)} of the building material sample. Also C_Ra is called the effective radium concentration^{(53, 54)}, as this is the fraction of total radium which contributes to radon exhalation⁽⁵⁵⁾. The effective radium content was calculated by using the following relation^{(15, 56)}:  

$${\mathit{C}}_{\mathrm{Ra}}({\mathrm{Bq}\mathrm{kg}}^{-1})=\frac{\mathit{h}\mathit{\rho }\mathit{A}}{{\mathit{KT}}_{\mathit{e}}\mathit{M}}$$

(6)

where, h is the distance between the detector and the top of the solid sample, ρ is the radon track density (track cm⁻²), K is the calibration factor for radon (0.16 track cm⁻²d⁻¹/Bq m⁻³) which was calibrated in previous work⁽⁵⁷⁾ and T_e is the effective exposure time (in days), which is related to the actual exposure time T and decay constant for ²²²Rn according to the following equation^{(58, 59)}:  

$$T_e=\left[T-\frac{1}{{\mathrm{\lambda }}_{\mathrm{Rn}}}\left(1-e^{–{\mathrm{\lambda }}_{\mathrm{Rn}}T}\right)\right]$$

(7)

The exposure time, in these measurements, was 90 days.

Radon exhalation from the building materials contributes towards indoor radon. In this regard, previous studies have been carried out in different parts of the world and extensive data are available in the literature. In the current survey, most of the houses in the studied area are built of blocks (mixtures of cement sand and gravel aggregates in definite proportions). Some of the houses were made up of bricks and Sand and most of them also contain interior decoration made of ceramics, gypsum, marble and granite. These houses are in fact constructed on the Earth’s crust, which is essentially composed of rocks and soil.

RESULTS AND DISCUSSION

The surface radon exhalation rates from the solid slab samples ranged from 275.24 ± 5.43 to 1267.6 ± 11.84 mBq m⁻² h⁻¹, with a mean of 661.69 ± 8.56 mBq m⁻²h⁻¹, whereas the surface exhalation rates from the powdery samples ranged from 178.90 ± 6.76 to 1079.53 ± 9.70 mBq m⁻² h⁻¹, with a mean of 462.46 ± 10.02 mBq m⁻²h⁻¹. The mass radon exhalation rates from the powdery samples ranged from 5.51 ± 0.2 to 33.25 ± 0.30 mBq kg⁻¹h⁻¹ with a mean of 4.24 ± 0.31 mBq kg⁻¹ h⁻¹. The values of exhalation rates of building materials vary from one building material to another. This variation can be attributed to the difference in the nature of the samples, and effective radium content in the samples.

To estimate the contribution of building materials to indoor radon concentrations⁽¹⁰⁾, a room with an air exchange rate of 0.5 h⁻¹ was assumed as mentioned above. The radon concentration contribution to indoor radon in the room due to exhalation from the solid slab samples building materials was found to range from 1.10 ± 0.02 to 5.07 ± 0.05 Bq m⁻³, with a mean of 2.65 ± 0.03 Bqm⁻³ (Table 1). The smallest accumulated radon concentration per day in a room with its entire floor and walls decorated with M2, denoting Yemen marble, would be 26.42 Bq m⁻³. Thus, on average, the sample M2 would contribute only slightly to the radon concentration in Yemen homes. On the other hand, the largest accumulated radon concentration per day was 121.656 and 103.634 Bq m⁻³ for samples Gra1 and Gra2, denoting Yemen granite and china granite, respectively.

Table 3.

Radon contribution to indoor radon, surface and mass exhalation rates, annual effective doses and effective radium content, together with the statistical uncertainties (1σ) for the powdery different building material samples.

S. no.	Sample ID	Size (cm3)	Radon surface exhalation rate (mBq m−2 h−1)	Radon contribution (Bq m−3)	Radon mass exhalation rate (mBq kg−1 h−1)	Effective radium content (Bq kg−1)	Annual effective dose (μSv y−1)
1	C1	342.65	207.96 ± 7.88	0.83 ± 0.03	6.40 ± 0.24	0.69	20.96
2	C2	338.80	243.12 ± 6.07	0.97 ± 0.02	7.49 ± 0.19	0.79	24.51
3	C3	338.80	507.66 ± 10.73	2.03 ± 0.04	15.64 ± 0.33	1.66	51.17
4	WC1	338.80	276.76 ± 6.07	1.11 ± 0.02	8.52 ± 0.19	0.90	27.90
5	WC2	342.65	455.67 ± 10.85	1.82 ± 0.04	14.03 ± 0.33	1.50	45.93
6	WC3	342.65	553.53 ± 9.39	2.21 ± 0.04	17.05 ± 0.29	1.83	55.80
7	Sa1	346.50	178.90 ± 6.76	0.72 ± 0.03	5.51 ± 0.21	0.60	18.03
8	Sa2	346.50	428.14 ± 10.41	1.71 ± 0.04	13.19 ± 0.32	1.43	43.16
9	Sa3	346.50	339.46 ± 6.62	1.36 ± 0.03	10.45 ± 0.20	1.13	34.22
10	Sa4	346.50	633.04 ± 35.51	2.53 ± 0.14	19.50 ± 1.09	2.11	63.81
11	Sa5	346.50	463.31 ± 6.76	1.85 ± 0.03	14.27 ± 0.21	1.55	46.70
12	Gy1	331.10	325.70 ± 6.76	1.30 ± 0.03	10.03 ± 0.21	1.04	32.83
13	Gy2	327.25	441.91 ± 10.98	1.77 ± 0.04	13.61 ± 0.34	1.39	44.54
14	Gr1	327.25	526.01 ± 7.22	2.10 ± 0.03	16.20 ± 0.22	1.66	53.02
15	Gr2	346.50	449.55 ± 8.62	1.80 ± 0.03	13.85 ± 0.27	1.50	45.31
16	Gr3	342.65	474.02 ± 10.52	1.90 ± 0.04	14.60 ± 0.32	1.57	47.78
17	So1	342.65	740.08 ± 9.57	2.96 ± 0.04	22.79 ± 0.29	2.44	74.60
18	So2	342.65	1079.53 ± 9.70	4.32 ± 0.04	33.25 ± 0.30	3.56	108.82

C, cement; WC, white cement; Sa, sand; Gy, gypsum; Gr, gravel; So, soil.

For the powdery materials, they ranged from 0.72 ± 0.03 to 4.32 ± 0.04 Bq m⁻³. The sand sample (Sa1) showed the lowest value, and the daily accumulated radon concentration in a room was estimated at 17.16 Bq m⁻³. On the other hand, the soil sample (So2) gave the highest value observed in this type of study. Where the daily accumulated radon concentration in a room was estimated to be 103.63 Bq m⁻³.

To study the contribution of building materials to indoor radon concentrations with air changes per hour (ACH), two scenarios were considered:

Case I: radon exhalation from a granite countertop

Consider a granite countertop (0.67 m × 2.50 m × 0.025 m) installed in a kitchen with an area of 20 m² and a height of 2.5 m⁽⁶⁰⁾, with the assumption that 25% of the kitchen volume is occupied by kitchen wares and furniture, and assuming as well that the kitchen is ventilated with the minimum required ACH of 0.3 h⁻¹, for granite countertops having the highest radon exhalation rate (30.42 Bq m⁻² d⁻¹) observed in this study, as shown in Table 2, the radon concentration in the kitchen will be 0.37 Bq m⁻³. It can be concluded that kitchen granite countertops, under normal ventilation, contribute very little to the radon concentration for the range of radon exhalation rates reported here. Similar assessments were reported in the literature^{(19, 60)}. Based on more data available recently, it was assessed that the average radon concentration in Yemen homes is 4 Bq m⁻³⁽⁶¹⁾. It indicates that on average, granite countertops could contribute to <1% of radon concentration in Yemen homes.

Table 1.

Radon contribution to indoor radon, surface exhalation rate and annual effective doses, together with the statistical uncertainties (1σ) for the solid slab different building material samples.

S. no.	Sample ID	Radon surface exhalation rate (m Bq m−2 h−1)	Radon contribution (Bq m−3)	Annual effective dose (μSv y−1)
1	M1	527.53 ± 10.84	2.11 ± 0.04	53.18
2	M2	275.24 ± 5.43	1.10 ± 0.02	27.74
3	M3	513.77 ± 6.87	2.06 ± 0.03	51.79
4	M4	336.40 ± 6.28	1.35 ± 0.03	33.91
5	Gra1	1267.61 ± 11.84	5.07 ± 0.05	127.78
6	Gra2	1079.53 ± 13.51	4.32 ± 0.05	108.82
7	Ce1	827.23 ± 11.63	3.31 ± 0.05	83.39
8	Ce2	564.23 ± 6.46	2.26 ± 0.03	56.87
9	Ce3	382.27 ± 8.15	1.53 ± 0.03	38.53
10	Ce4	342.52 ± 5.70	1.37 ± 0.02	34.53
11	RB1	519.89 ± 8.00	2.08 ± 0.03	52.40
12	RB2	593.28 ± 6.04	2.37 ± 0.02	59.80
13	T	758.43 ± 6.87	3.03 ± 0.03	76.45
14	CB1	889.93 ± 8.27	3.56 ± 0.03	89.70
15	CB2	1047.42 ± 12.53	4.19 ± 0.05	105.58

M, marble; RB, red brick; CB, concrete block; Gra, granite; Ce, ceramic; T, tiles.

Case II: radon exhalation from a floor area

If a room has a floor area of 20 m² and a height of 2.5 m as assumed in scenario 1 and its entire floor is decorated with ceramic tiles, slate or granite, and 10% of the room volume is occupied by furniture, the radon concentration, Bq m⁻³, due to radon exhalation from the floor can be determined by Eq. (4). The resulting radon concentrations are calculated for materials of various radon exhalation rates and for various air change rates, as shown in Table 2. One can infer from Table 2 that even if the entire floor was covered with a material of a relatively high radon exhalation rate, as granites studied here 30.42 Bq m⁻² d⁻¹, it would contribute only 1.83 Bq m⁻³ to a tightly sealed house (minimum required ACH of 0.3 per hour). This is only 1% of the action level for radon in dwellings⁽⁶²⁾. Higher ventilation rates will reduce the contribution proportionally. In the worst case scenario and when the mechanical ventilation system failed completely and no natural ventilation alternative was provided (ACH = 0), the indoor radon concentration would rise but not above an action level of 200 Bq m⁻³ for radon in dwellings⁽⁶²⁾.

The effective radium content in powdery samples was calculated using Eq. (6). And the results are provided in Table 3. RECs range from 0.6 to 3.56 Bq kg⁻¹ with a mean value of 1.52 Bq kg⁻¹, whereas in the solid slabs samples this parameter could not be determined. All the values of effective radium content in all samples under test were found to be quite lower than the permissible value recommended by Organization for Economic Cooperation and Development⁽⁶³⁾.

Table 2.

Estimate steady-state radon concentration (Bq m⁻³) due to radon exhalation from floor material with various air change rates.

Sample	EA (Bqm−2d−1)	ACH = 3	ACH = 1	ACH = 0.3	ACH = 0.15	ACH = 0
Marble	13.02	0.08	0.24	0.78	1.53	31.97
Ceramic	19.85	0.12	0.36	1.20	2.33	48.75
Tiles	21.92	0.13	0.40	1.32	2.58	53.81
Granite	30.42	0.19	0.56	1.83	3.58	74.69

Finally, it is necessary to estimate the radon emanation potential from building materials to evaluate the radiation risk to the inhabitants. Thus, the assessment of the annual effective dose expected to be received by populations due to radon and its progenies were based on the calculations of radon exhalation rates. The minimum, maximum and mean annual effective doses were found to be 27.74, 127.78 and 66.70 μSv y⁻¹ for the slabs samples, and 18.03, 108.82 and 46.62 μSv y⁻¹ for the powdery samples. The slabs samples giving the lowest values of the annual effective dose were marble (M2) from Yemen (Taiz); the lowest dose value from the powdery samples was attributed to the Sa1, denoting Yemen sand. The highest dose from the slabs samples came from granite in Yemen and the highest dose in the powdery samples came from So2, denoting Yemen. The results obtained are shown in Tables 3 and 1. The results of the current study have been compared with already published data (Table 4).

Table 4.

Comparison between surface exhalation rates (E_A) and mass exhalation rates (E_M) of some building materials in different countries.

Material	Country	EA (mBq m−2 h−1)	EM (m Bq kg−1 h−1)	Reference
Cement	Sudan	379	4.5	(65)
	Indian	263.3–401.6	12.1–18.4	(66)
	Libya	523.2	20.4	(10)
	Yemen	243.12–492.37	6.405–15.164	Present work
White cement	Libya	550.9	20.8	(10)
	Indian	417–437	13.8–20.1	(66)
	Yemen	276.76–553.53	8.524–17.048	Present work
Sand	Pakistan	205–291		(67)
	Pakistan	366–649		(7)
	Libya	722.2	27.4	(10)
	Yemen	178.90–633.04	5.510–19.497	Present work
Ceramic	Sudan	240		(65)
	Canada	8.33–125		(60)
	Libya	69.5–73.7		(10)
	Yemen	382.27–827.23		Present work
Granite	Libya	733.2–17 221.2		(10)
	Greece	1240–3540		(68)
	KSA	333–1250		(69)
	Yemen	1079.53–1267.61		Present work
Gypsum	Egypt	26.28		(30)
	Libya	147.5–365.5	17.4–43.1	(10)
	Yemen	325.70–441.91	10.031–13.610	Present work
Brick	Pakistan	184–231		(7)
	Pakistan	245–365		(67)
	Libya	123.7–1625.1		(10)
	Yemen	519.89–593.28		Present work
Gravel	Canada	45.83–4245.83		(19)
	Pakistan	168–322		(7)
	Yemen	449.55–526.01	13.85–16.20	Present work
Marble	Libya	142.3–970.6		(10)
	Italy	65.4		(43)
	Canada	4.17–25		(60)
	Algeria	35–66		(70)
	Yemen	275.24–527.53		Present work
Soil	Libyan	58.3–506.9	1.2–17.4	(14)
	KSA	45–8400	135–251	(59)
	India		6–56	(36)
	Pakistan	114–416		(67)
	Egypt	480–15 370	8.31–233.70	(31)
	Yemen	740.08–1079.53	22.794–33.248	Present work

CONCLUSION

• The two highest values of radon exhalation observed in the studied solid slabs samples came from the Yemeni granite and the Chinese granite. For this reason, these two materials are not favorable for decorative use and should be replaced by other alternatives.

• For the powdery sample, the soil was found to have a high level of radon exhalation. However, there exists only a small number of houses which are made of mud alone. Without ventilation, elevated radon levels could arise. In general, the current results are within the world-wide range of values found in soil, and this range is within the safe limits recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation⁽⁶⁴⁾.

• The effective radium content and annual effective dose equivalents are within the safe limits recommended by Organization for Economic Cooperation and Development (OECD)⁽⁶³⁾, European Commission (EC)⁽³⁾ and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)⁽⁶⁴⁾.

Generally speaking, building materials used in home construction and decoration make no significant contribution to indoor radon for a house with adequate air exchange. It is then possible to establish the data base for all building materials sold in the market by using this technique instead of spending much money and resources to perform a large-scale nationwide indoor radon screening measurement.