Introduction

Natural radiation can be found in the human environment in a variety of locations. Radon-222 (222Rn) is a radioactive gas produced by the decay of uranium-238 (238U) series and daughter of radium-226 (226Ra) with a short half-life (3.8 days) which is a radioactive colourless inert gas [1]. It has a high solubility in organic compounds, with a solubility value of 12.7 in toluene scintillation at 20 °C [2]. Natural gas is now a common energy source because of its low carbon dioxide and nitrogen oxide emissions [3].

On the other hand, liquid petroleum gas (LPG) and natural gas are significant radon sources [4]. Natural gas is typically supplied from various wells and fields, and as a result, the proportions provided from multiple sources will vary over time [5, 6].

Table 1 shows the radiation dose sources contribution from natural radioactive substances. As seen in Table 1, the main dose contribution is due to Rn gas. Radon and its decay components are the most significant contributors to radiation dose. Radon is a significant contributor to the general population’s ionising radiation dosage and lung cancer [7]. Recent research in Europe, North America, and Asia on indoor radon and lung cancer shows that radon can cause many lung cancer cases in the general population [8]. Many variables affect indoor radon concentrations, including construction materials, indoor-outdoor temperatures, relative humidity, air turbulence, air flow and ventilation volume, and geological formations. The other pathway to 222Rn entrance in residential houses is natural gas using for cooking and heating.

Table 1 The radiation dose sources contribution [3]
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International and national organisations such as the USEPA, WHO, ICRP, and HPA set indoor radon concentration limits worldwide. The USEPA suggests radon levels of less than 148 Bq m−3 in the home and 400 Bq m−3 in the workplace [9].

To limit the health risks associated with indoor radon radiation, the WHO suggests a reference standard of 100 Bq m−3. However, the preferred reference standard does not exceed 300 Bq m−3 if this level cannot be met under the current country-specific conditions. The European Union (EU) has agreed to the ICRP-65’s proposed action rate of 500–1500 Bq m−3 [10]. The Action Level should be 200 Bq m−3, and the Target Level should be 100 Bq m−3, according to the HPA, expressed as the annual average radon concentration in the household. The Health and Safety Executive (HSE) in the United Kingdom has set a radon action standard for workplaces of 400 Bq m−3.

Some of the studies have also presented the 222Rn concentration and radiological risk in building materials [11,12,13] and in water [14,15,16], in soil and rock [17], in the air [18].

This study’s objectives were to apply a new design and method to evaluate 222Rn radioactivity concentration in piping and capsule natural gas to residential houses and calculate natural gas contribution to indoor 222Rn concentration. Also, the evaluation of radiological hazrads due to 222Rn in indoor environment was calculated.

Materials and methods

A total of 5 natural gas samples, three compressed natural gas (CNG) and two liquefied petroleum gas (LPG) were selected to perform this experiment in Nicosia and Kyrenia, Cyprus.

To measure 222Rn gas, an AlphaGUARD model PQ 2000 pro (SAPHYM Co) was used. The passage of 222Rn gas from a filter to the ionisation chamber is the basis of this device’s operation. This radon monitor is suitable for the continuous monitoring of radon concentrations between 2 and 2,000,000 Bq m−3. It is both suited for short or long-term examination inside (e.g. in buildings) and outdoor and is capable of flow mode in a 1- or 10-min measuring cycle. Radon measurements were taken in a specially designed cubic chamber (70 × 50 × 60 cm) with sealed walls. An AlphaGUARD monitor was placed in the cubic chamber. The cubic chamber was connected to the vacuum pump to create a negative pressure (approximately 0.1 bar) inside the chamber. The natural gas flow was sent to the chamber by controlling pressure and temperature gauge. Due to the detector’s sensitivity to hydrocarbon materials, measurements were performed with relatively low gas pressure. In this study, the flow mode with 10 min measuring cycle was used for at least one hour. The volume of natural gas regarding the most common method that standard cubic metre is a quantity of gas, which at the pressure of 1013.25 mbar and temperature of 15 °C has a volume of 1 m3 [19]. The background of 222Rn concentration value at pressure 0.1 bar was measured and subtracted from all samples. The calibration of AlphaGUARD was carried out using a standard solution of 226Ra, code 71L07 was supplied from Amersham and diluted to activity concentration of 24.1(12) × 10−2 Bq mL−1. The 226Ra solution was in polyethylene and crammed into a leak-proof glass bulb, where 222Rn accumulated over time. In SAPHYM Co Frankfurt radon chamber [20], AlphaGUARD was factory calibrated. The NIST 226Ra SRM-4968/CP-100 principal standard source was used to calibrate a factory reference unit AlphaGUARD in the factory laboratory. This unit was used as an operational transfer standard device. The calibration of the AlphaGUARD used in this work was performed at the factory against the transfer device.

The schematic diagram in Fig. 1 shows the radon concentration measured by the active setup method. The gas samples were measured after one h in (n = 4–5) times to get average results. By Alpha View/Expert software, the final activity of 222Rn gas (ARn) with decay correction was computed. However, the radon portion releasing from natural gas can be calculated with the following formula.

Fig. 1
figure 1

Schematic diagram showing the radon concentration measurements of natural gas samples by an active setup method

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$${C}_{Rn-Gas}\left({\rm Bq\;m}^{-3}\right)={A}_{Rn}\times [\alpha {R}_{C}+\beta {R}_{H} ]$$
(1)

and

$${C}_{Cooking}=\alpha {R}_{C} \left({C}_{T}\right)$$
(2)
$${C}_{Heating}={\beta R}_{H} \left({C}_{T}\right)$$
(3)

where \({ C}_{Rn-Gas}\) is radon released from natural gas consumption \(\left({\rm Bq\; m}^{-3}\right);\)  \({A}_{Rn}\) is radon activity concentration mixed with natural gas (\({\rm Bq \;m}^{-3}\)); \({C}_{cooking}\) is natural gas consumption for cooking (\({\rm m}^{3}\;{\rm year}^{-1}) ;\; {C}_{Heating}\)is natural gas consumption for heating (\({\rm m}^{3}{\rm year}^{-1})\); \({C}_{T}\)is natural gas consumption per year; \({R}_{C}\) and \({R}_{H}\) are radon releasing factor from cooking and heating, respectively. The parameters \(\alpha\) and \(\beta\)are the natural gas consumption coefficient for cooking and heating, respectively.

Figure 2 shows the value of those parameters for 222Rn concentration calculation from natural gas consumption. These parameter values were selected according to Energy Resource Guide, Turkey. At the same time, these parameters can be defaulted by local conditions and consumption scale.

Fig. 2
figure 2

The flow chart and default parameters to determine 222Rn concentration in natural gas

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The average annual effective dose received due to radon gas and its daughters (mSv year−1) is derived from Eq. (4) [10, 21, 22]:

$$D\left({\rm mSv\;year}^{-1}\right)=5.56\times {10}^{-6}\times {C}_{F}\times {E}_{F}\times {C}_{Rn-Gas}\times T\times {Q}_{F}$$
(4)

where D is annual effective dose (mSv year−1); \({C}_{F }\) is dose conversion factor; for members of the public is 1.1 m Sv. (mJ h m−3)−1 and 1 Bq m−3 = 5.56 × 10−6 mJ m−3; \({E}_{F}\) is equilibrium factor for radon progeny; 0.4; \({A}_{Rn}\) is radon concentration (Bq m−3); T is time in hours for a year; (365 × 24 = 8760 h); and \({Q}_{F}\) is coefficient of occupancy; 0.8.

Excess lifetime cancer risk (ELCR) is the chance of a person contracting cancer if they are subjected to a given dose of radiation during their lifetime. By multiplying the parameters of annual effective dose D \(\left({\rm mSv\;year}^{-1}\right)\), the average period of life (DL = 70 years), and risk factor (RF = 0.05 Sv−1), the risk of life- cancer was estimated by the following equation [20]:

$$ELCR=D\times RF\times DL$$
(5)

Results

In this research, a new method for collecting and analysing radon gas from a natural gas source was introduced. The 222Rn concentration in natural gas samples with the system’s pressure and temperature are presented in Table 2. The error of calculation was considered within one standard deviation. The 222Rn concentrations in CNG gases were found to be in the range of 42.16 ± 3.11 to 94.55 ± 6.24 Bq m−3 with an average value of 74.54 ± 5.58 Bq m−3. The 222Rn concentrations in LPG gases were found to be in the range of 108.71 ± 9.27−146.47 ± 12.48 Bq m−3 with an overall average value of 127.59 ± 11.51 Bq m−3.

Table 2 The 222Rn measured concentration (\({A}_{Rn})\), 222Rn partial released to home (CRn−Gas), annual effective dose rate (D) and excess lifetime cancer excess risk (ELCR) from natural gas samples
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222Rn released to home (CRn−Gas), average annual effective dose (D), and excess lifetime cancer risk (ELCR) values are presented in Table 2. Minimum CRn−Gas concentration value 26.88±2.54 Bq m−3 was observed in CNG-1 sample, and maximum CRn−Gas concentration value 93.74 ± 8.41 Bq m−3 was observed in LPG-1 sample. The average concentration value of CRn−Gas (64.26 ± 5.54 Bq m−3) was calculated.

The minimum and maximum annual effective dose (D) value were found to be 0.46 mSv year−1 in CNG-1 and 1.61 mSv year−1 in LPG-1, respectively. The average annual effective dose value was calculated as 1.11 mSv year−1 (Table 2). Minimum excess lifetime cancer risk (ELCR) value (0.000016) was recorded in CNG-1, whilst maximum ELCR value (0.000056) was measured in LPG-1. The average ELCR value was calculated as 0.000039 (Table 2).

Discussion

In view of the different radon concentrations in natural gas, these changes depend on parameters such as the natural gas extraction area. 222Rn is decay product of 226Ra element which generating from 238U elements. 238U is present in soil, rocks and Earth’s bedrock. Whereas the natural gas exists in bedrocks and due to high pressure and temperature the radon mobility from rocks to natural gas occurs. Since the concentration of 238U in bedrocks varies from place to place on earth. So, the natural gas used in houses has different content of radon gas.

Natural gas 222Rn concentrations and annual effective dose calculations were carried out in 5 dwelling houses. It is observed that the average of 222Rn concentrations in LPG gas samples were higher than the CNG gas samples.

The results of the 222Rn concentration in the natural gas of the study are compared with the values of other areas reported in literature. Although 222Rn concentrations range of this study was less than the 222Rn concentrations value reported in Texas, Panhandle (370–1924 Bq m−3), in Colorado (407–1665 Bq m−3), USA [23]; Netherlands [24] (150,000–380,000 Bq m−3); and Algeria [25] (1085–5800 Bq m−3). Whereas the results of this study were comparable with 222Rn concentrations in China Beijing (49–88 Bq m−3) [26]. The results of this study were within the range reported from Thailand (16–197 Bq m−3) [27] and Poland (30–1400 Bq m−3) [28].

Figure 3 shows a boxplot of 222Rn concentration with WHO reference level of 222Rn concentration in dwelling houses. As seen in this Figure, the 222Rn concentration in all CNG samples is less than the reference level, while the 222Rn concentration in all LPG samples is more than the reference level.

Fig. 3
figure 3

The boxplot and WHO reference level for indoor 222Rn concentration

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The higher values than reference may be attributed to U and Th content in geological media and soil porosity. Also, radon emanation coefficient can be the factor that affected the radon concentrations in LPG gas.

It was observed that the annual effective dose due to natural gas in study area was lower than the ICRP 2010 reference range of 3 to 10 mSv year−1 [28] and the WHO suggested reference limit of 10 mSv year−1 (WHO 2009) [29] (Fig. 4).

Fig. 4
figure 4

The chart of annual effective doses (D) with ICRP 2010 reference range limit

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The risk factor of developing cancer increases by 0.004%, 0.04%, 0.4%, and 4% with the increment of ELCR values corresponding to the effective dose of 1, 10, 100, and 1000 mSv year−1, respectively [30]. The excess lifetime cancer risk (ELCR) for radon exposure ranged from 1.6 × 10−5 to 5.6 × 10−5 with an average of 3.7 × 10−5 (Fig. 5), whereas those values are lower than the world’s average (2.9 × 10−4) [31].

Fig. 5
figure 5

The chart of Excess lifetime cancer excess risk (ELCR) from natural gas samples with UNSCEAR, 2000 reference range limit

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Conclusions

In this study, the activity concentrations of 222Rn in natural gas samples have been measured by a direct method. 222Rn concentration in LPG gas samples was higher than WHO reference level, whilst 222Rn concentration in CNG gas samples was lower than WHO reference level. Also, the indoor 222Rn contribution from natural gas regarding cooking and heating consumption was calculated. The annual effective dose (D) and the excess lifetime cancer risk (ELCR) for radon exposure from natural gas pathways was lower than the world’s average reported by UNSCEAR, 2000 reference range limit. This study shows the importance of using a suitable filter to reduce the amount of 222Rn in the natural gas purification process, and the authors of this study recommend it.