1 Introduction

Indoor air pollution has recently attracted a great deal of attention. Due to the trend toward a reduction of ventilation and infiltration rates in buildings and the use of synthetic chemicals in the indoor environment, indoor pollution has become even more serious. Natural radon isotopes (219Rn, 220Rn, and 222Rn) are produced within the grains of the bedrock of the Earth's crust, which contains the ores of uranium and thorium and their daughters (218Po, 214Pb, 214Bi, and 214Po. The 218Po and the 214Po are alpha emitters, while the 214Pb and 214Bi are beta/gamma emitters) in equilibrium. Uranium and thorium ores concentrations widely vary in the bedrock of the Earth’s crust. Because of its short half-life (3.92 s) and because of the very low abundance of its forerunner (radium-223), radon-219 can be complete disregarded in survey studies. The abundance of (uranium-238) 238U in the Earth's crust is slightly lower than that of (thorium-232) 232Th [1]. However, the production rates of 220Rn (from thorium series) and 222Rn from uranium series) are nearly the same because 238U has a shorter half-life and is less abundant than 232Th. The half-life of 222Rn (3.82 d) is much longer than that of 220Rn (55 s) [2]. Hence, the decay of 220Rn in its cradle is highly probable before it can reach the atmosphere; thus, 220Rn and its daughters are normally neglected when the “radon problem” is examined, even though their contributions are not as trivial as those of 219Rn [3]. Consequently, the entrance rate of radon (222Rn) into the environment is much greater than that of 220Rn. Radon is a radioactive noble gas that is colorless, odorless, and tasteless and when decays it gives alpha particles with energy of 5.59 MeV. Since 238U is a very widely distributed element in the Earth's crust, consequently radon is widely spread and become part of our environment [4]. The health effect of radon comes from its short-lived daughters, which are metallic radioactive nuclei. Two of these, 218Po and 214Po, emit alpha particles with energies of 6.0 and 7.78 MeV, respectively. If the decay process occurs inside the lung, the energy of decay may be deposited and absorbed in the tissue lining the lung and disturbed the genes functions. Therefore, living at an elevated radon concentration for a long time increases the probability of inducing lung cancer [5].

The subject of indoor radioactivity evaluation due to radon in Jordan and worldwide has attracted much research interest. Oni et al [6]. estimated the lifetime fatality risk from indoor radon in several offices at Nigeria University. Abumurad and Al-Omari [7], studied indoor radon levels in Irbid and health risks from internal doses, and Alqadi et. Al., [8], measured radon exhalation rates from hot water springs in Jordan, their result for annual effective doses range from 10 to 12 µSv.y−1 due to inhalation of radon released from the water. Ahmad et al. [9], studied radon concentrations in Jordan and their results at the global average of 46 Bq.m−3.

According to the World Health Organization (WHO) [10], in many countries, smoking and radon exposure are the first and second most important causes of lung cancer, respectively. The proportion of lung cancers attributed to radon is estimated to range from 3 to 14% [10].

Depending on the national average radon level and smoking prevalence, the risk of lung cancer increases by approximately 16% per 100 Bq/m3 in the long-term average radon concentration [10].

Furthermore, smokers are estimated to be 7 times more at risk from radon than nonsmokers are [11].

In some countries, if the radon level in any resident exceeds 150 Bq.m−3, corrective measures are recommended [12]. Research has shown that 16% of lung cancer cases are caused by irradiation with radon or its progeny products [12]. US EPA [13] reported approximately 280 of the 2300 cases of lung cancer per year may be associated with radon [1].

The study area (KufrKhal) is part of a tourist area (Jerash Governorate) in northern Jordan, that was not previously a subject of a comprehensive serious radon survey before I started this study in 2023. Therefore, the aim of this study is to measure the radon activity concentration using CR-39 plastic detectors and consequently to calculate both the annual effective dose (AED) and the excess lung cancer risk (ELCR) in the KufrKhal region.

2 Methodology

2.1 Dosimeters

There are many ways to measure radon concentration; active method or passive method, which does not use power. The alpha track detector is a solid-state nuclear track detector (SSNTD, type CR-39 which is a trademark name of poly-allyl-diglycol-carbonate (C12H18O7)) normally used in passive dosimeters. Detectors CR-39 record the trajectories of heavy charged particles (Z > 1) in dielectric solids [14]. The passive radon dosimeters used in this work consists of a cup with of cylindrical shape (truncated cone) as shown in Fig. 1 [14].

Fig. 1
figure 1

Schematic diagram for dosimeters using the closed-can technique for radon measurements [14]

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When alpha particles with suitable energy (between 0.1 MeV and 20 MeV) and incident angle strike CR-39 surface, it will damage the chemical bond between the carbon atoms. These damaged bonds called tracks. The used CR-39 detectors have thickness of 0.5 mm, and initially zero backgrounds, as stated by the manufacturer: (PAGE MOULDINGS (PERSHORE) LIMITED, Company number 03069139, Hill House, 1 Little New Street, London, EC4A 3TR).

Furthermore, a pilot group of dosimeters were prepared (unexposed) and carried to the KufrKhal during planting the experimental dosimeter (exposed). The pilot group brought back to the lab and stored in a freezer for three months. Both of pilot group and retrieved experimental dosimeters etched and counted for radon tracks. The average pilot readings subtracted from the average experimental readings to make sure zero background of the exposed ones.

2.2 Study area

The origin of the name is “kufr,”which is an Arabic word meaning “agricultural village,” and “khal” means grape vinegar. Therefore, the name KufrKhal refers to the “village of grapes,” and the village is still famous for growing grapes in addition to olive trees. KufrKhal is located 18 km north of Jerash city, approximately 5 km west of the international road connecting Irbid city and the capital Amman (Fig. 2). The rock layers of the study area characterized by sedimentary rocks such as limestone that is found in the form of Coquina rocks of the Upper Cretaceous period in many areas of Jordan and KufrKhal is an example. However, limestone is considered a weak habitat for uranium ores and oxides. For the purpose of the study, we anticipated the natural division of the village; three hills and the village center: Jabal al-Sanad (Hill 1, elevation 1120 m), Jabal al-Ahed (Hill 2, elevation 950 m), and al-Jabal al-Gharbi (Hill 3, elevation 1050 m), and the center (Center, elevation 900 m) The coordinates of the study area are 32.357677°N, 35.884467°E.

Fig. 2
figure 2

Location of the study area. KufrKhal has coordinates of 32.357677°N, 35.884467°E

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Random samples of houses representing each division were selected. Approximately 117 dosimeters distributed among the selected houses. The dosimeters were placed in the living rooms, where the families meet, for three months; from Sept 1 to Nov 1. For quality control and interpretation, proper placement of detectors was essential. Therefore, the detector was placed at least 20 cm away from the walls, 90 cm from doors or windows, and 120 cm above the ground [15]. At the end of the exposure time, the detectors were retrieved and chemically etched (using 6 M of NaOH solution at 70° for eight hours) and then the track density, ρ (track.cm−2) was determined using the following equation:

$$\rho =\frac{Average\; number\; of\; tracks\; per\; view}{View\; area\; in\; {cm}^{2}}=\frac{{N}_{Avg}}{A}$$
(1)

The radon concentration (C) in a room was compared to the radon concentration (Co = 90 × 103 Bq.m−3) in the radon calibration chamber (at Birmingham University, UK) as follows

$$\frac{C.t}{\rho }=\frac{{C}_{o}.{t}_{o}}{{\rho }_{o}}=k\to C=k\frac{\rho }{t}$$
(2)

where ρo is the track density in the calibration chamber (3.302 × 104 tracks.cm−2), to is the exposure time in the chamber (2 days), and t is the detector exposure time in a room (90 days), and the calibration factor was calculated to be 1.064 × 103 (Bq.m−3.day)/(track.cm−2). These measurements analyzed to determine the accumulated radon concentrations. Measuring radon in indoor air is crucial for assessing potential health risks such as annual effective dose (AED).The formula used to calculate (AED) is [11, 12]:

$$\varvec{AED}\left({\varvec{mSv}.\varvec{y}^{ \mathbf{- 1}} } \right) = \varvec{C*F*O*T*DCF}$$
(3)

where C is the radon concentration (Bq.m−3), F is the equilibrium factor (0.4), O is the occupancy factor (0.8), T is the indoor occupancy time (in hour per year, 8760 h.y−1), and DCF is the dose conversion factor (9.0 × 10–6 mSv.h−1). (Bq.m−3)−1. Therefore, when these number values are used, Eq. 2 reduces to the following:

$$ {\varvec{AED}}\left( {{\varvec{mSv}}.{\varvec{y}}^{ - 1} \user2{ }} \right) = \mathbf{0.02523}\varvec{*C} $$
(3′)

And the excess lung cancer risk (ELCR) due to the inhalation of radon gas in the study area is given by the following [11, 12]:

$$ \varvec{ELCR} = \varvec{AED}\left({\varvec{mSv}.\varvec{y}^{ \mathbf{- 1}} } \right)\varvec{*DL}(\varvec{y})\varvec{*RF}\left( {\varvec{Sv}^{\mathbf{ - 1}} } \right) $$
(4)

where DL is the life expectancy for Jordanians (75 y) and RF is the risk factor (0.055 \({Sv}^{-1}\)). Equation 4 can be written as follows

$$ \varvec{ELCR} = \mathbf{0.1043}\varvec{*C} $$
(4′)

whereas, the results of these calculations will help indicate the need for remedial action or not.

3 Results and discussion

We scanned all collected detectors (under an optical microscope with proper magnification), at least twenty times with different views to determine the number of tracks per view (and then tracks per cm2). The arithmetic mean and standard deviation for radon concentration, AED, and ELCR were calculated. Table 1 summarize the obtained results. Radon concentrations in Hill 1 range from 17.1 to 114.7 Bq.m−3 with an average of 64.4 Bqm−3, in Hill 2 the range from 35.2 to 120.8 Bqm−3 with an average of 71.5 Bqm−3, in Hill 3 the range from 18.1 to 121.8 Bqm−3 with an average of 67.4 Bqm−3, and in the center area the range from 31.2 to 128.8 Bqm−3 with an average of 75.5 Bqm−3 the all average radon level in KufrKhal was 69.2 Bqm−3. Obvious, the village center has the biggest mean radon concentration (75.5 Bq.m−3) in the village. This may be due to: its old building that was constructed from stones and mud, and contain many cracks that provide canals for radon to seep inside. Furthermore, most of these buildings occupied by small elder families who maintain doors and windows close (i.e. poor ventilated homes) that cause radon to accumulate [16]. While the lowest mean was 17.1 Bq.m−3 in the newly built, ventilated (air circulation from inside to outside) homes in the Hill 1 area.

Table 1 Max, Min, AVG, SD, AED, and ELCR of the radon levels in KufrKhal village
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Figures 3, 4, show the distribution of the radon levels in areas Hill 1 and Hill 2. Most of the houses in these areas characterized by low radon concentrations. Their curves are skewed to the left. Figure 5 and 6 represent radon distribution in Hill 3 and village center. The curves are slightly skewed to the right, which indicate that most of the houses in these areas have high radon level. From radon levels and according to some models [12, 17], one can calculate (using Eqs. 3 and 4) the annual effective dose (AED) and excess lung cancer risk (ELCR).

Fig. 3
figure 3

Variation of radon concentration in the study area; Hill 1

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Fig. 4
figure 4

Radon variation levels in the study area Hill 2

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Fig. 5
figure 5

Variation of radon concentrations in the study area Hill 3

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Fig. 6
figure 6

Variation of Radon concentrations in the study area Village Center

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ICRP [18] recommends an action level of 200 Bq m−3 (corresponding to an annual dose of 3 mSv y−1) for radon in dwellings. Based on this recommendation, less than 2% of the sampled houses in this study exceeded the limit of the action level. For comparison, Table 2 lists the indoor radon levels in different localities [19].

Table 2 Comparison of the mean indoor radon concentrations in different countries worldwide. From [19]
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4 Conclusion

Indoor radon levels (69.2 Bq.m−3 in average) in 117 sample homes in KufrKhal remained below the national standard limit of 200 Bq.m−3. Furthermore, this average is comparable to worldwide average (74.3 Bq.m−3) EU average (72.0 Bq.m−3) and higher than US average (46.3 Bq.m−3) [12, 20]. The International Commission on Radiological Protection (ICRP) [18] recommended monitoring indoor radon levels in public facilities as a part of broader radon surveillance measures.

The calculated annual effective dose (AED = 1.8 mSv.y−1), and excess lifetime cancer risk (ELCR = 0.2%) are also acceptable and similar to the global values (1.3 mSv.y−1) [12]. However, high values in some dwellings in the center of KufrKhal are likely due to that dwellings built directly on the ground without cellar in between so radon can easily sneak into indoor. In addition, dwellers do not ventilate their homes every day to naturally mitigate radon [21]. Furthermore, a positive relationship exists between the increase in indoor radon concentration and the decrease in elevation. The results from this study show that the measured levels of radon gas are within the safe limits. Accordingly, living in the dwellings of KufrKhal village poses no significant risk to the inhabitants.