Introduction

Naturally occurring radionuclides of terrestrial origin emerge mostly from the primordial radionuclides (238U, 232Th) that have considerable long half-lives (Aközcan 2014; Akkurt et al. 2015; Akkurt 2009; Çetin et al. 2016; Demir et al. 2017; Günay 2018; Günay et al. 2018a, b; Kara et al. 2017; Kara et al. 2016; Mavi and Akkurt 2010; Seçkiner et al. 2017; Uyanık et al. 2013). Radon is the member of the radioactive chain of uranium and produced by alpha decay of radium. Almost 54% of the natural radiation that people are often exposed in their daily lives is due to radon isotopes, especially 222Rn.

The abundance of these radionuclides is related to the rock types in which the soil is formed. The Earth’s crust consists of many different types of rocks. For this reason, terrestrial radiation is at different levels in the soil of every region in the world. In addition to regional levels, radon can migrate through the fractures in the crust without undergoing chemical reactions depending on its relatively long half-lives and inert gas structure.

Although the outdoor radon concentration levels are very low, it can accumulate in galleries, caves, mines, and dwellings. The most common places for radon accumulation in a building are the basement floors and cellars, which are the closest parts of the buildings to the soil and rocks. Due to the variability of radon concentration in the environment, indoor radon levels exhibit a considerable regional variation. There are several main factors that affect the formation of radon concentration in the building, such as radium content of the building materials, the penetration of the radon gas in the ground into the building from the pores and cracks at the base of the building, radon entry from open windows and doors due to the differences in pressure and temperature inside the building, and dissolved radon access to the dwellings due to water usage.

The primary hazards of radon are due to the inhalation of its short-lived decay products, which emit 훽, 훾, and 훼 particles. Radon progenies become radioactive aerosols by adhering to dust and other particles in the air, the radioactive decaying radon gas being converted into particles that can be trapped by the lung. Therefore, inhalation of short-lived radon progeny in the ambient can cause internal exposure and becomes an important health risk for human being (WHO 2010). The International Commission on Radiological Protection (ICRP) links 10% of total lung cancers to radon.

Considering these health risks, the determination of radon levels in living spaces and working places is an essential worldwide subject (Kuluöztürk et al. 2018). Systematic and area-wide assessments of radon risk are desirable to reduce public exposure to indoor radon (Kemski et al. 2009).

Over the past decades, investigations on radiation levels and especially on radon have been performed worldwide. Several radon detectors, which have alpha spectroscopic analysis systems, are being used for indoor radon surveys such as passive detectors like electret ion chambers, charcoal detectors, etched track detectors, and active systems like scintillation cells and ionization chambers (Durrani and Ilic 1997a; Durrani and Ilic 1997b; Günay et al. 2018b; Kulalı and Akkurt 2015; Kulalı et al. 2014; Mnich et al. 2004).

The primary objective of this research is the determination of radon concentration levels by using an active radon monitor and the investigation of radiation exposure depending on the time spent in the chosen areas.

Materials and methods

Study area

This study was carried out on Akfirat campus of Istanbul Okan University. Istanbul Okan University is one of the universities with a high student density in Istanbul with 26,000 students. Due to the high student density, more people can be affected by radiation caused by radon gas. Because of this, this study was conducted on the Akfirat campus of Istanbul Okan University (Fig. 1).

Fig. 1
figure 1

Akfirat Campus of Istanbul Okan University (Google Earth 2018)

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The study areas lie within the latitudes 40o 56′ 57″ to 40° 57′ 14″ north latitudes and 29° 23′ 13″ to 29° 23′ 41″ east longitudes.

222Rn activity concentration measurements by using Alpha GUARD were made at different locations of the university, as shown in the map Fig. 2.

Fig. 2
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Map of the measuring points (Google Earth 2018)

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This study was conducted in the Faculty of Health Sciences, Faculty of Art, Design and Architecture, Faculty of Education, Faculty of Law, Student Dorm, and Activity Center on the campus (Fig. 2). Two different measuring points were identified in the buildings where radon measurements were made. Six measurements were taken over a total of 1 h to take measurements every 10 min at all measurement points. The average radon concentrations of each measurement point were calculated using the results of these measurements. Then, the average radon concentrations were determined by taking the average of two different measurement points on the same floor. In other words, the radon concentrations on a floor were calculated using averaging of 12 different measurements.

Experimental method

The activity measurements are based on the counting of alpha particles emitted from the radon gas. Radon measurement methods generally converge in three main sections. These measurement methods have passed the literature as instantaneous, integral, and continuous measurements.

For continuous radon measurements, electronic detectors are used. In this survey, portable radon monitor “AlphaGUARD” was selected for the radiological assessment of the university campus buildings. The AlphaGUARD is an instrument designed mainly for active measurements of radon concentrations and it is equipped with other metrological sensors for pressure, relative humidity, and temperature. AlphaGUARD is a device made of steel with an ionization chamber, with 750 V between the anode and cathode. The total volume of the device is 620 cm3 and the effective volume is 560 cm3. The detector’s battery measures approximately 10 days. But if it is connected to the source of power, this period can be extended considerably. AlphaGUARD can measure between 6 Bq m−3 and 2.106 Bq m−3. However, it is an advantageous device because the error margin is 3%. AlphaGUARD can measure in the air, soil, and water using various apparatuses. AlphaGUARD also measures temperature, outdoor air pressure, and humidity.

In this method, measurements are made in short periods (such as 10 min) continuously without interruption and measurements made can be recorded electronically. There are many electronic devices manufactured for this. The main ones are Barasol detector, Clipperton detector, and AlphaGUARD device (Fig. 3).

Fig. 3
figure 3

AlphaGUARD instrument used for 222Rn measurements

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Result and discussion

In this study, radon measurement was made in the basement of the Health Sciences Faculties (Health-B), ground floor of the Health Sciences Faculties (Health-GF), ground floor of the Faculty of Art, Design, and Architecture (Art-GF), first floor of the Faculty of Art, Design, and Architecture (Art-1F), ground floor of the Faculty of Engineering (Engineering-GF), first floor of the Faculty of Engineering (Engineering-1F), ground floor of the Faculty of Law (Law-GF), first floor of the Faculty of Law (Law-1F), ground floor of the Faculty of Education (Education-GF), first floor of the Faculty of Education (Education-1F), ground floor of the Student Dorm (Dorm-GF), first floor of the Student Dorm (Dorm-1F), ground floor of the Activity Center (AC-GF), and first floor of the Activity Center (AC-1F).

Tables 1 and 2 show the results of all the measurements performed by AphaGUARD. Temperature, humidity, and pressure are also shown in the tables.

Table 1 222Rn activity concentrations as well as other parameters in ground floor
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Table 2 222Rn activity concentrations as well as other parameters in the first floor
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The concentration of radon in the basement of the Faculty of Health Sciences is 32.5 ± 8.5 Bq m−3. When the measurements on the ground floor are examined, it is seen that the minimum radon concentration is 13.2 ± 6.4 Bq m−3 in the Faculty of Health Sciences (Health-GF). Maximum radon concentration is 24.3 ± 8.5 Bq m−3 in the Faculty of Engineering (Engineering-GF). The average radon concentration is 17.4 ± 7.2 in the ground floor (Fig. 4). The temperature on the ground floor during the measurement ranges from 21 to 24 °C. The humidity on the ground floor ranges from 51 to 56%. The outdoor pressure on the ground floor ranges from 1001 to 1004 hPa.

Fig. 4
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222Rn activity concentration ranges in the ground floor of the buildings

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The radon concentrations calculated, temperature, humidity, and outdoor pressure for this study are shown in Table 2 for the first floor.

Minimum radon concentration is 5.2 ± 2.5 Bq m−3 in the first floor of the Faculty of Art, Design, and Architecture (Art-1F). Maximum radon concentration is 17.2 ± 6.5 Bq m−3 in the first floor of the Student Dorm (Dorm-1F). The average radon concentration is 8.5 ± 3.9 Bq m−3 in the first floor (Fig. 5). The temperature on the first floor during the measurement varied in the range from 21 to 26 °C. The humidity on the ground floor varied in the range from 52 to 59%. The outdoor pressure on the ground floor varied in the range from 1000 to 1003 hPa.

Fig. 5
figure 5

222Rn activity concentration ranges in first floor of buildings

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The annual effective radiation dose is calculated using the equation:

$$ \mathrm{AED}={C}_{\mathrm{Rn}}\times F\times d\times u\times t $$

Where;

AED:

Annual effective radiation dose (Sv/year),

C Rn :

Radon concentration (Bq m−3)

F :

Equilibrium factor between radon and its decay products (0.4 for the buildings (UNSCEAR 2000))

d :

Dose conversion factor (1.43 Sv/(J hm−3) (ICRP 1990)),

u :

Unit factor (5.56 × 10−9 J m−3/Bq m−3 (ICRP 1990)),

t :

Time spent annually inside the building (hour/year (h/year). If people stay in the building for 3 h a day, the time (3 × 365 = 1095) should be used as 1095 h/year.

In this study, it was thought that people were staying in the building for 3 h, 6 h, 12 h, and 18 h a day. Annual effective dose calculations were made using these times (3, 6, 12, and 18 h). The annual effective radiation dose received by a person is shown in Table 3, depending on whether they stay on the ground floor and the basement floor in the building for 3 h, 6 h, 12 h, and 18 h a day.

Table 3 The annual effective radiation dose received by a person on the ground floor in the buildings
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The annual radiation dose to be taken by radon for an average of 6 h a day in the building varies from 0.091 to 0.167 mSv/year in the ground floor. For the people living in the building 6 h per day on the ground floor, average radiation dose is 0.120 mSv/year. But the annual radiation dose to be taken by radon for an average of 6 h a day in the basement of the Faculty of Health Sciences is 0.223 mSv/year (Fig. 6).

Fig. 6
figure 6

The annual effective radiation dose on the ground floor in the buildings

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The annual effective radiation dose received by a person is shown in Table 4, depending on whether they stay on the first floor in the building for 3 h, 6 h, 12 h, and 18 h a day.

Table 4 The annual effective radiation dose received by a person on the first floor in the buildings
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The annual radiation dose to be taken by radon for an average of 6 h a day in the building varies from 0.036 to 0.118 mSv/year in the first floor. For the people living in the building 6 h per day on first floor, average radiation dose is 0.059 mSv/year (Fig. 7).

Fig. 7
figure 7

The annual effective radiation dose on the first floor in the buildings

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Conclusion

World Health Organization declares that radon exposure is a major and growing public health threat in the homes and recommends that countries adopt reference levels of the gas of 100 Bq m−3; nevertheless, limits are different for each region and country. The Turkey Atomic Energy Agency reported on Radiation Safety Regulation that the limits of inhaled radon in the homes is at an annual average of 400 Bq m−3 and 1000 Bq m−3 for work places (TAEA 2010).

According to the results of the survey on chosen area, Akfirat campus of the Istanbul Okan University, indoor radon activity concentrations are below the limits for each location. By using the activity values and exposure times, annual effective dose limits are calculated, and these values are also under the recommended level of 1 mSv. Low abundance of radon in the investigated rooms could be depending on building materials, sufficient ventilation, or soil structure in the region. There will be more radioactive decay as gas will accumulate continuously in closed or poorly ventilated locations. At sufficiently ventilated spaces, the concentration of the indoor radon concentrations reduces because of the air circulation.

The most important point observed in the measurements is revealed when the ground floor is compared with the first floor. The basement or ground floor is the point where the first contact of the buildings with the soil has higher radon concentrations. The lack of building insulation or the presence of cracks on the floors and walls will increase radon flow towards the inside of the building. Even though the new structures are insulated sufficiently, they may become worn and weakened over time so the activity levels due to radon sources need to be under control.