Seasonal Variation of Indoor Radon Concentration Levels in Different

sustainability

Article Seasonal Variation of Indoor Radon Concentration Levels in Diﬀerent Premises of a University Building

Pranas Baltrenas˙ 1, Raimondas Grubliauskas 2 and Vaidotas Danila 1,*

1 Research Institute of Environmental Protection, Vilnius Gediminas Technical University, Sauletekis avenue 11, LT-10223 Vilnius, Lithuania; [email protected] 2 Department of Environmental Protection and Water Engineering, Vilnius Gediminas Technical University, Sauletekis avenue 11, LT-10223 Vilnius, Lithuania; [email protected] * Correspondence: [email protected]

Received: 25 June 2020; Accepted: 29 July 2020; Published: 31 July 2020

Abstract: In the present study, we aimed to determine the changes of indoor radon concentrations depending on various environmental parameters, such as the outdoor temperature, relative humidity, and air pressure, in university building premises of different applications and heights. The environmental parameters and indoor radon concentrations in four different premises were measured each working day over an eight-month period. The results showed that the indoor radon levels strongly depended on the outside temperature and outside relative humidity, whereas the weakest correlations were found between the indoor radon levels and indoor and outdoor air pressures. The obtained indoor radon concentration and environmental condition correlations were different for the different premises of the building. That is, in two premises where the ventilation effect through unintentional air leakage points prevailed in winter, positive correlations between the radon concentration and outside temperature were obtained, reaching the values of 0.94 and 0.92, respectively. In premises with better airtightness, negative correlations (R = 0.96 and R = 0.62) between the radon concentrations and − − outside temperature were obtained. The results revealed that high quality air isolation in premises could be an important factor for higher indoor radon levels during summer compared to winter.

Keywords: indoor radon concentration; seasonal changes; airtightness; outdoor temperature; humidity; university building

1. Introduction Radon 222Rn is an odorless and colorless radioactive gas [1,2]. It is formed from the decay of 226Ra, which is a member of the 238U decay chain [3–5]. Ionizing radiation can cause harm to human health [6,7]. As radon is one of the natural ionizing radiation sources [8] and is the second cause (after smoking) of lung cancer, a great deal of effort is spent monitoring and forecasting the radon concentrations in the premises of buildings [9]. The radon concentration levels in a building differ depending on the characteristics of the climate (atmospheric pressure, relative humidity, and temperature) and building, as well as on what rocks the building is built [10,11]. In Lithuania, as in many other countries, the main source of radon in buildings is gas flow from the soil. The indoor radon concentration depends on radon exhalation from the soil and building materials, the isolation and height of premises, and the ventilation intensity [12]. Radon has seasonal variations [9,13]. One of the reasons why indoor radon concentration is higher in winter is because in winter, the windows and doors are generally closed, which results in increased accumulation of radon in closed premises [9]. The outdoor and indoor air exchange is an important parameter that influences the indoor radon concentration [14,15].

Sustainability 2020, 12, 6174; doi:10.3390/su12156174 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 6174 2 of 15

Radon gas infiltration from the soil into buildings is controlled by many physical and meteorological factors related to the environment, building construction, and the type of ventilation [16,17]. Differences between the indoor and outdoor temperature lead to the difference of air density and pressure between the premises of the building and outdoors. The difference between the indoor and outdoor pressure is the main driving force causing the air flow between the outside and inside of the building [18]. Another important factor influencing the radon concentration in premises is the indoor microclimate—the indoor temperature, indoor humidity, and air change rate in the premises [19]. The relationships between indoor radon concentrations and meteorological parameters have been investigated by numerous researchers and various results have been reported. In the majority of cases, higher indoor radon levels were observed in winter, when the outdoor temperature was low [10,20–22]. However, some studies observed elevated radon concentrations in the warm period of year [18]. For example, Hubbard et al. (1992) studied the effect of outdoor temperature on radon concentration in two dwellings and found that indoor radon concentration increased with increasing outdoor temperature. This was explained by the ventilating effect, which was dominant in that house in the winter period [18]. Kitto (2005) reported that the diurnal indoor radon levels in the basement of a single-family house were mostly influenced by the difference between the indoor and outdoor temperatures; however, little correlation between the indoor radon concentrations and barometric pressure and wind speed were found [23]. Xie et al. (2015) found that indoor radon levels were mostly influenced by the indoor–outdoor barometric pressure difference in a two-story building (correlation coefficient R = 0.67) [21]. Aquilina and Fenech (2019) studied the influence of various meteorological parameters on indoor radon levels in four different locations in the Maltese Islands and found that indoor radon levels mostly depended on the outdoor relative humidity and wind speed [24]. The majority of the studies on indoor radon levels and their seasonal variations were performed in dwellings [1,10,13,23,25]. However, some studies were performed on indoor radon levels in workplaces, such as offices and industrial buildings [26–29], hospitals [26,30,31], and schools [26,32]. Only limited data are available regarding indoor radon levels and their seasonal changes in university buildings [33]. The aim of this study was to analyze the dependence of the indoor radon concentrations on various meteorological parameters in four different premises of a university building that was built in 1985. The meteorological parameters and the radon concentrations were measured each working day and the average monthly values were calculated. During the study period, the premises of the university building were used as usual.

2. Experimental Methods

2.1. Description of the Building Location and Premises The nine-story university building where we analyzed the indoor radon concentrations is located in southeastern Lithuania (Vilnius city). The city is located at north latitude 54◦410 and east longitude 25◦160. Vilnius has a humid continental climate and high precipitation during the year (655 mm). The average temperature of the city is 6.4 ◦C. The outdoor meteorological conditions, including the outdoor relative humidity, with an accuracy of 3% and outdoor temperature, with an accuracy ± of 0.5 C, were measured using a Metrel Poly MI6401 device at the same time the radon level ± ◦ measurements were performed. In addition, the outdoor pressure measurements were performed using a pressure measuring instrument with an accuracy of 3 hPa. The average monthly outdoor ± temperature and outdoor relative humidity measured during the period of study (from November 2018 to June 2019) are presented in Figure1. In November, the monthly average outdoor temperature was 4.9 ◦C, and this decreased until January. In January, the average outdoor temperature was the lowest and reached 6.9 C. In later − ◦ months, during radon concentration measurements, the average outdoor temperature increased and was the highest in June, when it reached 18.8 ◦C. The highest outdoor relative humidity values were in Sustainability 2020, 12, x FOR PEER REVIEW 3 of 16

Sustainability 2020, 12, 6174 3 of 15

November and December, and reached 89.3% and 89.6%, respectively. Later on, during the indoor radon concentration measurements, the average outdoor relative humidity decreased. The lowest relative humidity values were in May and June, and reached 55.0% and 57.5%, respectively. Sustainability 2020, 12, x FOR PEER REVIEW 3 of 16

Figure 1. Average monthly outdoor temperature and relative humidity during the study.

In November, the monthly average outdoor temperature was 4.9 °C, and this decreased until January. In January, the average outdoor temperature was the lowest and reached −6.9 °C. In later months, during radon concentration measurements, the average outdoor temperature increased and was the highest in June, when it reached 18.8 °C. The highest outdoor relative humidity values were in November and December, and reached 89.3% and 89.6%, respectively. Later on, during the indoor Figure 1. Average monthly outdoor temperature and relative humidity during the study. radon concentrationFigure 1. Average measurements, monthly outdoorthe average temperature outdoor and relative relative humidity during decreased. the study. The lowest relative humidity values were in May and June, and reached 55.0% and 57.5%, respectively. TheIn November, studied building the monthly (Faculty average of Environmental outdoor temperature Engineering) was is present 4.9 °C, onand the this University decreased campus until The studied building (Faculty of Environmental Engineering) is present on the University (FigureJanuary.2, In No. January, 6) and the has average nine stories outdoor and notemperature basement. was Sand the and lowest sandy and loam reached soil types−6.9 °C. prevail In later in campus (Figure 2, No. 6) and has nine stories and no basement. Sand and sandy loam soil types themonths, studied during area. radon The buildingconcentration was built measurements, in 1985 and, the therefore, average isoutdoor not energy temperature eﬃcient. increased The external and prevail in the studied area. The building was built in 1985 and, therefore, is not energy efficient. The wallswas the of thehighest building in June, are madewhen fromit reached two layers 18.8 °C. of brickwork, The highest an outdoor insulation relative layer humidity (polystyrene) values between were external walls of the building are made from two layers of brickwork, an insulation layer them,in November plaster layers,and December, and paint. and reached 89.3% and 89.6%, respectively. Later on, during the indoor (polystyrene) between them, plaster layers, and paint. radon concentration measurements, the average outdoor relative humidity decreased. The lowest relative humidity values were in May and June, and reached 55.0% and 57.5%, respectively. The studied building (Faculty of Environmental Engineering) is present on the University campus (Figure 2, No. 6) and has nine stories and no basement. Sand and sandy loam soil types prevail in the studied area. The building was built in 1985 and, therefore, is not energy efficient. The external walls of the building are made from two layers of brickwork, an insulation layer (polystyrene) between them, plaster layers, and paint.

FigureFigure 2. 2. UniversityUniversity campus: campus: 1— 1—centralcentral building, building, 2— 2—buildingbuilding I, I,3— 3—auditoriumauditorium building building I, I, 4—4—laboratorylaboratory building building I, I,5— 5—laboratorylaboratory building building II, II, and and 6— 6—buildingbuilding II II(Faculty (Faculty of of Environmental Environmental Engineering).Engineering).

TheThe radon radon concentration concentration levels levels were were studied studied in in four four building building premises premises (hereafter (hereafter called called premise premises No.No. 1, 1,premise premises No. No. 2, premise 2, premises No. No.3, and 3, andpremise premises No. 4) No. that 4) are that shown are shown in Figure in Figure 3 (marked3 (marked with withred dots).reddots). Figure 2. University campus: 1—central building, 2—building I, 3—auditorium building I, 4—laboratory building I, 5—laboratory building II, and 6—building II (Faculty of Environmental Engineering).

The radon concentration levels were studied in four building premises (hereafter called premise No. 1, premise No. 2, premise No. 3, and premise No. 4) that are shown in Figure 3 (marked with red dots). Sustainability 2020, 12, 6174 4 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 4 of 16

(a)

(b)

(c)

(d)

FigureFigure 3. The3. The locations locations of of studied studied premises premises (red(red dots):dots): (a) the second second floor floor plan, plan, No. No. 1; 1;(b ()b the) the third third floorfloor plan, plan, No. No. 2; 2; (c )(c the) the fourth fourth floor floor plan, plan, No. No. 3;3; andand (d) the ninth ninth floor floor plan, plan, No. No. 4. 4. Sustainability 2020, 12, 6174 5 of 15

Premises No. 1 was located on the second ﬂoor of the building, premises No. 2 was on the third, premises No. 3 was on the fourth, and premises No. 4 was on the ninth ﬂoor. In general, during the study, the building premises were used in a normal way. Table1 shows the descriptions of the analyzed premises.

Table 1. Description of the premises.

Premises The Premises Floor Cubic Volume, m3 Type of Ventilation The Purpose of Premises Natural and forced No. 1 Second 680.7 Auditory premises convection Various laboratory equipment No. 2 Third 25.65 Natural convection storage place No. 3 Fourth 100.4 Natural convection Laboratory premises Old laboratory equipment No. 4 Ninth 67.0 Natural convection storage place

The cubic volume of premises No. 1 is 680.7 m3. In premises No. 1, windows are not typically opened; therefore, a forced convection system is used (each day in the afternoon for about 30 min). Air also enters the premises when opening or closing doors. premises No. 1 is an auditory room used for lectures. Approximately 20 lectures a week were held during the study period. premises No. 2 is used as auxiliary room for various laboratory equipment storage and is located on the third floor of the building. The cubic volume of the premises is 25.65 m3. In this premises, the windows are closed all year round, and the room is only ventilated by natural convection and when opening the doors. During the study, premises No. 2 was used every day for picking up and returning various equipment or materials. premises No. 3 is used for laboratory work and is located on the fourth floor of the building. The cubic volume of the premises is 100.4 m3. premises No. 3 is ventilated by natural convection. In this premises, the frequency of opening the windows depended on the season and they were opened only after the measurements were performed. premises No. 4 is a storage place of old laboratory equipment, the capacity of which is 67.0 m3. It is on the ninth floor of the building. The premises has a natural convection system. During the measurement period, premises No. 4 was used approximately three times per week and ventilated rarely.

2.2. Measurements of Environmental Parameters and Indoor Radon Concentrations We performed 8 months of indoor radon concentrations measurements, which began in November 2018 and ended at the end of June 2019 and took place in the four university premises. The indoor radon levels strongly depend on the building operating conditions, therefore the 8-month period was chosen to ensure equal conditions during radon measurements. In the months of July, August, and September, many staﬀ have holidays; in addition, the windows and entrance doors were often opened during these months. The radon measurements were conducted according to the standards ISO 11665-1:2019 and ISO 11665-5:2020 [34,35]. The radon concentrations in the premises were measured at the same time each working day at the beginning of the working hours (8:00 a.m.) using commercially available radon monitors (SARAD RTM2200) (Figure4). The measurement range of radon concentration (Bq /m3) of these devices is in the range of 0–10 MBq/m3. In the analyzed premises, the radon monitor was placed at the height of 1 m, away from windows and doors. The measurements were taken on an hourly basis. At the same time, the indoor environmental parameters were also measured by the same device; i.e., the indoor temperature, with an accuracy of 0.5 C, indoor pressure, with an accuracy of 0.5%, and indoor relative ± ◦ ± humidity, with an accuracy of 2%. The monthly average values of all measured data were calculated. ± The following correlations between the monthly indoor radon concentrations and environmental parameters were analyzed: (1) dependence of the indoor radon concentration on the indoor and outdoor pressure, (2) dependence of the indoor radon concentration on the indoor and outdoor relative Sustainability 2020, 12, 6174 6 of 15 humidity, (3) dependence of the indoor radon concentration on the indoor and outdoor temperature, andSustainability (4) dependence 2020, 12, x of FOR the PEER indoor REVIEW radon concentration on the indoor–outdoor temperature diﬀ6erence. of 16

FigureFigure 4. 4.Monitor Monitor SARADSARAD RTM2200RTM2200 for radon measurement. measurement.

2.3. StatisticalIn the analyzed Analysis premises, the radon monitor was placed at the height of 1 m, away from windows and doors. The measurements were taken on an hourly basis. At the same time, the indoor The statistical data analysis was performed using Microsoft Excel. Due to significant variations environmental parameters were also measured by the same device; i.e., the indoor temperature, of the daily indoor radon concentrations obtained on an hourly basis for the premises (typically, with an accuracy of ± 0.5 °C, indoor pressure, with an accuracy of ± 0.5%, and indoor relative higher indoor radon values were obtained on Mondays), the average monthly indoor radon values humidity, with an accuracy of ± 2%. The monthly average values of all measured data were (the monthly sample size ranged from 20 to 23 depending on month) and their standard deviations calculated. The following correlations between the monthly indoor radon concentrations and wereenvironmental calculated. Despiteparameters daily were indoor analyzed: radon (1) variation dependence to some of the degree, indoor no radon filtering concentration or treatment on of the the obtainedindoor dataand outdoor was performed. pressure, (2) dependence of the indoor radon concentration on the indoor and outdoorThe Pearson relative correlation humidity, coe (3)ffi dependencecient (Equation of the (1)) indoor was used radon to evaluate concentration the linear on strengththe indoor between and theoutdoor monthly temperature, indoor radon and concentrations (4) dependence and of the environmental indoor radon variables concentration and was on the calculated indoor–outdoor using the followingtemperature formula difference. [36]: Pn i=1(xi x)(yi y) R = q − q − , (1) 2.3. Statistical Analysis P P n (x x)2 n (y y)2 i=1 i − i=1 i − The statistical data analysis was performed using Microsoft Excel. Due to significant variations where,of theR —thedaily indoor Pearson radon correlation concentrations coefficient, obtained on an hourly basis for the premises (typically, n—thehigher number indoor ofradon pairs values (n = 8), were obtained on Mondays), the average monthly indoor radon values x—the(the monthly monthly sample valueof size environmental ranged from variable,20 to 23 depending on month) and their standard deviations x—thewere meancalculated. value Despite of environmental daily indoor variable radon variation of the study to some period, degree, no filtering or treatment of the y—theobtained monthly data was indoor performed. radon value, and y—theThe mean Pearson indoor correlation radon value coefficient of the study (Equation period. (1)) was used to evaluate the linear strength between the monthly indoor radon concentrations and environmental variables and was calculated usingThe the correlation following was formula considered [36]: strong when its absolute value was higher than 0.70. The determination coefficient (the square of the correlation, R2) was also calculated. ( )( ) = , The p value was calculated to test the probability𝑛𝑛 that there was no correlation between the indoor(1) ∑(𝑖𝑖=1 𝑥𝑥𝑖𝑖)−𝑥𝑥�� 𝑦𝑦𝑖𝑖−(𝑦𝑦�� ) radon levels and the studied environmental parameters.𝑛𝑛 2 𝑛𝑛 If the2 obtained probability value was lower 𝑅𝑅 �∑𝑖𝑖=1 𝑥𝑥𝑖𝑖−𝑥𝑥�� ∑𝑖𝑖=1 𝑦𝑦𝑖𝑖−𝑦𝑦�� thanwhere, the conventionalR—the Pearson (p correlation< 0.05), the coefficient, correlation coefficient was considered statistically significant. The p valuen—the was number calculated of pairs using (n Microsoft= 8), Excel as a function of the t-statistic (Equation (2)), which was calculatedx—the using monthly the following value of formula:environmental variable, —the mean value of environmental variable of the study period, y—the monthly indoor radon value, and R √n 2 𝑥𝑥̅ t = − (2) —the mean indoor radon value of the study√1 period.R2 The correlation was considered strong when− its absolute value was higher than 0.70. The 𝑦𝑦� where,determinationt—t statistic, coefficient (the square of the correlation, R2) was also calculated. n—theThe number p value of pairs was (calculatedn = 8), and to test the probability that there was no correlation between the R—theindoor correlation radon levels coe ffiandcient. the studied environmental parameters. If the obtained probability value was lower than the conventional (p < 0.05), the correlation coefficient was considered statistically significant. The p value was calculated using Microsoft Excel as a function of the t-statistic (Equation (2)), which was calculated using the following formula: Sustainability 2020, 12, 6174 7 of 15

3. Results and Discussion

3.1. Variations of Monthly Indoor Radon Concentrations The monthly averages of the radon concentrations obtained within the eight months are presented in Table1. The indoor radon concentrations were obtained each working day in the morning. Before the measurements (after the night), the windows were not opened. Generally, the measured indoor radon levels in the premises were relatively low, not exceeding the reference levels for indoor radon concentrations in workplaces (300 Bq/m3) provided in European Directive 2013/59/Euratom [37]. As it can be seen from Table2, the highest average value of the eight-month period (47.9 Bq /m3) was obtained in premises No. 4, which is on the top floor of the building; whereas the lowest average value of the indoor radon concentration, 25.6 Bq/m3, was obtained in premises No. 1, which is on the second floor of the building. In most studies, researchers reported that the higher the floor of the building [38], the lower the radon concentration is for that floor. However, such results were not obtained in this study. The results revealed that not only the height of the premises of the same building but also the type of ventilation and the purpose of the premises could be influencing parameters for the indoor radon level. The forced convection system used in premises No. 1 could be influenced by the higher air exchange rate compared to the exchange rate in other premises, where only natural convection systems were used. As premises No. 4 was used and ventilated rarely, the highest indoor radon levels were obtained there. This could also be the reason why limited indoor radon seasonal variation was obtained in premises No. 4. Xie et al. (2015) also found limited indoor radon seasonal variation in the basement they studied, which had negligible ventilation [21].

Table 2. The monthly average radon concentrations in the premises, Bq/m3 (average standard ± deviation).

Radon Concentration in the Premises, Bq/m3 Month No. 1 (Second Floor) No. 2 (Third Floor) No. 3 (Fourth Floor) No. 4 (Ninth Floor) November 2018 21.5 5.7 45.0 17.6 30.7 10.1 47.1 13.8 ± ± ± ± December 2018 20.9 8.2 47.2 16.1 34.4 11.9 45.6 18.2 ± ± ± ± January 2019 17.0 5.0 55.9 20.1 23.4 8.1 50.4 18.9 ± ± ± ± February 2019 16.5 5.4 39.9 12.7 25.7 9.9 50.8 19.7 ± ± ± ± March 2019 21.9 7.3 49.0 18.3 31.0 9.3 50.1 18.3 ± ± ± ± April 2019 24.0 8.5 41.1 17.2 34.9 9.6 53.7 17.6 ± ± ± ± May 2019 40.9 13.9 21.9 7.2 40.2 9.9 44.5 13.6 ± ± ± ± June 2019 41.8 14.3 20.1 8.3 46.1 17.3 41.2 14.7 ± ± ± ± Average 25.6 40.0 33.3 47.9

Generally, indoor radon levels are reported to be the highest in the winter period [9,21,38]. However, the results of this study showed different seasonal variations of radon levels in different premises. Such results could be caused by the difference of the quality of the wall and window insulation in the premises. Therefore, the obtained correlations between the indoor radon levels and meteorological parameters, which are analyzed and discussed below, were different for the different premises of the university building.

3.2. The Dependence of the Indoor Radon Concentrations on the Outdoor Temperature, Indoor Temperature, and Indoor–Outdoor Temperature Diﬀerence The dependencies of the indoor radon concentration on the outdoor and indoor temperature are presented in Figures5 and6, respectively. Sustainability 2020, 12, x FOR PEER REVIEW 8 of 16 Sustainability 2020, 12, x FOR PEER REVIEW 8 of 16 3.2. The Dependence of the Indoor Radon Concentrations on the Outdoor Temperature, Indoor Temperature, 3.2. The Dependence of the Indoor Radon Concentrations on the Outdoor Temperature, Indoor Temperature, and Indoor–Outdoor Temperature Difference and Indoor–Outdoor Temperature Difference SustainabilityThe 2020dependencies, 12, 6174 of the indoor radon concentration on the outdoor and indoor temperature8 of 15 The dependencies of the indoor radon concentration on the outdoor and indoor temperature are presented in Figure 5 and Figure 6, respectively. are presented in Figure 5 and Figure 6, respectively.

Figure 5. Average monthly indoor radon concentration in the premises as a function of the outdoor FigureFigure 5. AverageAverage monthly monthly indoor indoor radon radon concentration concentration in the in premises the premises as a function as a function of the outdoor of the temperature. outdoortemperature temperature..

Figure 6. Average monthly indoor radon concentration in the premises as a function of the Figure 6. Average monthly indoor radon concentration in the premises as a function of the indoor indoorFigure temperature.6. Average monthly indoor radon concentration in the premises as a function of the indoor temperature. temperature. As can be seen from Figures5 and6, depending on the premises, positive or negative correlations As can be seen from Figures 5 and 6, depending on the premises, positive or negative betweenAs indoorcan be radonseen levelsfrom andFigures outdoor 5 and and 6, indoor depending temperature on the were premise obtained.s, positive The indoor or negative radon concentrationcorrelations between in premises indoor No. radon 2 (R2 levels= 0.92, andR =outdoor0.96, pand< 0.01)indoor and temperature No. 4 (R2 =were0.39, obtained.R = 0.62, The correlations between indoor radon levels and outdoor−2 and indoor temperature were obtained.2 − The p indoor radon concentration in premises No. 2 (R = 0.92, R = −0.96, p < 0.01) and No. 4 (R = 0.39, R = indoo= 0.10)r radon tended concentration to decrease when in premise the outdoors No. 2 temperature (R2 = 0.92, R increased = −0.96, p and, < 0.01) similarly, and No. tended 4 (R2 to = decrease0.39, R = R−20.62, p = R0.10) tendedp to decreaseR 2when theR outdoor temperaturep increased and, similarly, tended to (−0.62,= 0.86, p = 0.10)= tended0.93, to< 0.01decrease and when= 0.44, the outdoor= 0.66, temperature= 0.075, respectively) increased and, with similarly, increased tended indoor to decrease (R2 =− 0.86, R = −0.93, p < 0.01 and R2 = −0.44, R = −0.66, p = 0.075, respectively) with increased temperature.decrease (R2 = On 0.86, the R contrary, = −0.93, p for < 0.01 premises and R No.2 = 0.44, 1 and R No.= −0.66, 3, the p = indoor 0.075, radonrespectively) levels increasedwith increased with indoor temperature. On the contrary,R for2 premisesR No. 1 andp No. 3, theR indoor2 radonR levelsp increased theindoor increasing temperature. outdoor On temperature the contrary, ( for =premises0.89, No.= 0.94, 1 and< No.0.01 3, and the indoor= 0.84, radon= levels0.92, increased< 0.01, with the increasing outdoor temperatureR2 (R2 = 0.89,R R = 0.94,p p <0.01 andR2 R2 = 0.84,R R = 0.92,p p < 0.01, respectively)with the increasing and indoor outdoor temperature temperature ( (=R20.84, = 0.89,= R0.92, = 0.94,< p 0.01<0.01 and and R2= =0.82, 0.84, R= = 0.91,0.92, p< < 0.01,0.01, respectively) and indoor temperature (R2 = 0.84, R = 0.92, p < 0.01 and R2 = 0.82, R = 0.91, p < 0.01, respectively).respectively) and In general, indoor whentemperature the outdoor (R2 = 0.84, temperature R = 0.92, increased, p < 0.01 and the indoorR2 = 0.82, temperatures R = 0.91, p < in 0.01, the respectively). In general, when the outdoor temperature increased, the indoor temperatures in the studiedrespectively). premises In general, also increased. when the The outdoor obtained temperature absolute correlation increased, values the indoor between temperatures the indoor radonin the levels and indoor temperature were lower (except for premises No. 4) compared to the correlations between the indoor radon levels and outdoor temperature. This may be due to the fact that the variations of indoor temperatures in the premises were substantially lower compared to the variations Sustainability 2020, 12, x FOR PEER REVIEW 9 of 16

studied premises also increased. The obtained absolute correlation values between the indoor radon Sustainability 2020, 12, 6174 9 of 15 levels and indoor temperature were lower (except for premises No. 4) compared to the correlations between the indoor radon levels and outdoor temperature. This may be due to the fact that the ofvariations outdoor temperature. of indoor temperatures In the study ofin Xiethe et premises al. (2015), were no clear substantially correlations lower were compared found between to the indoorvariations radon of levels outdoor and temperature. indoor temperatures In the study [21]. of Xie et al. (2015), no clear correlations were found betweenFigure indoor7 shows radon the dependencieslevels and indoor of thetemperatures indoor radon [21]. concentration on the indoor–outdoor temperatureFigure di 7ﬀ erence.shows the dependencies of the indoor radon concentration on the indoor–outdoor temperature difference.

Figure 7. Average monthly indoor radon concentration in the premises as a function of the Figure 7. Average monthly indoor radon concentration in the premises as a function of the indoor– indoor–outdoor temperature difference. outdoor temperature difference. As can be seen from Figure7, the indoor radon concentration in premises No. 2 and No. 4 tendedAs to increasecan be seen (R2 =from0.89, FigureR = 0.94, 7, thep < indoor0.01 and radonR2 = concentration0.32, R = 0.57, inp =premises0.14, respectively) No. 2 and withNo. 4 2 2 thetended increasing to increase indoor–outdoor (R = 0.89, R temperature = 0.94, p < 0.01 diff erence.and R = In0.32, contrast, R = 0.57, for p premises = 0.14, respectively) No. 1 and No.with 3,the theincreasing indoor radon indoor levels–outdoor decreased temperature (R2 = 0.89, difference.R = 0.94, In pcontrast< 0.01, and for Rpremises2 = 0.81, No.R = 1 0.90,and pNo.< 0.01,3, the 2 − 2 − respectively)indoor radon with levels the increasing decreased indoor–outdoor (R = 0.89, R = temperature −0.94, p < 0.01 diff erence.and R = 0.81, R = −0.90, p < 0.01, respectively)The obtained with absolute the increasing correlation indoor values–outdoor between temperature the indoor radondifference. levels and the indoor–outdoor temperatureThe obtained difference absolute were slightly correlation lower values compared between to the the absolute indoor correlation radon levels values and between the indoor the – indooroutdoor radon temperature levels and outdoordifference temperature. were slightly The lower outdoor compared temperature to the was absolute a more correlatio dominantn factor values thanbetween the temperature the indoor di radonfference levels in influencing and outdoor the variationtemperature. of the The radon outdoor levels temperature in the premises. was On a more the otherdominant hand, slightlyfactor than lower the correlations temperature could difference be obtained in influencing due to the factthe thatvariation two variables of the radon (indoor levels and in outdoorthe premises. temperature) On the were other measured hand, slightly on an hourlylower correla basis usingtions twocould di ffbeerent obtained instruments. due to the fact that twoThe variables majority (indoor of previous and outdoor studies [temperature)21] found increased were measured indoor radon on an levels hourly in winterbasis using periods two compareddifferent to instruments. warmer seasons. In our study, this tendency was also found in premises No. 2 and No. 4. In mostThe cases, majority the indoor of previous radon concentrations studies [21] found were higher increased in winter, indoor as radon the increased levels in indoor–outdoor winter periods temperaturecompared to di ffwarmererence causedseasons. increased In our study, gas flow this fromtendency the soil was to also buildings. found in In premises contrast, No. in the 2 and other No. two4. premisesIn most cases, (No. 1the and indoor No. 3), radon elevated concentrations radon levels we werere higher found in in winter, the warmer as the period. increased There indoor are – veryoutdoor few studies temperature [18] that dif foundference that cause increasedd increased inside–outside gas flow temperature from the soil di fftoerence buildings. caused Ina contrast, decrease in inthe indoor other radon two concentrations.premises (No. 1 and No. 3), elevated radon levels were found in the warmer period. ThereHubbard are very et al.few (1992) studies studied [18] that the radonfound levelsthat increased in two Swedish inside–outside dwellings temperat and foundure difference that an increasedcaused a indoor–outdoor decrease in indoor temperature radon concentrations. difference caused a decrease in radon concentrations in both studiedHubbard dwellings. et Inal. our(1992) study, studied the di thefferent radon trends levels were in two obtained Swedish in studied dwellings premises and found of the samethat an building.increased In indoor their– study,outdoor Hubbard temperature et al. difference (1992) explained caused a that decrease there arein radon different concentrations mechanisms in forboth howstu thedied variation dwellings. of temperatureIn our study, di thefference different can trends change were the indoorobtained radon in studied concentrations. premises Increasedof the same indoor–outdoorbuilding. In their temperature study, Hubbard difference et al. can (1992) increase explained air flow that through there are the different building mechanisms shell and, for at the how samethe time,variation reduce of the temperature indoor radon difference concentration can (hereafterchange the called indoor the ventilationradon concentrations. effect) or can increaseIncreased the soil-gas entry to a building, therefore increasing the indoor radon concentration (hereafter called Sustainability 2020, 12, x FOR PEER REVIEW 10 of 16

indoor–outdoor temperature difference can increase air flow through the building shell and, at the Sustainabilitysame time,2020 ,reduce12, 6174 the indoor radon concentration (hereafter called the ventilation effect)10 or of 15can increase the soil-gas entry to a building, therefore increasing the indoor radon concentration (hereafter called the radon source effect) [18]. These two mechanisms are not individual thecomponents, radon source as e ffoftenect) [ 18both]. These mechanisms two mechanisms can occu arer with not individualincreased indoor–outdoor components, as oftentemperature both mechanismsdifferences. can occur with increased indoor–outdoor temperature differences. InIn our our studied studied building, building, in in premises premises No. No. 1 and1 and No. No. 3, 3, the the ventilation ventilation eff effectect prevailed prevailed in in winter, winter, whilewhile in premisesin premises No. No. 2, the2, the radon radon source source eff ecteffect prevailed. prevailed. Architectural Architectural style style can can have have a significanta significant influenceinfluence on on indoor indoor radon radon levels levels [25 [25].]. Di Differentfferent trends trends in in the the premises premises of of the the studied studied building building can can be be explainedexplained by dibyff erentdifferent airtightness airtightness attributes attributes of the of premises. the premises. In the winterIn thewith winter a high with indoor–outdoor a high indoor– temperatureoutdoor temperature difference, airdifference, leakage couldair leakage occur could through occur possible through unintentional possible unintentional air leakage pointsair leakage in premisespoints No.in premises 1 and No. No. 3. 1 Duringand No. the 3. warmDuring period, the warm when period, the indoor–outdoor when the indoor–outdoor temperature temperature difference wasdifference similar and was air similar circulation and air through circulation unintentional through leakageunintentional points couldleakage not points occur could at the samenot occur rate, at elevatedthe same radon rate,concentrations elevated radon wereconcentrations detected inwere those detected premises. in those In premisespremises. No. In premises 2, due to No. better 2, due airtightness,to better airtightness, higher indoor higher radon indoor levels radon were levels detected were during detected the during cold season. the cold In season. premises In No.premise 4, bothNo. ventilation 4, both ventilation and radon and source radon eff sourceects could effects occur could since occur the obtainedsince the absoluteobtained correlationabsolute correlation values werevalues the lowest.were the Symonds lowest. etSymonds al. (2019) et and al. Meyer(2019) (2019)and Meyer found (2019) that, in found houses that, with in better houses airtightness with better andairtightness higher energy and performance higher energy characteristics, performance higher charac indoorteristics, radon levelshigher were indoor obtained radon compared levels were to housesobtained with compared lower energy to houses performances with lower [39 ,energy40]. performances [39,40]. 3.3. The Dependencies of the Indoor Radon Concentrations on the Outdoor and Indoor Relative Humidity 3.3. The Dependencies of the Indoor Radon Concentrations on the Outdoor and Indoor Relative Humidity The dependencies of the indoor radon concentration on the outdoor relative humidity are The dependencies of the indoor radon concentration on the outdoor relative humidity are presented in Figure8. presented in Figure 8.

Figure 8. Average monthly indoor radon concentration in the premises as a function of the outdoor Figure 8. Average monthly indoor radon concentration in the premises as a function of the outdoor relative humidity. relative humidity. Outdoor relative humidity decreases with increasing outdoor temperature. As can be seen Outdoor relative humidity decreases with increasing outdoor temperature. As can be seen from from Figure1, the measured outdoor relative humidity was inversely proportional to the outdoor Figure 1, the measured outdoor relative humidity was inversely proportional to the outdoor temperature, and therefore the obtained correlations were opposite compared to the correlations temperature, and therefore the obtained correlations were opposite compared to the correlations between the indoor radon levels and outdoor temperature. As shown in Figure8, for premises No. 2 between the indoor radon levels and outdoor temperature. As shown in Figure 8, for premises No. 2 and No. 4, the radon concentrations increased (R2 = 0.79, R = 0.89, p < 0.01 and R2 = 0.16, R = 0.40, and No. 4, the radon concentrations increased (R2 = 0.79, R = 0.89, p < 0.01 and R2 = 0.16, R = 0.40, p = p = 0.33, respectively) with increasing outdoor relative humidity, while for premises No. 1 and No. 3 0.33, respectively) with increasing outdoor relative humidity, while for premises No. 1 and No. 3 (R2 (R2 = 0.80, R = 0.89, p < 0.01 and R2 = 0.64, R = 0.80, p < 0.05 respectively), the radon levels decreased. = 0.80, R = −−0.89, p < 0.01 and R2 = 0.64, R = −0.80, p < 0.05 respectively), the radon levels decreased. The absolute values of the correlations obtained between the indoor radon levels and outside relative The absolute values of the correlations obtained between the indoor radon levels and outside humidity were lower than the absolute correlation values between the indoor radon concentration relative humidity were lower than the absolute correlation values between the indoor radon and the outdoor temperature; therefore, in this study, outdoor temperature was the dominant factor determining indoor radon concentrations. Outdoor relative humidity could also be inﬂuenced by Sustainability 2020, 12, x FOR PEER REVIEW 11 of 16 Sustainability 2020, 12, 6174 11 of 15

concentration and the outdoor temperature; therefore, in this study, outdoor temperature was the varyingdominant amounts factor of dete precipitationrmining indoor during radon the study concentrations. period. In theOutdoor scientiﬁc relative literature, humidity contradictive could also be diﬀerencesinfluenced of absoluteby varying values amounts of correlations of precipitation between during indoor the study radon period. levels andIn the outdoor scientific variables literature, (i.e.,contradictive temperature differences and relative of humidity) absolute values are provided of correlations and either between of them indoor can be radon stronger levels depending and outdoor on thevariables location (i.e., and temperature the building and [11 ].relative humidity) are provided and either of them can be stronger dependingThe indoor on relative the location humidity and isthe dependent building on[11] many. factors, such as the outdoor air temperature and outdoorThe relativeindoor humidity.relative humidity The dependencies is dependent of indoor on many radon factors, concentrations such as and the the outdoor indoor air relativetemperature humidity and are outdoor presented relative in Figure humidity.9. The dependencies of indoor radon concentrations and the indoor relative humidity are presented in Figure 9.

FigureFigure 9. Average 9. Average monthly monthly indoor indoor radon radon concentration concentration in the in premisesthe premises as a as function a function of the of indoorthe indoor relativerelative humidity. humidity. The indoor relative humidity had a relatively lower variation during the study period compared The indoor relative humidity had a relatively lower variation during the study period to the variation of outdoor relative humidity. The radon concentrations correlated positively with compared to the variation of outdoor relative humidity. The radon concentrations correlated the indoor relative humidity in premises No. 1 (R2 = 0.34, R = 0.58, p = 0.13) and No. 3 (R2 = 0.56, positively with the indoor relative humidity in premises No. 1 (R2 = 0.34, R = 0.58, p = 0.13) and No. 3 R = 0.75, p < 0.05), i.e., in premises where, during winter, possible increased air exchange could occur (R2 = 0.56, R = 0.75, p < 0.05), i.e., in premises where, during winter, possible increased air exchange through air leakage points. The negative correlation values (R2 = 0.41, R = 0.64, p = 0.087) between could occur through air leakage points. The negative correlation values− (R2 = 0.41, R = −0.64, p = the indoor radon concentration and indoor relative humidity were obtained in premises No. 2 and 0.087) between the indoor radon concentration and indoor relative humidity were obtained in No. 4, with better airtightness and where the indoor radon levels were elevated during the cool period. premises No. 2 and No. 4, with better airtightness and where the indoor radon levels were elevated Negative indoor radon correlations with indoor humidity (in buildings where higher indoor radon during the cool period. Negative indoor radon correlations with indoor humidity (in buildings levels appeared during the autumn–winter season) were also observed by Xie et al. (2015) [21]. where higher indoor radon levels appeared during the autumn–winter season) were also observed During winter, the ventilation effect through leakage points in premises No. 1 and No. 3 was by Xie et al. (2015) [21]. confirmed by the obtained indoor relative humidity values for that premises. The indoor relative During winter, the ventilation effect through leakage points in premises No. 1 and No. 3 was humidity strongly depends on the outdoor meteorological parameters (temperature and outdoor confirmed by the obtained indoor relative humidity values for that premises. The indoor relative humidity) and on how much air comes in to the premises from outside. During the winter, when cold humidity strongly depends on the outdoor meteorological parameters (temperature and outdoor outdoor air enters warm places (building), the relative humidity drops substantially. During the study, humidity) and on how much air comes in to the premises from outside. During the winter, when the lowest monthly outdoor temperature was obtained in January (Figure1); therefore, the indoor cold outdoor air enters warm places (building), the relative humidity drops substantially. During the relative humidity in the studied premises was the lowest. study, the lowest monthly outdoor temperature was obtained in January (Figure 1); therefore, the In January, the indoor relative humidity in premises No. 1 reached 29.1% and in premises No. 3 indoor relative humidity in the studied premises was the lowest. reached 29.2%. The low values of indoor humidity in these premises in January indicate that, during In January, the indoor relative humidity in premises No. 1 reached 29.1% and in premises No. 3 winter, the air from outside entered the premises and resulted in the ventilation effect. Since the reached 29.2%. The low values of indoor humidity in these premises in January indicate that, during outdoor absolute air humidity is low during the winters, the entering of air into the premises caused a winter, the air from outside entered the premises and resulted in the ventilation effect. Since the decrease in the indoor relative humidity. In January, the monthly indoor relative humidity in premises outdoor absolute air humidity is low during the winters, the entering of air into the premises caused No. 2 was 40.8%, indicating lower (compared to premises No. 1 and No. 3) amounts of fresh air a decrease in the indoor relative humidity. In January, the monthly indoor relative humidity in entering from outside during winter. Therefore, the increased radon gas flow during winter prevailed premise No. 2 was 40.8%, indicating lower (compared to premises No. 1 and No. 3) amounts of fresh air entering from outside during winter. Therefore, the increased radon gas flow during winter SustainabilitySustainability2020 2020, 12, 1,2 6174, x FOR PEER REVIEW 1212of of 15 16 Sustainability 2020, 12, x FOR PEER REVIEW 12 of 16 prevailed in that premise. In premise No. 4, where both ventilation and radon source effects could inoccur,prevailed that premises.in January, in that In premise.the premises obtained In No.premise monthly 4, where No. indoor 4, both where humidity ventilation both valueventilation and reached radon and 34.6%. source radon esourceffectscould effects occur, could inoccur, January,The in January, theobtained obtained the correlations obtained monthly monthly indoorbetween humidity indoor the indoorhumidity value reachedradon value concentrations 34.6%.reached 34.6%. and indoor relative humiditTheThe obtainedy wereobtained weaker correlations correlations compared between between to the the correlations indoor the indoor radon between concentrationsradon theconcentrations radon and concentrations indoor and relative indoor and humidity outdoorrelative wererelativehumidit weaker humidity,y were compared weaker except tocompared thefor premise correlations to the No. correlations between4, which thewas between radon rarely the concentrations used. radon In concentrationspremise and No. outdoor 4, andcorrelations relative outdoor humidity,betweenrelative humidity, exceptthe indoor for except premisesradon forlevels No. premise and 4, which indoor No. was4, environmental which rarely was used. rarely Invariables premises used. weIn No. repremise stronger 4, correlations No. compared 4, correlations between to the thecorrelationsbetween indoor the radon betweenindoor levels radon the and indoor indoorlevels radon and environmental indoor levels environmental and variables outdoor were environmental variables stronger we comparedre variables. stronger to thecompared correlations to the betweencorrelations the indoor between radon the levelsindoor and radon outdoor levels environmental and outdoor environmental variables. variables. 3.4. The Dependence of the Indoor Radon Concentrations on the Outdoor and Indoor Pressure 3.4.3.4. The The Dependence Dependence of of the the Indoor Indoor Radon Radon Concentrations Concentrations on on the the Outdoor Outdoor and and Indoor Indoor Pressure Pressure The dependencies of the indoor radon concentrations on the indoor and outdoor pressure are presentedTheThe dependencies dependencies in Figures 10 of andof the the 11. indoor indoor radon radon concentrations concentrations on on the the indoor indoor and and outdoor outdoor pressure pressure are are presentedpresented in in Figures Figures 10 10 and and 11 11..

FigureFigure 10. 10. AverageAverage monthly monthly indoor indoor radon radon concentration concentration in the in premises the premises as a function as a function of the outdoor of the outdoorpressure.Figure pressure.10. Average monthly indoor radon concentration in the premises as a function of the outdoor pressure.

Figure 11. Average monthly indoor radon concentration in the premises as a function of the indoorFigure pressure. 11. Average monthly indoor radon concentration in the premises as a function of the indoor pressure.Figure 11. Average monthly indoor radon concentration in the premises as a function of the indoor Wepressure. observed that there were no significant correlations between the indoor radon concentration and theWe outdoor observed and indoorthat there air pressurewere no in premisessignificant No. correlations 1 and No. 2.between However, the strong indoor positive radon concentrationWe observed and the that outdoor there and were indoor no air significant pressure in correlationspremises No. between1 and No. the2. However, indoor strongradon concentration and the outdoor and indoor air pressure in premises No. 1 and No. 2. However, strong Sustainability 2020, 12, 6174 13 of 15 correlation coefficients between the indoor radon concentration and outdoor and indoor pressure (R = 0.71, p = 0.05 and R = 0.70, p = 0.053, respectively) were obtained in premises No. 3, where the ventilation effect did occur in winter. Negative correlation coefficients (R = 0.57, p = 0.14 and R = 0.59, − − p = 0.12) between the indoor radon concentrations and the outdoor and indoor pressure were obtained in premises No. 4. Kitto (2005) and Rowe et al. (2002) [23,41] also observed that pressure had little effect on the indoor radon levels, while Ramola et al. (2000) observed different correlation values between the indoor radon levels and atmospheric pressure in different houses [11]. In this study also, no clear correlations between the indoor radon levels and outdoor and indoor pressures were obtained. The relationship between the indoor radon levels and indoor–outdoor pressure difference was analyzed; however, no significant correlations (R < 0.25 in all cases) were obtained.

4. Conclusions The present study reports the results of indoor radon level dependencies on various outdoor and indoor environmental parameters for four separate premises of a university building. We found that the indoor radon levels was mostly influenced by the outdoor temperature. The obtained absolute values of correlations between the indoor radon levels and outdoor temperature were the highest compared to the other studied correlations between the indoor radon concentrations and meteorological parameters. In the premises that was rarely used and ventilated, little seasonal radon variation was observed. In this premises, the indoor radon levels were mostly influenced by the indoor temperature and indoor relative humidity. The research results also showed that the indoor radon variation trends were different in different premises of the same building. Differing characteristics of each premises, such as the airtightness, can result in different correlations between the indoor radon level and outdoor temperature. In premises where possible cracks or unintentional air leakage points occurred, positive correlations between the indoor radon concentrations and outdoor temperature were obtained, while in premises with good airtightness, the obtained correlations between the indoor radon level and outdoor temperature were negative. The study results indicate that, in educational premises with poorer air isolation, higher indoor radon concentration levels can be obtained in warm seasons compared to the winter period. During winter in these premises, a higher rate of natural ventilation could occur through various air leakage points due to the higher indoor–outdoor temperature difference. However, during the summer season, when the air is hot and stale, higher indoor radon levels could occur, thus creating lower air quality in educational premises. In such premises, we recommend air conditioners in the summer season to lower the indoor temperature and increase the air ventilation rate. In premises with better airtightness, elevated indoor radon levels, especially in the winter season, can be detected. In these buildings, mechanical air ventilation may be recommended during the winter season. This research is subject to some limitations. The number of premises in which the radon measurements were performed was small, and the seasonal indoor radon level variations were determined in only one educational building, which was built in 1985. Therefore, future research is needed to analyze the relationships between radon concentrations and meteorological parameters in premises of newly built or renovated energy-efficient educational buildings.

Author Contributions: Conceptualization, P.B. and R.G.; methodology, V.D. and R.G.; formal analysis, R.G.; investigation, V.D.; data curation, P.B.; writing—original draft preparation, V.D.; writing—review and editing, P.B. and R.G.; supervision, P.B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sustainability 2020, 12, 6174 14 of 15

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