The frequency distribution of the indoor radon concentrations was given in Fig. 4. It is fitting to a lognormal distribution.
The present data analysis showed that the indoor radon activity concentration varied from 6.5-151.1 Bq m− 3 with a mean value of 18 Bq m− 3 during the study period (Table 1). UNSCEAR 2000 has reported 40 Bq m− 3 as the arithmetic means for the distribution of indoor radon concentration. The European Commission proposed that the action level is 200 Bq m− 3 for new buildings (Clouvas et al. 2011). Turkey Atomic Energy Authority published a directive on radiation safety which includes the action level of 400 Bq m− 3 and 1000 Bq m− 3 for houses and workplaces, respectively (TAEK 2000). It is seen that the radon data is lower than the national and international reference values. This may be because the building is newly built, the room is well ventilated during the day, and many other reasons. It is well known that the primary factors that affect indoor radon concentrations are ventilation, season, height, building, age, and building material (Nazaroff et al. 1989). The concentrations are expected to be relatively low since the office where the measurements are made is located on the second floor.
Table 1
Basic descriptive statistics for Radon and meteorological parameters
| 222Rn (Bq m− 3) | Temperature (°C) | Air Pressure (mbar) | Relative Humidity (%) |
Total | 18744 | 18744 | 18744 | 18744 |
Number of data | 14652 | 18744 | 18744 | 18744 |
Missing | 4092 | - | - | - |
Min | 6.5 | 16.7 | 987.9 | 12.6 |
Median | 13.9 | 23.7 | 1010.1 | 37.9 |
Mean | 18.0 | 23.7 | 1010.1 | 37.8 |
Max | 151.1 | 32.5 | 1034.0 | 66.0 |
Std. Dev. | 12.23 | 2.0 | 10.5 | 5.9 |
Skewness | 2.0 | 2.01 | -0.82 | 0.18 |
Kurtosis | 8.3 | 0.06 | 0.04 | 0.27 |
Diurnal Indoor Radon Variations
This section considers discussions on the diurnal variations of indoor radon concentration at the teaching staff office. The diurnal indoor radon concentrations vary from 3.3 to 37.7 Bq m− 3. In Fig. 5, a violin chart is plotted to show the variation of hourly radon concentrations. It is seen in the graph that the extreme values increase due to the emergence of outliers, especially in the morning hours. It was noteworthy that indoor radon concentrations changed at different times of the day.
Additionally, concentrations are measured slightly higher in the morning (downtime and early hours of the day) and then decreased in the afternoon. On the other hand, it can be seen from the graph that the distributions differ significantly at different times of the day. The shape of the violin becomes thinner and longer between 3 and 9 am, while at other hours, the shape of the violin becomes more rounded. This difference may be due to office hours. The office remains closed at night and midnight without any regular ventilation, and radon concentrations are likely to be measured over a broader range. Pant et al. (2016) reported an increase in radon concentration from evening to early morning until it reaches its maximum value. Murty et al. (2010) mentioned that radon concentrations are higher in the early morning hours and decrease after that and reach the minimum value during early afternoon hours. Moreover, they pointed out that Radon is strongly dependent on atmospheric pressure, relative humidity, and air temperature.
The median and mean values were not equal for most of the hours. The distribution spread with the increase of outliers, especially in the morning hours. The slow variation of indoor radon (higher in downtime, and early hours of the day and then decreased in the afternoon) throughout the measured period shows similarity to the results of some researchers (Murty et al. 2010; Pal et al. 2015; Chen et al. 2016; Pant et al. 2016; Xie et al. 2017). This variation may be affected by the ventilation differences and the gap between indoor and outdoor temperatures.
Monthly Variations Of Indoor Radon And Meteorological Parameters
The monthly average of indoor radon concentrations, air temperature, relative humidity, and air pressure obtained within the 25 months are presented in Table 2. The monthly variations of each parameter were calculated by calculating the average of total monthly data. As can be seen from Table 2, indoor radon concentrations decrease in the spring and summer months of both years (2019 and 2020). This decline is apparent in June and July. It is thought that air circulation in the room increases with air conditioners in the office with the increasing temperatures. This view is consistent with the fact that there is no significant increase in office temperatures as in outdoor temperatures. Likewise, the humidity values measured inside the room during these months also increase.
On the other hand, concentrations are higher in November and December compared to other months. During these periods, the lack of air conditioners, the fact that the office's central heating starts to work, and the fact that the doors and windows are less open may be the high levels. Researchers suggest that the seasonal patterns of Radon might change depending on the location of the measurements and their surroundings (Hirsikko et al. 2007; Victor et al. 2019).
Table 2
The monthly average of indoor Radon and meteorological parameters
| 222Rn (Bq m− 3) | Temperature (°C) | Humidity (%) | Pressure (mbar) |
January 2019 | 19.1 | 24.7 | 25.8 | 1007.8 |
February 2019 | 15.7 | 24.3 | 25.5 | 1014.4 |
March 2019 | 14.6 | 24.3 | 26.8 | 1012.1 |
April 2019 | 16.0 | 23.9 | 28.7 | 1010.9 |
May 2019 | 14.5 | 23.7 | 36.2 | 1008.6 |
June 2019 | 14.9 | 22.3 | 47.9 | 1007.3 |
July 2019 | 16.6 | 22.7 | 45.4 | 1004.6 |
August 2019 | 15.4 | 23.9 | 45.9 | 1005.8 |
September 2019 | 14.1 | 24.3 | 45.7 | 1009.0 |
October 2019 | 20.1 | 22.5 | 52.6 | 1012.2 |
November 2019 | 17.9 | 22.9 | 46.7 | 1012.6 |
December 2019 | 16.2 | 24.0 | 33.8 | 1012.4 |
January 2020 | 17.8 | 23.7 | 23.8 | 1018.0 |
February 2020 | 15.0 | 23.9 | 27.4 | 1013.9 |
March 2020 | 18.9 | 22.8 | 29.8 | 1010.9 |
April 2020 | 22.5 | 19.3 | 37.6 | 1009.8 |
May 2020 | 21.4 | 23.9 | 41.2 | 1010.6 |
June 2020 | 15.8 | 27.2 | 40.7 | 1006.2 |
July 2020 | 15.0 | 26.9 | 42.7 | 1004.1 |
August 2020 | 17.6 | 24.3 | 48.3 | 1004.2 |
September 2020 | 15.3 | 24.7 | 53.7 | 1008.4 |
October 2020 | 27.1 | 22.6 | 50.9 | 1011.9 |
November 2020 | 23.6 | 23.3 | 31.3 | 1016.3 |
December 2020 | 21.3 | 24.0 | 32.6 | 1013.8 |
Seasonal Variations Of Indoor Radon And Meteorological Parameters
Since the study began in January, the 2019 winter season was calculated by taking the average of January and February. Similarly, the winter season of 2021 is calculated by taking the average of December 2020 and January 2021. Therefore, two-month data were made for winter season calculations for 2019 and 2021. The other seasons were calculated using the three months' data that formed those seasons. The seasonal variation of indoor radon concentrations is figured in a ridgeline chart (Fig. 6) which allows for studying the distribution of a numeric variable for several groups.
The Ridgeline chart shows that indoor radon concentrations vary in the broader range in autumn and winter than in other seasons. Maximum radon levels in spring and summer do not exceed 100 Bq m− 3. It can be seen from the graph that the seasonal distributions are almost similar, but the distributions do not fit the normal distribution. In addition, the distributions from a fluctuating graph consist of several peaks and different distributions overlapping. An exciting outcome of this chart is a comparison of the consecutive years. Although the concentration ranges and the shapes of the distributions are similar, radon concentrations showed sudden increases and decreases in 2020. Therefore, in 2020, it was observed that the distribution did not have a single peak but more than 4 small peaks. This dynamism continued in the winter season of 2021 as well.