Study on indoor air radon measurement in a university in Shanghai

: With the improvement of people's requirements for living environment, the effect of indoor radon radioactivity on health has been widely concerned. In this paper, taking a university in Shanghai as an example, indoor radon concentration and its related environmental parameters were measured in some of its houses to understand the level of indoor radon concentration in its campus. The results show that the average concentrations of student dormitory, office building, and academic building are 14.79±9.15 Bq/m 3 ,22.67±5.72 Bq/m 3, and 12.55±6.71 Bq/m 3 respectively. indoor radon is positively correlated with indoor temperature and humidity, and ventilation is also an effective method to reduce indoor radon.


Introduction
Radon is a widespread natural radiation source, and the radiation dose generated by the decay of radon gas alpha particles and their daughter bodies can cause radiation damage to the human respiratory system [1~3] . In daily life, the radiation dose generated by radon and its daughter decay accounts for about 50% of the total radiation dose generated by natural radiation sources [4] . Meanwhile, radon has become the second most carcinogenic factor after smoking.
Modern research shows that long-term exposure to indoor radon concentration will increase the risk of lung cancer. In addition, radon may trigger malformation, genetic malformation inheritance, leukemia, and other adverse consequences. Therefore, the effect of radon concentration in the indoor environment on human health has become a major problem that needs to be solved in the theoretical field [5] . Through the survey, it was found that the study of indoor radon on campus is still immature. To better understand the indoor radon concentration levels and factors affecting indoor radon in some buildings on campus, three typical buildings with relatively dense populations in colleges and universities were selected in this paper [6~7] .

Methods
The measurements located in three types of rooms. A total of 59 rooms were tested during one and half year measurment. The samples included indoor and outdoor radon concentrations, indoor and outdoor air temperature and relative humidity, and carbon dioxide. Parameters for the building characteristics included room geometry, number of floors, and air exchange rate calculated by CO 2 [8~9] . In addition, occupant conditions and activities were recorded for 1 hour during each * Corresponding author：chunxiao.SU@usst.edu.cn sampling period. In the actual measurement, the inhalation mode was adopted, in which the outdoor air radon concentration was measured in the first and the last hour. The suction port was placed near the window and 1.2 meters above the ground.Each measurement was made for 10-12 hours and the room is kept in normal use during the measurement period.
Radon gas measurement adopted RAD7 radon meter (SURRIDGE, USA) with a correction coefficient of 1.The measurement period was 1h. The number of measurement periods was 8. The measurement mode was set to AUTO. Indoor and outdoor temperature and relative humidity adopted Jianke Renda C0S-03, and a set of data was measured every 5min. The CO2 exhaled by the human body was collected with Xinster HT-2000 and converted to respiration rate according to the number of people in the room per hour [10~12] .

Indooor radon concentration
It can be seen from the figure that the log indoor radon concentration is normally distributed, and the maximum indoor radon value is 65.5 Bq/m³, the median is 12.93 Bq/m³, and the arithmetic mean and geometric mean are 14.68±12.41 Bq/m³.In this study, a total of 59 rooms were tested, including 47 student dormitories, 2 office buildings, and 10 academic buildings. Their average values were 14.79±9.15 Bq/m 3 , 22.67±5.72 Bq/m 3 , and 12.55±6.71 Bq/m 3 , respectively. The results showed that the tested values were all lower than the standard limit value [13] of 150 Bq/m 3 , probably because the outdoor soil radon concentration in Shanghai is lower and the indoor radon infiltration rate is less, resulting in the generally lower indoor radon concentration in Shanghai.

Seasonal changes and indoor radon
In this paper, three different room types with high daily utilization rates and high mobility of students on college campuses are selected as the research objects. Their indoor radon concentrations were measured, and the results were shown in Figure 2.

Fig.2. Indoor radon concentration at different seasons
Since in spring, the second test is mainly conducted on the previously measured rooms to determine the reasonableness of the values, the effect of season on indoor radon is not included in spring. It can be seen from Figure 2 that the indoor radon concentrations in summer and winter are higher than that in the transitional season, probably because the air conditioners are closed in the transitional season and the windows are opened for a longer period, resulting in a lower radon concentration. In summer and winter, because the air conditioners are opened, radon and its daughters will accumulate a lot, making the radon concentration high.

Building type
The test sites include three typical buildings with the average values of 14.79±9.15 Bq/m 3 , 22.67±5.72 Bq/m 3 , and 12.55±6.71 Bq/m 3 respectively. The indoor radon concentrations in the student dormitories gradually accumulates as the test time is extended because it is conducted at night. The radon concentrations in the teaching buildings are high or low because of a large number of personnel activities and the accompanying going to and coming from classes. During the whole testing process, the rooms are closed without strict limitation and the airflow in the room is not strictly controlled, so the test value only shows the normal indoor radon content.

Fig.4. Indoor radon concentration on different floors
The sources of indoor radon concentrations are mainly the building foundation, surrounding soil, and old building materials. For the rooms higher than 3rd floor, the contribution to indoor radon exposure by soil was considered as extremely low and neglected. It can be seen in the figure 4 that the indoor radon concentration gradually decreases on the 1st to 3rd floors. This supported our assumption on the soil infiltration. The indoor radon concentration gradually decreases on the 4th to 6th floor but not as significant as those from 1st to 3rd floor.

Indoor air temperature and relative humidity, air change rate
In this paper, the relationship between indoor radon and air temperature, relative humidity, and ventilation frequency was investigated by single-factor and Spearman correlation analysis. The results are shown in Table 1. It can be seen from the table that indoor air temperature and relative humidity are positively correlated with indoor radon concentration, ventilation and air change rates are negatively correlated with indoor radon concentrations, and ventilation and air change rates have obvious effects on reducing indoor radon concentrations. The air change rates are the main factor affecting indoor radon, and the average value is 4.6 t/h. It can be seen from the figure that air change rates are negatively correlated with indoor radon, of which about 1/2 is located in 0~1 times/h, which is because most people do not have the habit of opening windows for a long time. The air change rates are higher than 5 times/h but the indoor radon concentrations are higher, probably because although the windows are opened for ventilation, more people cause the temperature to indirectly leads to the increase of radon concentration.

Conclusion
To investigate the harm of radon radioactivity to the human body, this sampling was carried out on campus. After this test, we got the following conclusions: (1) The indoor and outdoor air radon measurement data are lower than the limit value and also lower than the national average level of indoor radon. (2) Indoor radon has a positive correlation with indoor humidity and carbon dioxide, but a significant negative correlation with air exchange rate. (3) Through testing, indoor radon distribution and its influencing factors are explored, and guidance for improving indoor radon environment is proposed.