Measurement of Radon Concentration in Selected Houses in Ibadan, Nigeria

Radon is a natural radioactive gas without colour or odour and tasteless. The World Health Organization (WHO) grouped radon as a human lung carcinogen. For this reason, there has been a lot of interest on the effects of radon exposure to people all over the world and Nigeria is no exception. The aim of this study is to investigate the radon concentration in selected houses in three local government areas of Ibadan. The indoor radon was measured in both mud and brick houses. Fifty houses were considered from the three Local government areas. A calibrated portable continuous radon monitor type (RAD7) manufactured by Durridge company was used for the measurement. A distance of 100 to 200 m was maintained between houses in all the locations. The living room was kept closed during the measurements. The mean radon concentration measured in Egbeda is 10.54 ±1.30 Bqm-3; Lagelu is 16.90 ± 6.31 Bqm-3 and Ona-Ara is 17.95 ± 1.72 Bqm-3. The mean value of the annual absorbed dose and annual effective dose for the locations in the three local government areas was 0.19 mSvy-1and 0.48 mSvy-1respectively. The radon concentration for location 10 in Ono-Ara local government exceeded the recommended limit. However, the overall average indoor radon concentration of the three local governments was found to be lower than the world average value of 40 Bqm-3. Hence, there is need for proper awareness about the danger of radon accumulation in dwelling places.


Introduction
Radon is a natural radioactive gas that is derived from the decay of 238 U. It exists ubiquitously, small in degree to be noticed but can only be measured using a detector. 238 Uranium is available in soil and rocks in limited quantity. It slowly disintegrates to other compounds like radium, which later turns to radon. Part of the radium migrates to the soil surface and finds their ways to the air, while other part remains at the sub-surface and penetrates the ground water [1,2]. Admit the radioisotopes that add to general background radiation, radon posed major risk to human health. About 55% of the yearly radiation dose incurred by the general public can be traced to it [3]. Alpha particles are heavy particles and can only move a short distance. It cannot permeate through our skin but it can be inhaled and settled in the lung tissue. The energy deposition in lungs is highly localized and this will likely elevate the risk of lung cancer [4]. The International Agency for Research on Cancer (IARC) and the World Health Organization (WHO) categorized it as a human lung carcinogen [5,6]. The ICRP suggests that all the countries of the world should carry out radon survey to find radon prone areas [7]. Due to dearth of information about the concentration and harmful effects [8,9] of radon in Nigeria, lot of people are working and living in places unaware of the dangers posed to their lives. Concentration of radon gas in dwellings has been found to depend on geological condition, meteorological condition, construction materials and ventilation [10]. The buildings in the study areas were built from cement, concrete, mud, brick, granite etc, thus it is nearly impossible not to detect activity of radon in the areas. Also, there is a large variability in indoor concentration levels between different countries, even between areas in the same country, due to variations in the climatic parameters, geology of the subsoil, and building characteristics [11]. 222 Rn exposure levels in large buildings such as commercial buildings, schools and multiunit residential structures may be different from exposure patterns in detached houses due to difference in operation going on in the building [3]. There is no specific regulation or safe limit value in Nigeria concerning indoor radon levels in either homes or workplaces. Therefore, it is necessary to take measurement with the RAD 7 radon detector and other passive detector [12] to measure radon levels inside homes. Hence, the aim of this study is to investigate the concentration of radon in homes (both mud and brick houses) in Egbeda, Lagelu and Ona-Ara local government areas of Ibadan, Oyo State, Nigeria and the results will be used to estimate the annual effective dose for the homes dwellers.

Materials and Method
Ibadan is located on latitude 7.39° N and longitude 3.9° E, southwestern Nigeria. It has mud houses in rural areas and brick houses in both rural and urban area. The elevation of the town ranges from 150 m in the valley to 275 m above sea level. It has eleven local governments out of which three were selected for this study. Rn can enter the ionization chamber. A personal computer (PC) Software (CAPTURE) allows a graphic presentation and calculation of the average concentration in the measured period. The measurements were carried out in fifty houses from the three locations: twenty were taken from Lagelu local Government (all brick houses), twenty five were taken from Ona-Ara local Government (fifteen mud houses and ten brick houses) and five mud houses were taken from Egbeda local Government. 100 to 200 m was maintained from one house to the other in all the locations. For all runs, the RAD7 was positioned on the table and on the floor in the center of the living room ( Figure  1). The living room was kept closed during the measurements. The average radon concentration was calculated by the system in RAD7 for every cycle (20 min), displayed on LCD and a printer printed out the reading.

Dose Estimation
The indoor radon concentrations measured in each location of the study were used to estimate the annual effective doses and annual absorbed dose. The annual absorbed dose was calculated using equation 1.
where D is the annual absorbed dose (mSvy -1 ); C Rn is the radon concentration (Bqm -3 ); D c is the dose conversion factor (9.0 ×10 -6 mSvh -1 per Bqm -3 ); F is the equilibrium factor (0.4); T is hours in a year (8760); H is the occupancy factor (0.4). The annual effective dose was calculated by using equation 2.
where E is the annual effective dose; D is the annual absorbed dose (mSvy -1 ); W R is the radiation weighting factor for alpha particles (20) according to [1]; W T is the tissue weighting factor for the lung (0.12) according to [1].

Results and Discussion
The radon concentration measured, estimated annual effective dose and annual absorbed in the study areas are presented in Table 1-3. The concentration of radon was found to vary from one location to the other. Figure 1 is the pictorial presentation of the radon concentration in the three local government areas. concentration of the radon was found to have the highest value in location 10 (which is mud house) with the mean radon concentration of 42.54 ± 1.32 this was found to exceed the recommended limit of 40 Bqm -3 . This may be because the measurement was done in the night when all the windows were closed and this increased the radon concentration level as they were trapped due to decrease in the cross ventilation.  The overall mean value of the indoor radon concentration in Ona-Ara was 17.95 Bqm -3 . In Ona-Ara it was found that radon concentration measured in some mud houses and the mean was higher when compared to those obtained in brick houses. Table 2 displayed measured concentration in Egbeda local Government. The highest radon concentration in Egbeda was obtained in location 1 with a mean value of 13.46 ± 4.43 Bqm -3 while the lowest value was obtained in location 3 with the average value of 8.76 ± 0.01 Bqm -3 . The highest value may be attributed to type of ventilation in location 1 the window is small and there is very low ventilation as a result of this, the radon concentration levels accumulate, while in location 3 there is a good ventilation in which the radon concentration mixed with the air to reduce the radon concentration level. The overall mean value of the indoor radon concentration in Egbeda was 10.45 Bqm -3 .  Table 3 displayed the measured concentration in Lagelu local Government, the highest radon concentration was found in location 8 with the mean concentration of 28.76 ± 1.44 Bqm -3 while the lowest value was in location 3 with the average value of 3.67 ± 4.45 Bqm -3 . The high value of radon concentration in location 8 may be due to the age of the building, due to crack on the ground which is likely to be one of the causes of increase in the concentration. Also in location 3, the reading were taken at the first floor of the building, the elevation of the building can also contribute to the decrease in radon concentration level. The overall mean value of indoor radon concentration in Lagelu was 16.90 Bqm -3 .
The radon concentration obtained in Ono-Ara local government was quite high when compared to the other two local governments which may be as result of the materials used in the construction of the houses and the mode of the construction of the building as the sizes of the window in this location was small compared to other locations. Meanwhile, the overall average radon concentration from the three (3) local Government Areas in Ibadan is 15.10 ± 3.11 Bqm -3 , which is lower than the world average value of 40 Bqm -3 [4,13]. The radon concentration obtained in these locations compared well with studies conducted by [14] where the concentration measured was 52 Bqm -3 , 28 Bqm -3 and 17Bqm -3 for Japan, Pakistan and Kuwait respectively. The correlation coefficient of Lagelu brick and Egbeda mud houses; Egbeda mud and Ona-Ara brick houses; Lagelu brick and Ona-Ara mud houses and Lagelu brick and Ona-Ara brick houses values are 0.45, 0.09, -2.26 and -0.28 respectively. The negative means the correlation coefficient between them are extremely low. The correlation coefficient value between the three locations is -5.53 and this means there is no correlation between them. The mean annual absorbed dose calculated was 0.13 mSvy -1 , 0.23 1  mSvy and 0.21 mSvy -1 for Egbeda, Ona-Ara and Lagelu respectively. Since the annual effective dose depends on mean indoor radon concentrations, the location with highest mean indoor radon concentration also has the highest value of the effective dose. The estimated mean annual effective was of 0.38 mSvy -1 , 0.54 mSvy -1 and 0.51 mSvy -1 for Egbeda, Ona-Ara and Lagelu respectively. Locations 1, 9, 10 and 11 exceeded the recommended limit of 1 mSvy -1 . However, the average annual effective dose obtained for the three study areas is below 1 mSvy -1 the world average value for normal background radiation.
The results obtained from RAD7detector showed that, the average radon concentration for the 3 local government areas surveyed in Ibadan was 15.10 ± 3.11 Bqm -3 which is lower when compared to the measurement reported by [9] conducted in some Secondary Schools in Oke-Ogun area of Oyo State with average radon concentration of 45 ± 27 Bqm -3 . This study revealed that proper ventilation in a building reduces the level of radon that will be recorded in a particular building, the more the ventilation the more the air enters into the room, the lower the radon level. The lifestyle of the individual living in this building is also important in some of these places people smoked cigarratte which increased the radon level. The floor of the room equally plays some role as observed here. The rooms in ground floor have higher value than those in first floor. The age of a building is equally important in the concentration obtained.

Conclusion
The first radon measurement in Lagelu, Egbeda and Ona-Ara Local Government Areas, Ibadan Southwestern Nigeria was conducted using RAD7. The mean value of indoor radon concentration of the three local governments was found to be 15.10 ± 3.1 Bqm -3 which is lower than the recommended limit of 40 Bqm -3 . The average annual effective dose estimated for this study was found to be 0.48 mSvy -1 which is lower than the recommended limit of 1 mSvy -1 . Therefore, the study provides preliminary data about indoor radon levels in the three local governments in Ibadan as a baseline study for a wider future national survey in the whole Ibadan city.