ESTIMATION OF CANCER RISK DUE TO INDOOR RADON AND RADON EXHALATION RATES FROM BUILDING MATERIALS IN SOUTH-WESTERN NIGERIA

Abstract

Indoor radon measurements in some homes of selected towns/cities in South-Western Nigeria have been carried out with the aim of estimating the Excess Life Time Cancer Risks (ELCR) using Solid State Nuclear Track Detectors (SSNTDs) specifically, CR-39 detectors. Closed can technique was used to measure radon exhalation rates from commonly used building materials to examine the contributions of the materials to indoor radon concentration in the study area. Exposure due to gamma radiation from natural radionuclide in the building materials was also investigated. Results of indoor radon concentrations for both the surveyed living rooms and bedrooms followed log-normal distribution as it is the case of most related indoor radon surveys. On the basis of the individual houses, the representative values of indoor radon concentration were found to be in the range of 14-291 Bq m-3 with an arithmetic mean of 64.78 ± 37.21 Bq m-3 and the geometric mean as 56 ± 2 Bq m-3. The overall average value (1.86 mSv) of Annual Effective Doses (AED) due to indoor radon was observed to be higher than the world average level (1.15 mSv) but less than lower limit (3-10) mSv of International Commission on Radiological Protection. The Excess Life-Time Cancer Risk (ELCR) evaluated in the study ranged from 6.01 to 16.36 per million person-years (MPy)-1 with an average value of 7.14 (MPy)-1. This implies that after exposure to indoor radon for 70 years, 7 people in every 1000 are likely to suffer the risk of developing lung cancer. The overall contribution of the surveyed building materials to the indoor radon is insignificant. The average values of the activity concentrations of 40K, 226Ra and 232Th are 286.81, 29.30 and 25.99 Bq kg-1 in the building materials respectively. These values were observed to be less than the corresponding world average values of 500, 50, and 50 Bq kg-1 given by United Nations Scientific Committee on the Effect of Atomic radiations respectively. While the highest contributions to the average activity concentration of 40K and 226Ra were observed in granite (GN) and for 232Th was observed in mud blocks (MB), the lowest contributions to the average activity concentrations of the radionuclides was found in wall paints. All the mean values of the radiological indices due to gamma exposure were found to be less than their corresponding upper limits, that is, radium equivalent activity (Raeq) less than 370 Bq kg-1, relative index level (RLI), gamma index level, external hazard index level (Hex), internal hazard index level (Hin) and alpha index (I ) all less than unity, the activity utilization index (AUI) < 2, only annual gonad dose equivalent (AGDE) is slightly higher than its recommended value. It can be inferred that the radiological health implications arising from the values of the indices obtained in this study are insignificant and the occupants of the study area are safe from the radiological points of view. However, it is recommended that some homes with relatively high level of indoor radon should establish effective ventilation and other mitigation techniques.