INDOOR RADON CONCENTRATIONS AND TERRESTIAL GAMMA DOSE RATES FOR ESTIMATING CANCER RISK IN IWO, OSUN STATE, NIGERIA

Abstract

Indoor Radon (IR) and Primordial Radionuclides (PR) which are major contributors to cancer incidence are sources of natural ionising radiation,especially from rocks used in building constructions. The assessment of the radiological implication of radiation from rocks on human population is important.Iwo is a community predominantly underlain by basement complex rocks which could be high in radioactivity. To the best of our knowledge, radiation measurements have not been previously carried out in the area. This study was designed to determine IR and PR in order to assess associated cancer risks in Iwo.

Indoor radon survey in 10 living and 32 bed-rooms in 24 randomly selected Concrete Houses (CH) based on accessibility and 5 rooms in 5 Mud Houses(MH),with varying ventilations,in six major areas in Iwo was carried out using CR-39 detectors during the rainy and dry seasons (2009˗2010).The detectors were placed 1.5 m above the ground in the rooms for 90 days,and thereafter were etched in 6.25 M NaOH solution at 90^( 0) C for 2 hours. Alpha particle tracks on the etched detectors were counted by optical microscope and converted to Radon Concentrations (RC) using standard formula.The annual Effective Dose (ED) was estimated using standard conversion factor. Sixty soil samples randomly extracted, as representative of the soil, at a depth of 20˗30 cm and 20 rock samples in the vicinity of the houses were collected.The samples were air-dried, crushed and sieved through 0.2 mm mesh.Two hundred grams of each sieved sample was transferred into a plastic container and sealed for 30 days to attain secular equilibrium. Radionuclide (40K, 226Ra and 232Th) concentrations in the samples were determined using gamma-ray spectrometer with NaI(Tl) detector. Radiological indices were calculated to estimate cancer risks. Data were compared with International Commission on Radiological Protection (ICRP) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) standards. Data were analyzed using descriptive statistics.

Radon concentrations in living and bed-rooms were respectively, 200.0±56.8 and 228.4±48.8 Bqm^(-3) during rainy season and 176.6±36.9 and 167.1±28.3 Bqm^(-3 )during dry season.The RC in CH and MH were 199.5±51.2 and 169.4±21.4〖 Bqm〗^(-3), respectively. The mean activity concentrations of 40K, 226Ra and 232Th in soil were 447.4±216.6, 13.0±7.8 and 25.2±15.4〖 Bqkg〗^(-1); and in rocks were 647.4±478.0,27.1±26.6 and 13.3±11.5〖 Bqkg〗^(-1), respectively. The mean ED for rainy and dry seasons, in living-rooms, were 5.0±1.4 and 4.5±0.9 mSvy^(-1) respectively; in bed˗rooms they were 5.8±1.2 and 4.2±0.7 mSvy^(-1),respectively. These values lie below the upper limit set by the ICRP for action in dwellings.The mean ED in soil and the rocks were (50.9±17.6 μSv) and (59.1±36.9 μSv),respectively. The average excess lifetime cancer risks were 0.021 and 0.016 (MPY)^(-1) during rainy and dry seasons respectively. The average cumulative lifetime cancer morbidity and mortality risks due to the radionuclides were respectively,5.0×10^(-4)and 3.4×10^(-4 ) for soil;and 7.2×10^(-4) and 4.9×10^(-4) for rocks which were below the upper limit recommended by ICRP and UNSCEAR.

The concentrations of indoor radon and primodial radionuclides in Iwo were low. The concentrations posed no cancer risk to the populace in the study area. . Keywords: Columbia Resin-39 detectors, Radon concentration, Primodial radionuclides,Natural ionising radiation. Word counts: 497