RISK ASSESSMENT DUE TO NATURALLY OCCURRING RADIOACTIVE MATERIALS IN KILIMAMBOGO, KENYA
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Date
2023-12
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Abstract
Man is continually exposed to ionizing radiation whose main origin is either natural
or artificial. The primordial radionuclides found in building materials are important
sources of radon and thoron in the indoor air. The main objective of this research was
to assess exposure due to radioactive elements in indoor air, crops and building
materials (soil and rocks), and determine the level of the risks. The concentration
levels of radon, thoron, and thoron progeny were measured in mud-walled, iron sheet
walled, and stone walled modern houses in Kilimambogo region, Kenya. Radon and
thoron concentration levels were determined using passive radon–thoron
discriminative monitors (RADUET), while thoron progeny levels were measured
using thoron progeny monitors. The activity concentration of 226Ra, 232Th and 40K
present in the different environmental samples like rocks, soil, and cassava crop were
determined using a Thallium- activated sodium-iodide scintillation (NaI (TI)) detector
Model TS-2L. Elemental concentration in water samples was determined using an
atomic absorption spectrophotometer (AAS). The mean external dose received
indoors by the residents due to radionuclides present in rock and soil samples was
measured to be 0.91 ± 0.26 mSv y-1 and 0.86 ± 0.19 mSv y-1 respectively. This is
comparable to the total exposure dose limit of 1 mSv y-1 recommended by ICRP for
members of the general public. This means that, soil and rocks in the region does not
pose any radiological risk to the residents of Kilimambogo region. Consequently, they
can be used for agriculture and construction of dwellings. The doses received due to
intake of cassava tubers and leaves was 1.82 ± 0.60 mSv y-1 and 0.81± 0.30 mSv y-1
respectively, totaling to 2.63 ± 0.70 mSv y-1. The ingestion dose contributes to 42.1%
of the total dose received by the resident from measured sources. Residents should
therefore reduce the intake of cassava. The annual effective dose received by the
residents of Kilimambogo and Gatuanyaga regions due to inhalation of radon and
thoron was determined for the three types of dwellings, from their respective
progenies and found to be 1.3 ± 0.2 mSv y-1, 1.1 ± 0.1 mSv y-1 and 1.4 ± 0.2 mSv y-1
for radon progeny and 2.4 ± 0.4 mSv y-1, 0.5 ± 0.1 mSv y-1 and 1.5 ± 0.2 mSv y-1 for
thoron progeny in mud-walled, iron sheet-walled and stone-walled houses
respectively. It was observed that, residents living in iron sheet - walled houses with
cemented floors received the lowest doses from inhalation of the radon isotopes hence
such houses are safer than other types of dwellings. Residents living in mud houses
and uncemented houses should consider improving the ventilation of their houses to
reduce radiation exposure. The lifetime cancer risks for the residents due to intake of
metal carcinogens, Cd, Ni and Pb in borehole and surface water was also estimated.
The cumulative cancer risk in the borehole samples was 0.12 ± 0.06 and 0.19 ± 0.09
in the dry and wet seasons respectively. Those from surface water sources were 0.10
± 0.07 and 0.21 ± 0.06 in the dry and wet seasons respectively. In both seasons, the
cancer risks were much greater than the acceptable risks of 10-6 to 10-4 by a factor of
1000 and above. This indicates that about 10% of the residents are likely to die of
cancer related ailments. RESRAD computer simulation code was used to estimate the
doses received by the resident farmer and the quarry worker scenarios at time, t = 0
years. The doses received by the resident farmer and quarry worker scenarios were
0.22 mSv y-1 and 0.16 mSv y-1 respectively only for the external radiation pathway.
This risk is low and does not pose any significant health risk to the residents. The data
measured in this research will be used by future researchers as baseline study.