Radioactive elements found in rocks, soil, water, air, and in food from the earth make their way in our bodies when we breathe in air or eat foods which contain them. These naturally occurring radio isotopes such as carbon-14, potassium-40, thorium-232, uranium-238, polonium-218 and tritium (hydrogen-3) expose us to radiation from within our bodies.

Exposure to radiation is injurious to living tissues owing to its ionizing power in matter. This ionization can directly destroy living cells directly, by breaking up the chemical bonds of key biological molecules like DNA, or indirectly, by creating chemical radicles from water molecules in the cells, which can chemically attack biological molecules ( UNSCEAR, 1993 ). Somehow, these molecules are mended by use of natural biological procedures; effectiveness of this mending however depends on the size of damage. Obviously, if cell repair is not properly done or not repaired at all, the cell then may suffer either of these possible fates (Cember, 1996):

1) Cell demise

2) Cell impairment leading to somatic effects that are physical effects suffered by the irradiated person like cancer disease.

3) Genetic mutation, considered a permanent alteration of the cell and which is passed to the later generations.

The presence of radionuclides in natural environment has been noticed always. Wherever universally present in little amounts in building materials and earth, thorium together with uranium series, together with potassium 40 (⁴⁰K) are considered leading natural donors to external exposure from γ-radiation. (UNSCEAR, 2008).

Potassium, uranium and thorium radioactive elements together with any of their decay outcomes like radon and radium are examples of Naturally Occurring Radioactive Materials (NORM) considered long-lived. These elements have always existed in the atmosphere and earth’s crust. NORM issue relates to radon exposure in homes, particularly those built on granitic ground. By using building materials with relatively high activity concentration of ²²⁶Ra and building techniques that stipulate the influx of radon from the ground e.g. well insulated housing, the radiation dose to the population dose is still further increased (Aguko et al., 2013).

Approximation of the radiation dose distribution is key in gauging the health risk to a populace and serves as a reference point for registering changes in environmental radioactivity owing to anthropogenic activities (Obed et al., 2005).

Exposure to radiation for a long period of time is presumed to have some probabilities of cancer causing disease, thus everyone is at risk of contracting cancer. An additional risk that a person might have of contracting cancer disease due to long time exposure to materials causing cancer disease is called excess lifetime cancer risk (ELCR) (Qureshi et al., 2014).

For outdoor occupation evaluating the terrestrial gamma dose rate, estimation of the natural radioactivity level is very important for geological samples, usually determined from the ⁴⁰K, ²³²Th and ²²⁶Ra contents ( UNSCEAR, 2000 ). Activity concentrations of ⁴⁰K, ²³²Th and ²²⁶Ra in collected soils have been estimated mainly by gamma ray spectrometry, although the fission track registration technique has also been used for the analysis of uranium concentration of these samples ( Singh et al., 2005 ). They found the absorbed dose rate as found in air, calculated the gamma dose rate from concentration of radionuclides of ⁴⁰K, ²³²Th and ²²⁶Ra from which they deduced the effective dose the inhabitants receive annually.

Health facilities found within and around Kargi have continuously reported a number of cancer (disease) cases together with deaths year in year out hence prompting this research work. The main aim of investigating activity concentrations together with distribution of radionuclides naturally in soil from this area was to gauge radiological health hazard together with environmental radioactivity.

Calculations give specific activities of ²³⁸U and ²³²Th series as well as ⁴⁰K, expressed in Bq∙kg⁻¹ for samples obtained from Kargi area as summerised in Table 1. Obtained values for ²²⁶Ra, ²³²Th, ⁴⁰K together with Ra_eq varied from 3.30 to 18.12

Bq∙kg⁻¹, 0.00 to 17.80 Bq∙kg⁻¹, 36.87 to 667.77 Bq∙kg⁻¹, and 18.09 to 102.34 Bq∙kg⁻¹ with arithmetic mean and standard deviation as 7.40 ± 2.61, 7.98 ± 4.01, 351.74 ± 109.63 and 45.90 ± 12.65 respectively. This is seen from Table 1. Globally, recommended values are respectively 37.0 Bq∙kg⁻¹, 33.0 Bq∙kg⁻¹, 400.0 Bq∙kg⁻¹ ( UNSCEAR, 2008 ) and 370 Bq∙kg⁻¹ ( Lu & Zhang, 2006 ). Figure 3 gives a more illustrative level of the activities.

All mean activities for ²²⁶Ra, ²³²Th, ⁴⁰K together with Ra_eq were less than the global values recommendation.

Mean calculated absorbed dose rates for all soil samples was 23.87 ± 3.48 nGy∙h⁻¹ against the global median value of 54 nGy∙h⁻¹ ( UNSCEAR, 2000 ). The values varied between 16.06 ± 1.09 and 29.35 ± 1.32 nGy∙h⁻¹, figures which are less than the required value (Table 4). The AEDE (De) values were found lower than the global value of 1 mSv∙y⁻¹ ( ICRP, 2000 ) with a mean and standard deviation value of 0.14 ± 0.03 mSv∙y⁻¹, varying from 0.11 ± 0.01 to 0.18 ± 0.06 mSv∙y⁻¹.

Excess lifetime cancer risk is an added risk that one might have on contracting cancer disease if susceptible for longer time to cancer disease causing materials. Taking 70 years as mean life duration with a risk factor of 0.05 per Sv ( ICRP, 2007 ; Taskin et al., 2009 ) and a median annual effective dose rate of 0.14 mSv∙y⁻¹, then excess cancer risk is computed as 0.05%. This value is less than world agreed value of 0.145% ( Taskin et al., 2009

Elemental radionuclide concentrations in the samples were calculated from the activity concentrations in Bq∙kg⁻¹ by use of conversion factors in Equations (13)-(15). These results are presented in Table 5. From the table, the calculated elemental concentrations ranged from 0.74% to 1.51%, 0.68 to 3.04 ppm and 0.33 to 0.97 ppm for potassium, uranium and thorium respectively with respective arithmetic mean and standard deviation of 1.13% ± 0.21%, 1.98 ± 0.47 ppm, 0.59 ± 0.13 ppm.

Tzortzis & Tsertos (2004) and Al-Hamarneh & Awadalla (2009) noted that a high or low value of Th/U ratios as measured in some studied locations may be an indication of a uranium depletion or thorium enrichment due to natural processes alteration in that area. They approximated the theoretical normal continental crust values of Th/U elemental ratios to be 3.0. From Table 5, our Th/U results ranged from 1.19 ± 0.85 to 6.35 ± 1.72 with mean and standard deviation of 3.57 ± 1.13. Other correlation ratios of K/U together with K/Th varied from 0.84 ± 0.46 to 3.89 ± 2.13 together with 0.34 ± 0.09 to 1.94 ± 2.42 with mean and standard deviations of 2.15 ± 0.67 and 0.68 ± 0.30 respectively.

Correlations existing between activities ²³²Th and ²³⁸U, ⁴⁰K and ²³⁸U together with ⁴⁰K and ²³²Th showed a weak relationship existing on ²³²Th against ²³⁸U, ⁴⁰K against ²³⁸U together with ⁴⁰K against ²³²Th with correlation coefficients of 0.405, 0.134 and 0.319 respectively.

Respective mean values together with their standard deviations of representative alpha (I_α), representative gamma (I_γ), internal hazard (H_in), external hazard (H_ex) indices and Annual Gonadal Equivalent Dose (AGED) values, as seen from Table 3 are 167.32 ± 23.56, 0.12 ± 0.02, 0.14 ± 0.02, 0.18 ± 0.03 and 0.03 ± 0.01 mSv∙y⁻¹ with ranges from 122.70 ± 13.39 to 210.84 ± 62.88, 0.09 ± 0.01 to 0.16 ± 0.05, 0.11 ± 0.01 to 0.17 ± 0.05, 0.13 ± 0.01 to 0.23 ± 0.07 and 0.02 ± 0.01 to 0.05 ± 0.02 mSv∙y⁻¹. A pictorial representation of hazard indices is shown in Figure 4.

Results from the study give the mean activities for the area as 354.81 ± 67.06, 7.23 ± 1.67 and 8.03 ± 1.91 Bq∙kg⁻¹ against the world standard values of 400, 35 and 30 Bq∙kg⁻¹ for ⁴⁰K, ²²⁶Ra together with ²³²Th ( UNSCEAR, 2000 ) respectively. The area generally cannot be classified as a High Background Radiation Area (HBRA), thus good for human settlement.

Mean evaluated absorbed dose rate was below recommended values by almost half. Annual effective dose rate values were found lower than global value of 1

mSv∙y⁻¹, which, for the public exposure is the annual effective dose rate limit ( ICRP, 2000 ).

Hazard indices (H_in, H_ex, I_γ and I_α) together with Radium equivalent (Ra_eq) values for the studied area were lower than values recommended of 1 (H_in, H_ex and I_γ), 0.5 (I_α) and 370 Bq∙kg⁻¹ ( Lu & Zhang, 2006 ). Radiation risk associated with these soils can be considered negligible making the rocks and soils in the area safe for construction causing no important radiological threat to populace.

Excess cancer risk, calculated from the samples showed lower values hence minimal chance of getting cancer disease. The area is safe from cancer causing radionuclides.

Because Th/U value was higher than the recommended, the study can therefore conclude that there could have been a fractionation during weathering or involvement of metasomatic activity of the radioelements.

The authors thank Jomo Kenyatta University of Agriculture & Technology (JKUAT), Kenya Bureau of Standards (KEBS) for their mass measurements and oven drying facility, University of Nairobi’s Nuclear Science and Technology Institute (INST) for laboratory analysis and γ-ray spectrometer, Kargi administration, my family and Research assistant, Lydia Okemwa of Technical University of Kenya, TUK.

The authors declare no conflicts of interest regarding the publication of this paper.

Aguko, W. O., Kinyua, R., & Githiri, J. G. (2020). Natural Radioactivity and Excess Lifetime Cancer Risk Associated with Soil in Kargi Area, Marsabit-Kenya. Journal of Geoscience and Environment Protection, 8, 127-143. https://doi.org/10.4236/gep.2020.812008