Natural Radioactivity and Excess Lifetime Cancer Risk Associated with Soil in Kargi Area, Marsabit-Kenya

The main aim of investigating activity concentrations together with distribution of radionuclides naturally in soil from Kargi was to evaluate radiological health hazard together with environmental radioactivity. Research shows radionuclides as one source of exposure due to radiation with detrimental effects health wise for populations found in areas considered high background radiation. After collecting 117 soil samples from the area, analysis was done in order to measure their natural radioactivities due to 40 K, 232 Th and 226 Ra radionuclides. Measurements method of gamma spectrometry employing a high purity germanium (HPGe) detector was employed basically to evaluate the radiological hazard of radioactivities. For 40 K, 232 Th and 226 Ra, mean calculated activities were 353.19 ± 110.07, 7.98 ± 3.98 and 7.37 ± 2.60 Bq∙kg −1 respectively. Mean values of absorbed and effective dose rates, external and internal hazard indices together with radium equivalent activity were 23.82 ± 6.59 nGy∙h −1 and 0.14 ± 0.04 mSv∙y −1 , 0.12 ± 0.03 and 0.14 ± 0.04 and 45.90 ± 12.65 Bq∙kg −1 respectively. Comparing with approved global values, the values were found to be below the given global limits. Evidence of involvement of metasomatic activity of the radioelements or fractionation during weathering is seen as calculations give a higher value Th/U. Excess cancer risk, calculated from the samples showed lower values as compared to global standard values hence minimal chance of getting cancer disease. The area is safe from cancer causing radionuclides.


Introduction
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 ( 40 K) 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 226 Ra 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) .
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 40 K, 232 Th and 226 Ra contents (UNSCEAR, 2000). Activity concentrations of 40 K, 232 Th and 226 Ra 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 40 K, 232 Th and 226 Ra 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.
powder through a 0.6 mm mesh sieve was done, oven dried for 24 hours at 100˚C to remove water completely from these samples. Crushed powder samples were individually weighed, parked in a special marked containers (plastic) before being closed tightly for about 4 weeks which was enough time to allow 238 U and 232 Th to reach secular equilibrium with their daughters before measurement of radioactivity (Hassan et al., 2016).

Measurement of Activity Concentration of
Natural Radionuclide and Absorbed Doses A high-purity germanium (HPGe) gamma-ray detector, 76 mm outside diameter, active volume of 144 mm 3 with an efficiency and a resolution of 31.6% and 1.8 keV respectively was used for measurement of activity concentrations. individual sample was put in a marinelli beaker of 500 cm 3 , filled up to same IAEA standard reference soil level (RGK-1, RGU-1 and RGTh-1) and then positioned in a lead shielded detector, adopting 22,000 -62,000 seconds as counting time. Soil sample reference standards from International Atomic Energy Agency (IAEA) (RGU-1, RGTh-1, RGK-1 and IAEA-375 soil) were used for method validation together with calibration of spectrometer using Maestro software (IAEA, 2003).
A comparative method was then employed to compute the activity concentration of each radionuclide. There were five notable gamma lines, 40 K line, 214 P b and 214 B i lines from 238 U and 212 P b and 228 A c lines from 232 Th for each sample. A gamma line of 1461 keV gave activity of 40 K, the activity of 238 U from 352 keV and 609 keV gamma lines of 214 P b and 214 B i respectively and that of 232 Th from 238 keV and 912 keV gamma lines of 212 P b and 228 A c respectively.

Outdoor External Gamma Dose-Rate (D out )
The outdoor dose-rates (D out ) due to γ-radiations in air, measured 1 m above surface of ground for even spread of naturally occurring radionuclides ( 40 K, 232 Th and 226 Ra) were computed based on UNSCEAR 2000 provided guidelines.
Assuming that the contributions coming from other naturally occurring radionuclides to be unimportant, then D out can be computed from: where, C k , C Ra and C Th are the average activities of potassium, radium and thorium respectively in the sample and nGy•h −1 (nano Gray per hour) is the unit of the absorbed dose rates (D).

Indoor External Gamma Dose Rate (D in )
European Commission, 1999 gives a formula for calculating indoor external dose rate as: where, C k , C Ra and C Th are the average activities of potassium, radium and thorium respectively in the sample and nGy•h −1 (nano Gray per hour) is the unit of the absorbed dose rates (D).

Radium Equivalent Activity (Ra eq )
Radium equivalent (Ra eq ) activity, a weighted sum of 40 K, 232 Th and 226 Ra activities is established on approximation that 13 Bq•kg −1 of 40 K, 1 Bq•kg −1 of 238 U and 0.7 Bq•kg −1 of 232 Th produce the similar radiation dose rates. Avwiri et al. (2013) estimates radium equivalent activity as: where, ,1.43 U Th C C and 0.077 K C are activity concentrations in Bq•kg −1 or Bq•l −1 of 238 U, 232 Th and 40 K.

Annual Effective Dose (De)
Two main classification of annual effective dose are annual outdoor (D out ) and indoor (D in ) effective doses respectively. In estimating the yearly effective dose rates, coefficients of conversion from absorbed dose to effective dose of 0.7 Sv•Gy −1 together with 0.2 Sv•Gy −1 (UNSCEAR, 2008) proposed outdoor occupancy factor are used. Mustapha (1999) estimates occupancy factors, that is mean time consumed indoor and outdoor as 0.6 and 0.4 respectively. According to UNSCEAR, 2008 the respective global mean outdoor and indoor occupancy factors are 0.2 and 0.8. 1) Annual outdoor effective dose (De out ) In estimating the yearly effective dose (D e ), coefficients of conversion from absorbed dose as measured in air to effective dose (0.7 Sv•Gy −1 ) and an outdoor occupancy factor of 0.2 as proposed by UNSCEAR 2008 are used. The following formula therefore gives the effective dose rate: where, mSv•y −1 (milli sieverts per year) is the unit for annual effective dose rate (De) nGy•h −1 (nano Gray per hour) is the unit of the absorbed dose rates (D) hy•r −1 (hour per year) given by 24 hours (in a day) × 365 days (in a year) 2) Annual indoor effective dose (De in ) This can be taken as that dose a person receives while in the indoor environment and can be computed from formula 5 (UNSCEAR, 2000). Annual indoor effective dose depends on time of stay inside a building, dose factors for conversion together with gamma ray dose existing within buildings.
where, mSv•y −1 (milli sieverts per year) is the unit for annual effective dose rate (De) nGy•h −1 (nano Gray per hour) is the unit of the absorbed dose rates (D) hy•r −1 (hour per year) given by 24 hours (in a day) × 365 days (in a year)

Annual Gonadal Equivalent Dose (AGED)
UNCEAR (2000) considers bone marrow, bone surface cells and gonads to be organs of interest due to their radiation sensitivity. AGED increase has been known to interfere with bone marrow thereby red blood cells causing a cancer disease known as leukemia which is lethal. The AGED for the inhabitant using such material for building may be evaluated by the following equation (Avwiri et al., 2013):

Excess Lifetime Cancer Risk (ELCR)
Potential carcinogenic effects that are characterized by estimating the probability of cancer incidence in a populace of individuals for a particular lifetime from projected exposures and intakes is called Excess Lifetime Cancer Risk (ELCR). Taskin et al. (2009) and  calculates ELCR using Equations (7) and (8).

1) Hazard indices for external gamma radiation (H ex and I γ )
Two indices were used in assessing the gamma radiation excess from materials used in building to ensure these materials safety. From the building materials, a hazard index describing the external gamma radiation dose is given as (Beretka & Mathew, 1985;Raghu et al., 2017): where, ,

Ra Th
A A and K A are the activity concentrations, in Bq•kg −1 of Uranium ( 238 U), Thorium ( 232 Th) and potassium ( 40 K) respectively.
For safe use of building materials, H ex value needs to be less than unity, corresponding to 370 Bq•kg −1 , an upper limit of Ra eq . In order to substantiate whether European Commission (EC) guidelines for usage of building materials are met, EC proposed an index (I ᵧ ) given by: 2) Hazard indices for internal alpha radiation (H in and I α ) Alpha (I α ) and internal hazard (H in ) indices are the two indices used in assessing alpha radiation excess due to radon gas coming from building materials. Raghu et al. (2017) mathematically defines H in as in equation 11 below and can be used for internal radiation excess consideration owing to 222 Rn inhalation together with its short lived decay products from building materials.
For safe use of materials in building construction, the computed H in value should be less than unity.
For us to neglect the radiation hazard, values of the indices (H ex , H in ) as from Equations (9) and (11) respectively must be less than unity (Harb et al., 2010).
Equation (12) gives the quantity I α as:

Correlation between 238 U and 232 Th, 238U and 40 K and 232 Th and 40 K
According to Avwiri et al. (2013), the elemental concentrations (ppm) of U-238, Th-232 together with percentage potassium can be computed from activity concentrations of Th-232, U-238 together with K-40 in Bq•kg −1 as measured using the below conversion factors:

Results and Discussion
Calculations give specific activities of 238 U and 232 Th series as well as 40 K, expressed in Bq•kg −1 for samples obtained from Kargi area as summerised in Table 1.
Obtained values for 226 Ra, 232 Th, 40 K together with Ra eq varied from 3.30 to 18.12  (Lu & Zhang, 2006). Figure 3 gives a more illustrative level of the activities.
All mean activities for 226 Ra, 232 Th, 40 K 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;UNSCEAR, 2000). Table 4 tabulates the above values.
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.

Conclusion
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 40 K, 226 Ra together with 232 Th (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.