For a representative estimate of radon levels in residential areas, measurements were taken in occupied rooms in buildings. The absorbed dose quantifies this energy per unit mass of the organs and tissues affected. The absorbed dose in gray (Gy) is the energy in joule (J) compared to the mass in kilogram (kg) therefore (J/kg) with 1 gray (Gy) = 1 joule (J) per kilogram (kg). The regulations quantify exposure to ionizing radiation by an average dose over the whole body: the effective dose, expressed in sieverts (Sv). The effective dose balances the absorbed dose according to the radiotoxicity of the different types of radiation and the radiosensitivity of the organs and tissues. The lung is one of the most radiosensitive organs in the human body. The effective dose is an indicator of overall health detriment. The concentration of potential alpha energy of a mixture of progeny in the air is expressed in J/m³. Beyond the relationship between the radon concentration and that of potential alpha energy, 1 Bq/m³ = 5.56 × 10⁻⁶ mJ/m³. The Balance factor F depends on the level of ventilation: F = 0.4 in natural ventilation and F = 0.2 in forced ventilation. The annual absorbed dose due to indoor exposure to radon was estimated using the radon concentration (C_Rn), balance factor (F), occupancy factor (O), number of hours per year (T) and the dose conversion factor (D₂) [4] [8] .

The formula used to calculate the annual absorbed dose is given as follows:

D 1 ( mSv ) = C Rn ∗ F ∗ O ∗ T ∗ D 2 (4)

with: C_Rn the concentration in Bq/m³; F taken at 0.4; O estimated that people spend 80% of their time indoors; T = 24 hours × 365 days = 8760 h, and D₂ converts the radon concentration into a dose, 9 nSv/(Bq·m⁻³·h⁻¹) [1] [4] [8] [9] [10] [11] [12] [13] .

The dose received by the occupants is the equivalent dose on the type of radiation produced by radon as well as on the damage to organs and tissues. The effective dose equivalent was calculated using the alpha particle weighting factors for lung tissue based on ICRP, 2007.

The equivalent effective dose is estimated using the following equation:

E ( mSv ⋅ an − 1 ) = D ∗ W ∗ W T (5)

with: D represents the annual exposure dose (mSv), W a weighting factor for the alpha particle taking a value of 20 and W_T a weighting factor for the lung tissue with the value of [1] [4] [8] [9] [10] [14] .

The impact of indoor radon exposure on the health of residents has been quantified by the relative risk of lung cancer (RRLC) [4] [14] [15] given by the following equation:

RRLC = exp ( 0.00087352 C Rn ) (6)

This study was conducted only in residential areas in the city of Kaya. A data collection sheet has been prepared for each habitat containing questions on the geographical information of the place of residence, the date of the start and end of the measurements, the average values of the radon concentrations in the short term and in the long term measured every 24 hours. Table 1 gives the average, maximum, minimum and standard deviation of long-term concentration of radon in the studies houses Kaya.

The average long-term radon concentration in residential areas varies between 2.89 and 197.11 Bq/m³. The maximum concentrations in house were between 4 and 248 Bq/m³, and the minimum concentrations varies between 0 and 153 Bq/m³. The long-term average concentration in all the houses was below 100 Bq/m³, except for house 5 in sector 2. It is necessary to increase the ventilation in house 5.

Table 2 presents the average short-term and long-term radon concentrations in residential areas in Kaya town.

The short-term concentration varies between 3.78 and 198.22 Bq/m³ with an average of 25.68 Bq/m³ and the long-term concentration was between 2.89 and 197.11 Bq/m³ with an average of 28.47 Bq/m³. Both the short-term and long-term maximum values are above the WHO limit of 100 Bq/m³ [16] .

The results of the concentration measurements highlight the extremely variable nature of the average long-term and short-term radon concentration in the different residential buildings chosen in Kaya. According to Figure 3, the highest average concentrations of radon in houses at Kaya were around 200 Bq/m³ in the short term and in the long term. The histograms of the measurements show extreme values of radon concentration according to the houses.

In conclusion, the average radon concentration in house 5 was the highest in the short and long term and was around 200 Bq/m³, followed by house 16 which has an average concentration of 71.33 Bq/m³ in the long term and of 64.67 Bq/m³ in the short term. The high average concentration in House 5 may be related to building materials. It is necessary that this house can be ventilated to prevent the occupants from being exposed and developing the health effects.

Figure 4 shows us the variation in the long-term and short-term average concentration in house 5 of Kaya during the seven days of measurement. The concentration during the seven days of measurement was greater than 150 Bq/m³; which is above the WHO lower limit, but below the upper limit of 300 Bq/m³ [17] . The inhabitants of this house can be exposed to radon and develop the health effects.

The dose is the quantity of ionizing radiation interacting with the molecules of the human body by depositing an energy capable of damaging them. The absorbed dose quantifies this energy per unit mass of the organs and tissues affected and the effective dose quantifies the exposure to ionizing radiation by an average dose over the whole body.

Most of the dose received by the lung does not come from the radon gas itself but from the short-lived descendants with which it is in partial equilibrium. However, the conversion of radon exposure into a dose is the subject of debate within the radiation protection community. The respective views of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)

and the International Commission on Radiological Protection (ICRP) have been analyzed and the limit is 1 to 10 mSv respectively [17] [18] .

Table 3 gives the different absorbed and effective doses at short and long term.

The Table 3 show a significant variation of radon dose in different places of residence.

In the long term, the absorbed dose varied from 0.118 mSv to 4.975 mSv and the effective dose from 0.229 mSv to 12.002 mSv. In the short term, the absorbed dose varies between 0.095 mSv and 5.001 mSv and the effective dose is between 0.283 mSv and 11.935 mSv.

The maximum value absorbed and effective doses were determined in house 5. The absorbed dose in the lungs in the short term and long term in house 5 was 5.001 mSv and 4.973 mSv respectively, that in house 16 in the short term and long term was 1.631 mSv and 1.800 mSv respectively. The absorbed dose in the other houses was less than 1 mSv.

The short-term effective lung doses in houses 5, 6, 7, 14, 16, 17, 18 and 19 were above the “normal” background level of 1.1 mSv per year proposed by UNSCEAR-2000 [8] . In the long term, the effective doses in the lungs for houses 5, 6, 7, 14, 16, 17, 18, 19, 20 and 21 were above the “normal” background level of 1.1 mSv per year proposed by UNSCEAR-2000. Which shows that the level of exposure in houses 5, 6, 7, 14, 16, 17, 18, 19, 20 and 21 was higher than the level proposed by UNSCEAR-2000, hence the need to take measures of radiation protection to lower concentrations.

The short-term and long-term effective lung dose in house 16 was above the lower limit of 3 mSv per year recommended by ICRP-23. In house 5, the short-term and long-term effective dose to the lungs was 12.002 mSv and 11.935 mSv respectively and above the upper limit of 10 mSv per year recommended by ICRP-23 [19] . The occupants of house 5 are susceptible to radon-related illnesses. Houses 5 and 16 are still to be checked in terms of ventilation. House 5 is a potential danger for the occupants.

Chronic exposure to radon explains the appearance of cancer in occupants spending a lot of time in places where the concentration is very high. The most well-known effect associated with radon exposure is a risk of contracting lung cancer. It is noted that radon is the second cause of lung cancer after smoking. Smokers exposed to radon have a 10 to 15 times risk of developing lung cancer compared to a non-smoker. A dosimetric model represents the deposition of short-lived radon descendants, in the form of nanometric alpha (α) particles free fraction or attached to atmospheric dust in the respiratory tract according to their thermodynamic and aerodynamic properties and the resulting irradiation. If radon is inhaled, it decays into solid radioactive descendants that settle in the lungs. These solid radioactive daughters which constitute the energy of this radiation can fracture the double helix of the Deoxyribonucleide Acid (DNA).

The variations in relative risk of lung cancer in the different houses are approximately from 1.003 to 1.188 with an average of 1.026. The relative risk of lung cancer in house 5 is 1.188 higher than in the other houses. The mean relative risk of lung cancer is low; which does not constitute a carcinogenic risk for the inhabitants of the houses studied.

Figure 5 shows the relative risk of lung cancer according to each of the 21 houses studied.

The Lung Cancer Relative Risk Histogram clearly shows the different levels of risk in all the homes studied. On the histogram, we observe that the relative risk of lung cancer in house 5 was close to 1.2 and was the highest among the houses studied. The second highest value of the relative risk of lung cancer was obtained in house 16. The relative risk of lung cancer for the others houses were around one (01).

In conclusion, the variations in relative risk of lung cancer in the different houses are approximately from 1.003 to 1.188 with an average of 1.026 which is negligible.

Table 4 presents the average radon concentration in Kaya residential areas and that of other studies carried out in Burkina, Ghana, Sudan, Morocco and China.

The average radon concentration in Kaya is slightly higher than that obtained in homes in the city of Ouagadougou [4] . The average radon concentrations in similar studies carried out in Ghana, Sudan, Morocco and China show that the average concentration of homes in Kaya is low compared to that obtained in these countries.