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Probabilistic human health risk assessment from groundwater fluoride contamination in Main Ethiopia Rift.
| Scientific Reports | Abera A, Aseffa A, Mengistie B, Malmqvist E, Isaxon C, Sahilu G. | 15:28571.
Children Endemic fluorosis areas Ethiopian Rift Valley Groundwater Dental Fluorosis Skeletal Fluorosis Human Study
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Abstract

Fluoride toxicity has become a significant global public health concern, with drinking water being a major source of exposure. This study aimed to determine groundwater fluoride concentration and assess its non-carcinogenic health effects on human health. A longitudinal study design was applied to select water samples in dry and wet seasons from Adama City and Wenji Gefersa town of Ethiopia. Groundwater fluoride concentration was measured using an ion-selective electrode. Total hazard analysis was assessed based on the chronic daily oral intake and dermal absorbed dose of fluoride. Analyses were conducted using ArcGIS, an Excel spreadsheet and Statistical Packages for the Social Sciences (SPSS). This study reported that groundwater fluoride concentration ranged from 0.3 mg/L to 38 mg/L, with the mean annual concentrations of Adama City and Wenji Gefersa Town being 1.9 mg/L and 23 mg/L, respectively. Fluoride concentrations reported at 70% and 45% of groundwater samples during the wet season and dry season were above World Health Organization (WHO) guidelines for drinking water. Total hazard index values among sampled locations varied from 0.17 to 30.43. Three-fourths of infants, 99% of children, 62% of adolescents, and 45% of adults had a risk of developing a non-carcinogenic health effect. This study demonstrated fluoride contamination of groundwater sources pose the residents for higher probability of developing non-carcinogenic health effects on their lifetimes. Application of locally available defluorination technology is paramount to safeguard the community.

Introduction

Groundwater is widely used as a source of drinking water worldwide1. It is more microbiologically safe and better protected from pollution and evaporation than surface water1. Due to geological or anthropogenic sources of pollution, groundwater may, however, contaminants undesirable to human, such as microplastics2, Polychlorinated Biphenyls (PCBs)3, bisphenol4, radionuclides5, nitrate6 and fluoride6. Fluoride is a double sword element which is beneficial and potentially hazards to human health, which is the 13th most abundant in the environment7. Fluoride-bearing rocks release fluoride to groundwater during water-rock interaction8. Globally, more than 200 million people are exposed to groundwater fluoride concentrations that exceed the World Health Organization (WHO) guideline of 1.5 mg/L for drinking water9. Such exposure leads to 70 million people worldwide suffering from skeletal fluorosis10. In Africa continent, 81 million people are potentially exposed to fluoride greater than 1.5 mg/L in groundwater9. East Africa Rift Valley (EARV) regions, such as Kenya11, Ethiopia11 and Tanzania11 water sources, have substantially greater fluoride health hazards than other continents12 due to the nature of volcanic rocks. Ethiopia is the 4th country in Africa where 9.6 million population is potentially at risk of fluorosis due to fluoride groundwater concentration exceeded WHO guideline9. Moreover, Ethiopia Rift Valley (ERV) revealed an exposure of 18 mg/L of fluoride in groundwater was associated with dental fluorosis12.

An average of 75–90% of ingested water fluoride is absorbed, mainly via the gastrointestinal tract10, and from there transported through the circulatory system in the human body13. Consumption of 0.8 to 1.5 mg/L fluoride in drinking water improves bone density, prevents caries, acts as an antibacterial agent in the mouth, and prevents osteoporosis10. In contrast, excessive fluoride (i.e. greater than 1.5 mg/L) intake is associated with diseases such as dental fluorosis, skeletal fluorosis, and systematic defects in small intestinal microvilli14, and cognitive impairment in children15. Fluorosis can be non-skeletal, causing neurological symptoms16, high blood pressure17, effects on fetal development18, and child attention-deficit hyperactivity disorder19. Furthermore, evidence reported a substantial linear intelligence quotient (IQ) decrease for fluoride exposure above 1 mg/L in drinking water20. The plausible mechanisms may involve cellular damage, cell cycle arrest, DNA damage21, endoplasmic reticulum stress, and inflammatory cell response22. Animal study reported that disruptions of key signaling pathways, neuron inflammation and changes the neurotransmitters due fluoride induced mitochondrial damage23.

Water sources with high concentrations of fluoride are undesirable for drinking and bathing purposes24. ERV is part of EARV which is characterized by high groundwater fluoride concentration exceeds WHO guidelines25. Some evidence indicates the maximum fluoride concentration is up to 40 mg/L26. According to the systematic review and meta-analysis of ERV mean pooled concentration of fluoride is 6.03 mg/L with a maximum value of 75 mg/L27. This region is highly affected by the high prevalence of dental fluorosis due consumption of fluoride-enriched groundwater sources12. ERV residents access water supply for drinking and domestic purpose from groundwater even though it bears high fluoride concentration28. For example, the fluoride concentration in Adama district varied from 1 to 10 mg/L with a mean value of 4.2 mg/L29. As a result, the community was exposed to fluoride with different concentrations from conception to adulthood due to water scarcity and less migration. Aforementioned exposure to fluoride sought updated evidence on quantity of groundwater fluoride and its associated health risk among different categories of population to apply different defluorination techniques. Additionally, Southeast of Ethiopia drinking water source contains 3.15 mg/L of fluoride that surpassed WHO drinking water guideline30.

Ethiopia Standard Agency (ESA) recommended compulsory Ethiopia standard of 1.5 mg/L of fluoride for drinking water for regulatory purpose31. However, Ministry of Water and Energy and Wateraid Ethiopia non-governmental organization suggested the permissible limit of 3 mg/L thus admitting a certain degree of inevitable health effects32. Despite the route for contaminants drinking water served as a potential contributor for mineral water nutrient such Zinc, Fluoride, Magnesium and Sodium33,34. There are a few local and national initiatives in Ethiopia to rely on WHO guideline for drinking water sources, but these attempts have not been successful specifically rift valley area35. Poor quality of drinking water sources in Ethiopia rift area make challenge to achieve Sustainable Development Goals (SDGs), SDG 3 (Global Health and Well-being) and SDG 6 (Clean Water and Sanitation)36. Still, substantial evidence on groundwater fluoride concentration and its health risks are significantly improve the public health and decision making process. Different approaches have been applied to estimate fluoride human health risk from deteriorated water quality37,38. One such approach is health risk assessment (HRA), which associates the health effects with the daily intake of chemicals, e.g. fluoride, via water. It is expressed by a total hazard index (THI) value. A THI value greater than or equal to one is an indicator of a high probability of developing non-carcinogenic health effects. Whereas its value less than one is considered no health risk among the exposed population24. Non-carcinogenic health effects of fluoride vary among population categories but infants and children are more vulnerable39. There is a lack of evidence concerning spatiotemporal variability of fluoride concentration and the potential non-carcinogenic health effects on infants, children, adolescents, and adults have not been well studied. Quantification of fluoride health risks significantly sort-out severely affected population categories such as infants and children. Different pathways such as ingestion and dermal exposure are assessed to determine non-carcinogenic effects of groundwater fluoride. Hence, the aim of this study is, (a) to measure temporal variation of groundwater fluoride concentration at Adama City and Wenji Gefersa Town, and (b) to estimate its potential non-carcinogenic health effects of groundwater fluoride exposure among different age groups. This study’s findings come up with potential evidence for decision-makers such as governmental and non-governmental organizations. Moreover, encourages the policymaker to revise existing Ethiopian fluoride-national guidelines in drinking sources specifically fluoride endemic region.

Materials and methods

Study setting description

This study was performed in the Awash Basin Adama district in Ethiopia (Fig. 1). Adama-Wenji basin is characterized by permeable volcanic and alluvial sediment deposit which plays a significant role in groundwater discharge and recharges process of fluoride40. Adama City is a part of Main Ethiopia Rift (MER) where rhyolitic volcanic rocks and their weathered products (fluvio-lacustrine sediments) are predominant geogenic sources for high contents of fluoride in groundwater41. Furthermore low calcium bearing aquifer also common in MER which led to high contents of fluoride42. The basin experiences a rainy season from June to September and a dry climate for the rest of the year with an annual mean temperature of 21.6 °C43. To provide Adama City with drinking water, there are six groundwater wells, which are used in addition to the treated main water supply44. The Wenji Gefersa town residents access drinking water supply mainly from seventeen public and private groundwater wells and rarely from treated public tap water as well as few private tap waters. The projected total population of Wenji Gefersa town and Adama City are 29, 886 and 214, 000 respectively45.

Fig. 1
figure 1

Description of study area Oromia Regional State Ethiopia, Eastern Africa.

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Groundwater sample collection and geo-locating

A total of six and ten groundwater samples were collected from Adama City and Wenji Gefersa Town respectively during the wet season (August to September) and during the dry season (January to February) (Fig. 2).

Fig. 2
figure 2

Groundwater sampling location at Adama City and Wenji Gefersa Town.

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A longitudinal study design was performed to determine fluoride concentration. A two-season sampling was done to accommodate the temporal variability of the groundwater fluoride concentration. The sample locations were selected from study sites randomly and geo-recorded by using the smart Global Position System coordinate locator46. The samples were collected using a 50 ml polyethylene bottle which was rinsed with distilled water as well as with the actual groundwater before collecting the sample. The collected samples were stored at less than 4 °C during the field campaign, transportation and storage at the Addis Ababa University Water Resources Excellency Center Laboratory according to the drinking water sample collection protocol described by the United States of America Environmental Protection Agency (USEPA)47.

Determination of fluoride concentration and quality control

The Ion Selective Electrode (ISE) method was used to determine groundwater fluoride concentration by using the Crison-GLP 22 instrument in samples mixed at a 1:1 volume ratio with a total ionic strength adjustment buffer (TISAB) solution. This method can be used for measuring total solubilized fluoride in waters, wastewater, and in soil extracts. It is preferred to determine fluoride concentration in drinking water due to its high selectivity, specificity and low detection limit nature48. A range of pH 5-5.5 was maintained during mixing the samples with buffer solution, to optimize the analyses49. A triplicate analysis was performed for each sample and an average value was recorded. Calibration of Crison-GLP 22 was performed before analysis, by using the 5 standard solutions (0.5, 1.5, 10, 15 and 20 mg/L of fluoride) of fluoride stock standard solution (1,000 mg/L fluoride) (CRISON INSTRUMENTS, S.A).

Exposure and health risk assessment of fluoride

The fluoride exposure is assessed as the chronic daily intake (CDI) through drinking water and dermally absorbed dose (DAD). A probabilistic non-carcinogenic health risks assessment was calculated according to the US EPA assumption of standard, which is the most practical method used to compare health risk analysis for chemicals in drinking water50. The study population was classified into four age categories: infants (< 2 years), children (2 to < 6 years), adolescents (6 to < 16 years) and adults (>16 years) with quantitative values explained by Table 150. The exact location of study participants is not geocoded, but all study participants were permanent inhabitants of the towns and thus allocated their exposure based on the mean concentration of the towns they inhabited.

Non-carcinogenic health risk assessment was estimated by using hazard quotients for oral (HQoral) and dermal exposure (HQdermal).

Firstly, the CDI [mg/kg/day] was calculated using Eq. 151.

$$CDI=\left( {CW \times IR \times EF \times ED} \right)/\left( {BW \times AT} \right)$$
(1)

Where CW signifies the concentration of fluoride in groundwater (mg/L), IR is the ingestion rate (L/day), ED is the exposure duration (years), EF is the exposure frequency (days/years), BW is the average body weight (Kg) and AT is the average time (days) (Average time over which exposure is averaged).

$${\text{DAD}}=\left( {{\text{TC}} \times {\text{Ki}} \times {\text{EV}} \times {\text{ED}} \times {\text{EF}} \times {\text{SSA}} \times {\text{CF}}} \right)/\left( {{\text{BW}} \times {\text{AT}}} \right)$$
(2)

Secondly, the dermally absorbed daily dose (DAD, mg/kg/day) was calculated by using the Eq. 251.

Where TC is the contact duration (h/d) 0.4, Ki indicates the dermal adsorption parameters (cm/h: 0.001 cm/h), EV indicates the bathing frequency (times/day) 1 considered as time in a day, SSA is the skin surface area (cm2) 12,000 and 16,600 cm2 for children and females and 16,600 cm2 for males, CF indicates the conversion factors (0.001) and AT indicates the average time (days).

Table 1 Parameters used to estimate health risk of fluoride in drinking water50.
Full size table

The results from Eqs. 1 and 2 were used to estimate the non-carcinogenic health risk51.

$${\text{HQoral}}={\text{CDI}}/{\text{RfD}}$$
(3)
$${\text{HQdermal}}={\text{DAD}}/{\text{Rfd}}$$
(4)

US EPA refers to the reference dose (RfD) of fluoride as 0.06 mg/kg/day which is no health risk52. The summation of HQoral and HQdermal is known as THI, in the case of this study for the non-carcinogenic health risk of fluoride through oral ingestion and dermal exposure expressed follows (Eq. 5).

$$THI=HQdermal+HQoral$$
(5)

Methodological considerations

The strength of this study is demonstrated updated evidence on fluoride concentration in groundwater which is accessible to communities. Wenji Gefersa Town has one public tap where the water is scarce during the dry season and thus inaccessible. Additionally, a few households are accessing tap water every month or every two months from a private tap. Groundwater from a deep well found in Adama City center is blended into the main drinking water supply reservoir. Furthermore, it is distributed through car and horse-driven carts to different parts of the City. The limitation of this study is the lack of a geocoded location for each study participant, but the communities are permanent inhabitants of the towns. Additionally, exposure preference to beverages (tea, coffee, and alcohol)53 and food54 are not addressed by this study.

Statistical analysis

The data were entered into Microsoft Excel and exported to IBM (2011) IBM SPSS (version 22) Statistics for Windows, Version 20. IBM Corp., Armonk, which was used for the descriptive statistics such as average, standard deviation, minimum, and maximum. The non-parametric spearman rank correlation was applied to determine the relationships between fluoride contamination and other factors such as location, depth and altitude. Statistical significance was considered at a p-value of less than 0.05. The spearman correlation coefficient was varied from – 1 to + 1. A – 1 implied the perfectly negative correlation, + 1 implied a perfect correlation and 0 indicated no correlation among different parameters. The map of sampling locations was developed by using Arc GIS 10.7.1 software (ESRI, Redlands, CA, USA). All GPS data which was collected with degree, minutes, and second coordinate system were converted to the Universal Transverse Mercator (UTM) coordinates system.

Results

Groundwater fluoride concentration

In this study, groundwater samples were analyzed from Adama City and Wenji Gefersa Town to determine the fluoride concentration in the wet and dry seasons. Groundwater pH and temperature ranged from 5.04 to 7.67 and from 21.5 to 29 °C respectively in the wet season.

Fig. 3
figure 3

Ground fluoride concentration at Adama City and Wenji Gefersa Town.

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The fluoride concentration varied from 0.3 mg/L to 38 mg/L season and 1.1 mg/L to 24 mg/L during the wet and dry seasons respectively (Fig. 3). The mean annual fluoride concentrations for Adama City and Wenji Gefersa town were 1.9 mg/L and 23 mg/L respectively (Table 2).

Table 2 Descriptive statistics of Adama City and Wenji Gefersa town groundwater fluoride (mg/L) during wet and dry seasons.
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This study applied the WHO public health guideline for fluoride (1.5 mg/L) to explain the health effects of fluoride in drinking water sources. Of groundwater samples, 70% (during the wet season) and 45% of samples (during the dry season) had a fluoride concentration surpassing 5 mg/L (Supplementary Table S1). Furthermore, 17% and 20% of water samples in dry and wet seasons were at the WHO optimum range (0.5-1.5 mg/L) respectively.

In Adama, there are strong negative correlation between sample location such as latitude (p = 0.38), longitude (p = 0.85) and elevation (p = 0.14) and depth of groundwater (p = 0.31) on wet season groundwater fluoride contamination at significant level of the p <0.05 (Fig. 4). Furthermore, the same is true for dry season but with varied p-values for each variable (Supplementary Table S2). But the correlations are not statistically significant except elevation during dry season with p value of 0.028.

Fig. 4
figure 4

Spatial distribution of the Adama City (a) wet (b) dry seasons groundwater fluoride.

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According to spearman’s rank correlation coefficient longitude (p = 0.61), elevation (p = 0.65), and depth (0.95) are correlated negatively but not significant at the p <0.05 for wet season Wenji Gefersa Town groundwater fluoride contamination. Whereas latitude (p = 0.17) is moderately correlated but not significant at the p <0.05 (Fig. 5). Furthermore, dry season groundwater fluoride contamination is moderately correlated with latitude (p = 0.27), longitude (p = 0.45) and elevation (p = 0.87) but not significant at the p <0.05. Depth (p = 0.28) is negatively correlated with dry season groundwater fluoride contamination at the p <0.05 but not significant. (Supplementary Table S3).

Fig. 5
figure 5

Spatial distribution of the Wenji Gefersa Town (c) wet, (d) dry seasons groundwater fluoride.

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Human health risk assessment

This research evaluated the two important pathways (CDI and DAD) of groundwater fluoride exposure of infants, children, adolescents and adults at Adama City and Wenji Gefersa town during dry and wet seasons. The highest and lowest values CDI were 1.51 mg/kg per day at Wenji Gefersa Town for adolescents and 2.4E-03 mg/kg per day at Adama City for infant (Supplementary Table S4). In Adama City, DAD values for infants, children, adolescent and adults were 3E-04 mg/kg per day, 9.6E-04 mg/kg per day, 8.3E-06 mg/kg per day and 8.9E-05 mg/kg per day respectively. Whereas, Wenji Gefersa Town for infants (4.8E-04 mg/kg per day), children (2.3E-03 mg/kg per day), adolescent (1.3E-04 mg/kg per day) and adults (8.9E-05 mg/kg per day) (Table 3).

Table 3 Description of average CDI (mg/kg per day) and DAD mg/kg per day) among study groups at Adama City and Wenji Gefersa Town.
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Fluoride health risk assessments are assessed based on THI (HQoral and HQdermal) values to estimate non-carcinogenic health effects. This study evaluated HQder, HQoral and THI among different age categories. In Adama City annual average THI for infants, children, adolescents and adults were 1.52, 10.77, 1.27 and 1.06 respectively. Additionally, Wenji Gefersa Town for infants, children, adolescents and adults were 21.71, 130.36, 15.34 and 12.78 respectively (Table 4). For Wenji Gefersa Town THI varied from 8.01 to 81.75 and 15.05 to 153.45 in wet and dry seasons respectively. Furthermore Adama City THI varied from 1.03 to 10.58 at wet season and 0.45 to 4.52 dry seasons (Table 4).

Table 4 Description of HQdermal, HQoral and THI of study groups at Adama and Wenji Gefersa.
Full size table

Non-carcinogenic health effects due to groundwater fluoride exposure. For example, 58% of infants, 99% of children, 51% of adolescents, and 35% of adults had THI > 1 in the wet season. In the dry season, 85% of infants, 99% of children, 73% of adolescents, and 60% of adults had THI > 1. Wenji Gefersa Town residents have a higher probability of developing non-carcinogenic health effects than the population of Adama City. Variation was observed in the likelihood of developing non-carcinogenic health effects according to the THI value in the wet season for the study population of Adama City and Wenji Gefersa town. For example, the minimum THI value for Wenji Gefersa town is 70% while for Adama City is 33% (Supplementary Table S5).

Discussion

Groundwater fluoride concentration

This study assessed groundwater fluoride concentration during the wet and dry seasons a variation in Adama City and Wenji Gefersa Town water sources. These variations are due to the difference in aquifer types, for example, Wenji Gefersa town is alluvium while Adama City is fractured crystalline aquifers. In Adama, there are strong negative correlation between sample location such as latitude (p = 0.38), longitude (p = 0.85) and elevation (p = 0.14) and depth (p = 0.31) on wet and dry seasons groundwater fluoride contamination at significant level of the p <0.05. Groundwater fluoride concentration in Adama City and Wenji Gefersa Town is higher during the dry and wet seasons, respectively. Dry season high fluoride concentration in Adama City agrees with the previous study51. Evidence indicates that fluoride concentration depends on the depth of the well; shallow wells are highly vulnerable to anthropogenic pollution like fluoride such as agricultural activities51. However, another study reported that the fluoride concentration decreases with an increase of well depth55. Adama City and Wenji Gefersa Town groundwater fluoride contamination is negatively correlated with the depth of the well. This study found that Adama dry season maximum fluoride concentration was 3.55 mg/L, while the wet season was 1.49 mg/L. But the vice versa is true for Wenji Gefersa Town. According to spearman’s rank correlation coefficient for wet season Wenji Gefersa Town groundwater fluoride contamination is negatively correlated with sample location and depth. Study from semi-arid India groundwater fluoride concentration is less than both Adama City and Wenji Gefersa Town56. Wet season high fluoride concentration in Wenji Gefersa Town is subject to anthropogenic sources of pollution due to runoff. High concentration of fluoride in Wenji Gefersa town is due to the interior part of the well not being lined, compared to Adama, which is lined at the top as well as protected by the concrete box. Being not lined facilitates the soil-groundwater interaction and contributes to high fluoride concentration discharge. In this study catchment, large sugarcane plantation farms are apply herbicides and fertilizers, which is an anthropogenic source of fluoride in groundwater55.

This study’s findings revealed that the mean groundwater fluoride concentration is greater than in countries located in the Global Rift Valley, such as India57, Iran58, and Uganda59. This study mean fluoride concentration of 14.42 mg/L, which is higher than the mean of 6.03 mg/L according to the systematic review executed by Demelash et al., in ERV27. Wenji Gefersa Town Fluoride concentration is higher than compared to the fluoride in Adama Town and its vicinities whereas Adama Town fluoride is reported29. The maximum fluoride concentration of 38 mg/L suggests that the Adama-Wenji Gefersa region is a hotspot for fluoride exposure due to geological situations compared to other non-rift valley areas60. In this region, 20% of the groundwater samples were within the WHO guidelines but other study reported that 7.5% of groundwater fluoride samples exceeded WHO permissible level61. Indeed, the residents in the study area are at risk of different types of fluorosis. Furthermore, exposure to high drinking water fluoride concentration significantly reduces IQs of children20, and affects their cognitive function15.

Human health risk assessment

A finding from Tanzania, consumption of 4.7 mg/L of drinking water fluoride is associated with a 96.3% prevalence of dental fluorosis62. 50% of groundwater samples in Adama-Wenji Gefersa are greater than 5 mg/L of fluoride. Certainly, the residents are exposed to a fluoride concentration that has the potential to cause fluorosis. In the Adama-Wenji Gefersa region, the THI values were highest among infants, adult, children, and adolescents, were 0.95–30.43, 0.64–21.11, 0.84–20.49, and 0.77–20.26, respectively, in descending order. Recently, Haji et al. reported THI values for groundwater fluoride exposure in the Southern Main Ethiopian Rift varies from 0.75 to 8.44, 0.34–3.84 and 0.27–3.01 for children, women, and men, respectively28. This indicated that the Northern-Central Ethiopian Rift recorded higher drinking water related fluoride non-carcinogenic health effects compared to the southern part.

Resident living in the Adama and Wenji Rift region has a higher probability of developing non-carcinogenic health effects of fluoride compared to another non-rift valley region. Previous studies from Global Rift Valley countries such as India36 and Iran58 have reported mean values of HQoral for children, adolescents, and adults. This study finding reported that children and adults have higher than infants and adolescents regarding to probability of developing non-carcinogenic health effect. Similar findings have been reported from different parts of the Global Rift Valley, such as Iran63, Pakistan64, Ghana65.

Adama-Wenji Gefersa seasonal variation revealed that in the wet season, 58% of infants, 99% of children, 51% of adolescents, and 35% of adults had a higher risk of non-carcinogenic health risk according to the THI values. While in the dry season, this was true for 85% of infants, 99% of children, 73% of adolescents and 60% of adults. Adama City residents had a higher probability of developing non-carcinogenic health effects in the dry season. Similarly, Adimalla et al. reported that in India, men and women were 67.65% and 79.41%, respectively, with a value of THI greater than one, but for children the risk was even higher, i.e., 82.35%66. Water sources at the vicinities of Adama Town finding in line with this study i.e. it accounted 83% for children, 73% for women and 57% for men of samples exceed non-carcinogenic health threat of fluoride THI > 1. Another study reported that children and adolescents were at a higher probability of developing non-carcinogenic health effects than the rest of the population63. Furthermore, a study from ERV reported that the effects of dental fluorosis leads to children being reluctant to smile and speak as well as being challenged with food choices and with less chewing efficacy12. THI values and their related health hazards from this study’s findings could be expected due to the consumption of high fluoride though groundwater. The health risk of fluoride was estimated based on drinking water through oral and dermal pathways. This leads underestimation of exposure due to a lack of data from food, toothpaste, tea, and beverage sources. This study has no socio-demographic data for the actual study groups. But national age composition can support the age categories that are used to perform the risk assessment. Still, there is 4% of Ethiopians are age 65 or older, and age categories less than or equal to 65 account for 96%67. Furthermore, daily water consumption of the resident was varied from 0.25 to 5 l/day27 which make the water consumption rate used to estimate CDI and THI is valid to predict non-carcinogenic health risk.

Implication

The annual mean groundwater fluoride concentration in the Adama City and Wenji Gefersa Town were 1.9 mg/L and 23 mg/L respectively. Wenji Gefersa Town is higher than the Ethiopia National (3 mg/L) suggested by MoWE and the WHO (1.5 mg/L) guideline for drinking water fluoride. There is no sustainable and applicable monitoring of fluorination or defluorination of drinking water supply in suburban and rural part of Ethiopia but only few in cities. However, there are attempts to apply locally adopted defluorination techniques in the ERV. But still, residents at MER are subjected to higher groundwater fluoride concentration as well as exposure due to scarcity of alternative safe water sources. Dental and skeletal fluorosis is well known public health problems due to drinking water fluoride, but other systematic effects are overlooked. This study finding highlight the probability of developing non-carcinogenic health effects due to groundwater fluoride. Accordingly, infants, children and adolescents are at higher risk of developing different fluoride related health problems. Fluoride exposure assessments, and quantifications of health risks, play a profound role to motivate policies and developing guidelines to protect public health.

Conclusion

Poor quality of groundwater source is a challenging health problem in MER due to high fluoride concentration. In this region only 20% of water samples were acceptable ranges (i.e. 0.5 and 1.5 mg/L) of fluoride concentration according to WHO public health guideline. Wenji Gefersa Town mean groundwater fluoride concentration is fifteen times higher than the WHO drinking water guideline. But Adama City is higher than Ethiopia’s and WHO drinking water guideline. Fluoride concentration varied across the study sites and depth of the well. So that different age groups posed no risk or higher non-carcinogenic health risk. High groundwater fluoride concentration is contributed to a significant chronic daily intake of fluoride though drinking as well as dermal exposure while bathing and washing. Sensitive groups of communities such as infants, children and adolescents are at higher risk of developing non-carcinogenic health effects. This study finding indicated an alarming stage of developing health effects due to consumptions of fluoride contaminated groundwater sources for drinking and domestic purpose. Furthermore, THI risk evaluation through oral and dermal exposure due to groundwater alone underestimates the actual risk of fluoride exposure. Therefore, applying a more inclusive risk evaluation of fluoride exposure via food and beverages will improve.

Data availability

The data presented in this study are available on request from the corresponding author.

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Acknowledgements

The authors acknowledge the support given by Surafel Girma, Eyuel Asemahegn Bogale, Ali Gebeyewu, Fregenet Tesfaye (PhD), Eden Eritrea Desalegn and Mohammedsalih Hussen Ibrahim.

Funding

This research accessed financial support for field water sample collection and laboratory analysis by the Armauer Hansen Research Institute under Biomedical Science Student PhD Project (BSSP), Swedish International Development Cooperation Agency (SIDA).

Author information

Authors and Affiliations

  1. Ethiopia Institute of Water Resources, Addis Ababa University, 150461, Addis Ababa, Ethiopia

    Asmamaw Abera & Bezatu Mengistie

  2. Armauer Hansen Research Institute, 1005, Addis Ababa, Ethiopia

    Asmamaw Abera & Abraham Aseffa

  3. Africa Center of Excellence for Water Resources Management, Addis Ababa University, 1176, Addis Ababa, Ethiopia

    Asmamaw Abera

  4. Division of Occupational and Environmental Medicine, Lund University, Lund, 22362, Sweden

    Ebba Malmqvist

  5. Institute of Design Sciences, Lund University, Lund, 22362, Sweden

    Christina Isaxon

  6. Nano Lund, Lund University, Lund, 22362, Sweden

    Christina Isaxon

  7. School of Civil and Environmental Engineering, Addis Ababa Institute of Technology, Addis Ababa University, 385, Addis Ababa, Ethiopia

    Geremew Sahilu

Contributions

A.A.K. (Asmamaw Abera Kebede), E.M., C.I., G.S., and A.A. ( Abraham Aseffa) conceptualized the study design. A.A.K. drafted the manuscript. A.A.K., B.M., A.A., E.M., G.S., and C.I. conducted statistical analysis and designed the figures. A.A.K., B.M., A.A., E.M., G.S., and C.I. performed data quality control. A.A.K., E.M., G.S., and C.I. Supervised data collection. A.A.K., B.M., A.A., E.M., G.S,. and C.I. revised the manuscript. All authors approved the final version of the manuscript.

Corresponding author

Correspondence to Asmamaw Abera.

Ethics declarations

Competing interests

The authors declare no competing interests.

Ethical approval

The study has received ethical approval from Ethiopia Institute of Water Resources Addis Ababa University (Protocol No. EiWR043/11/18) and Armauer Hansen Research Institute (Protocol No. PO37/17). Additionally, a support letter was obtained from the Adama City water supply and wastewater management authority. The data was handled with appropriate confidentiality.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary Material 1

Cite this article

Abera, A., Aseffa, A., Mengistie, B. et al. Probabilistic human health risk assessment from groundwater fluoride contamination in Main Ethiopia Rift. Sci Rep 15, 28571 (2025). https://doi.org/10.1038/s41598-025-13821-7

FULL-TEXT STUDY ONLINE AT https://www.nature.com/articles/s41598-025-13821-7