Abstract

The purpose of this study was to determine the fluoride concentrations in drinking water of the Khaf County in Eastern Iran. Moreover, health risk assessment of three age groups (children, teens and adults), sensitivity analysis and uncertainties in the risk estimates were carried out using Monte Carlo simulation. For this reason, drinking water in 33 villages and 5 cities of the Khaf County were collected during March to September 2018. Fluoride contents in drinking water samples were assayed by using a HACH-DR6000 spectrophotometer. Fluoride content in drinking water from urban area and rural area ranged from 0.50 to 0.91 mg L−1 and 0.24 to 2.31 mg L−1. Among the population of the 33 villages, about 17 villages, corresponding to 51%, receive fluoride concentrations less than 0.5 mg L−1 (minimum allowable concentration recommended by WHO), while the population of 4 villages, corresponding to 12%, receive fluoride concentrations higher than 1.5 mg L−1 (the maximum allowable concentration of fluoride in drinking water recommended by the WHO). Moreover, our findings showed that the drinking water ingestion rate, fluoride concentration in water, and the fraction of skin in contact with water were the most important variable in calculating the Hazard quotient (HQ).

INTRODUCTION

Nowadays, a readily available access to safe drinking water resources is an important aspect of public health in many countries around the world including Iran (Garg & Malik 2004; Fallahzadeh et al. 2018). Iran is a vast country, and has diverse climatic conditions in comparison to other countries. Khorasan Razavi is located in the northeast of Iran. In the southern part of this province, because of its close proximity to the desert, a dry climate is dominant (Keshavarzi et al. 2006; Madani 2014; Soltani et al. 2016). Providing safe and quality drinking water will have a significant impact on sustaining life and social health. Generally, potable water contains both essential and non-essential elements. A lot of elements such as calcium, manganese, and fluoride that exist in drinking water are essential for metabolic activities as well as human health (Nerbrand et al. 2003; Li et al. 2009; Kazi et al. 2018a, 2018b). However, increasing the concentration of trace elements in relation to the required level in the body can lead to health problems. Contamination of drinking water with fluoride and its potential health implications remain a major public health issue in many countries, especially in Iran (Yousefi et al. 2018). Therefore continuous monitoring and control of drinking water quality is very important (WHO 2004).

Fluoride (F) is one of the mineral elements that exist in the earth's crust. This F ion is highly reactive due to the presence of a free electron in its upper electron layer, and is mostly present in compounds such as apatite and cryolite in nature (Kır et al. 2016). Contamination of surface and groundwater by fluoride can be traced to natural resources (availability and solubility of fluoride minerals and fluoride-bearing rocks) and anthropogenic resources (hydrofluoric production, fertilization, electroplating of metals, the electronics manufacturing industry) (Tor 2006; Xu et al. 2011). The World Health Organization (WHO) has recommended the minimum and maximum allowable concentration of fluoride in drinking water as 0.5 and 1.5 mg L−1 respectively. Defluoridation processes are recommended for water supplies where their fluoride concentration is higher than the standards (Kazi et al. 2018a, 2018b).

Also, according to the United States Public Health Service (USPHS) the maximum allowable concentration of fluoride in water depends on climate conditions because air temperature affects the amount of water consumed and consequently the exposure to the amount of fluoride (USPHS 1963; WHO 2004; Miretzky & Cirelli 2011). The optimal water fluoride concentration depends on several parameters such as methods of food processing and cooking, amount of food and water intake, dietary habits of the community, local climatic conditions, and physio-chemical parameters of water (Grimaldo et al. 1995; Kaseva 2006; Maheshwari 2006). Although drinking water is the main source of human exposure to fluoride, food, dental products, and pesticides are other important sources for dealing with fluoride (Garg & Malik 2004; National Research Council 2007).

Fluoride fluctuations can be associated with health problem of consumers, so that low fluoride concentrations cause tooth decay, and high concentrations of this element in water increase the risk of dental and bone fluorosis. According to previous studies, nearly 80% of prevalence and incidence of diseases all over the world are related to low water quality. Also contamination of drinking water with fluoride accounts for 65% of endemic fluorosis (World Health Organization 2011). Moreover, long term exposure to high fluoride concentrations may lead to various diseases such as dental and skeletal fluorosis, low hemoglobin levels, skin rashes, gland damage, gastrointestinal problems, infertility, abortions, neurotoxicological effects, and Alzheimer's disease (Sehn 2008; Choi et al. 2012; Choi et al. 2015). Research reported that more than 30 million people in China suffer from fluorosis and about 100 million people are exposed to it (Chen et al. 2010). In another study conducted by Huang et al. (2017), the authors reported that the concentration of fluoride in groundwater in California is more than 1.5 mg L−1 (Huang et al. 2017).

Moreover, the variations of fluoride concentration in drinking water are one of the main problems in drinking water quality in Iran. In some cities in Iran, the amount of fluoride in drinking water is higher than 1.5 mg L−1, which can lead to fluorosis (Asgari et al. 2012). For example Qasemi et al. (2018) reported that 55% of the rural areas in Gonabad and 4.7% of the rural areas in Bajestan had fluoride levels below the minimum recommended value of WHO (Qasemi et al. 2018). In another study, Ghaderpoori et al. (2018) illustrated that the fluoride levels from all sampling stations in the water distribution network of Mashhad in Eastern of Iran were below 0.5 mg L−1 (Ghaderpoori et al. 2018). Therefore, the aim of this study was to determine the fluoride concentration of drinking water in 33 villages and five cities of the Khaf County, which is located in the southern part of Khorasan Razavi province, from March to September 2018. Moreover, we assessed the potential health risks from fluoride ingestion through drinking water for infants, children, teenagers and adults. The results of this study could be useful for future planning of water resources as well as public knowledge about health problems related to high fluoride concentrations.

MATERIALS AND METHODS

The present study was descriptive-analytical and was performed experimentally on drinking water resources of Khaf County in Khorasan Razavi Province of Iran for determination of the fluoride ion concentration. The Khaf County contains five cities including Khaf, Nashtifan, Qasemabad, Salami, and Sangan. The climate of this region is hot and dry; that is, hot summers and cool winters. The mean annual temperature and rainfall in Khaf County are 18 °C and 120 mm respectively and the annual average of rainfall is about 255.7 mm. At the 2016 census, the county's population was 140,000 in 26,542 families. In the present study, all drinking water resources (38 wells and springs: five cities and 33 villages) were sampled in spring and summer of 2018 and their fluoride concentration was determined. The sampling locations as well as the map of Khaf County are shown in Figure 1.

Figure 1

Map of the study area and sampling stations in Khaf County, Eastern Iran.

Figure 1

Map of the study area and sampling stations in Khaf County, Eastern Iran.

Prior to sampling and accessing the drinking water resources, necessary coordination with the Water and Wastewater Administration was carried out and the required permissions were obtained. One-liter polyethylene containers were used to sample water from the studied areas. The containers were rinsed with acid and distilled water, and thoroughly cleaned before sampling. The date, time, and location of sampling were recorded on all the sampling containers. Then, samples were transferred to the laboratory by storing the cold chain (in 4 °C). Finally, fluoride concentrations in water samples were determined within 24 h after collection in the laboratory using a HACH-DR6000 spectrophotometer machine with SPADNS reagent. The detection limit for fluoride concentration was 0.02–0.2 mg L−1 (method 8029). Moreover, in this study, Arc GIS 10.4.1 software (Esri, Berkeley, CA, USA) was used for spatial distribution of fluoride in the studied areas.

In this study, the daily exposure to fluoride by drinking water was estimated using Equations (1) and (2) (USEPA 2004; Fakhri et al. 2018): 
formula
(1)
 
formula
(2)
In this regard, EDIing estimates daily intake of fluoride consumed per day by drinking water and EDIderm estimates the amount of fluoride received by skin absorption based on mg kg−1 day−1. Cw is the concentration of fluoride in drinking water in mg L−1, IRw is the drinking water ingestion rate based on L/day, EF is the exposure frequency based on day/year, ED is the exposure duration in terms of years, BW is the body weight in kg, AT is the averaging time in days, SA is the surface area of skin in terms of cm2, Kp is the coefficient of skin permeation (Cm/h), fluoride is the fraction of the contact surface of the skin with water (without unit) and ETs is the exposure time when showering (h/day). The Hazard quotient (HQ) of the non-carcinogenic risk estimate for fluoride exposure through drinking water and dermal exposure is calculated using Equation (3). 
formula
(3)

RfD in this equation expresses the reference fluoride dose by a specific exposure pathway in mg kg−1 day−1. Based on the USEPA database, the amount of RfD, oral reference dose, through drinking water consumption is 0.06 mg kg−1 day−1 (USEPA 2004). HQ < 1 implies a negligible risk of non-carcinogenic impacts, whereas HQ >1 indicates potential non-cancer-causing health effects. Based on the USEPA, RfD represents the reference dose of fluoride in a specific exposure pathway (mg kg−1 day−1).

Monte Carlo simulation and sensitivity analysis

We used sensitivity analysis in Monte Carlo simulation to identify important and effective parameters in the output model (Table 1). Crystal Ball software was used to simulate Monte Carlo. We also used 1,000 trails to analyze the sensitivity analysis. In this study, Crystal Ball (version 11.1.1.1, Oracle, Inc., USA) was used to simulate Monte Carlo and perform sensitivity analysis with 1,000 trails.

Table 1

Parameters used for the probabilistic risk model

Parameters (units)Distribution typeValues
ChildrenTeensAdults
Skin surface area (cm2Lognormal 7422 ± 1.25 14321 ± 1.18 18182 ± 1.10 
Body weight (kg) Lognormal 16.68 ± 1.48 46.25 ± 1.18 57.03 ± 1.10 
Ingestion rate (L/day) Normal 1.25 ± 0.57 1.58 ± 0.69 1.95 ± 0.64 
Average time (days) Fixed value 2190 2190 9125 
Exposure frequency (day/year) Triangular Min: 180
Mode: 345
Max: 365 
Min: 180
Mode: 345
Max: 365 
Min: 180
Mode: 345
Max: 365 
Exposure duration (year) Fixed value 
Dermal permeability constant (cm/h) Fixed value 1 × 10−3 1 × 10−3 1 × 10−3 
Exposure time in the shower (h/day) Lognormal 0.13 ± 0.0085 0.13 ± 0.0085 0.13 ± 0.0085 
Fraction of skin in contact with water Uniform Min: 0.4 Min: 0.4 Min: 0.4
Max: 0.9 
Fraction of fluoride absorbed in gastrointestinal tract Fixed value 
Oral reference dose (mg kg−1 day−1Fixed value 0.06 0.06 0.06 
Parameters (units)Distribution typeValues
ChildrenTeensAdults
Skin surface area (cm2Lognormal 7422 ± 1.25 14321 ± 1.18 18182 ± 1.10 
Body weight (kg) Lognormal 16.68 ± 1.48 46.25 ± 1.18 57.03 ± 1.10 
Ingestion rate (L/day) Normal 1.25 ± 0.57 1.58 ± 0.69 1.95 ± 0.64 
Average time (days) Fixed value 2190 2190 9125 
Exposure frequency (day/year) Triangular Min: 180
Mode: 345
Max: 365 
Min: 180
Mode: 345
Max: 365 
Min: 180
Mode: 345
Max: 365 
Exposure duration (year) Fixed value 
Dermal permeability constant (cm/h) Fixed value 1 × 10−3 1 × 10−3 1 × 10−3 
Exposure time in the shower (h/day) Lognormal 0.13 ± 0.0085 0.13 ± 0.0085 0.13 ± 0.0085 
Fraction of skin in contact with water Uniform Min: 0.4 Min: 0.4 Min: 0.4
Max: 0.9 
Fraction of fluoride absorbed in gastrointestinal tract Fixed value 
Oral reference dose (mg kg−1 day−1Fixed value 0.06 0.06 0.06 

RESULTS

Fluoride concentration in drinking water

Based on Table 2, overall fluoride levels from all sampling sites in rural and urban residents living were 0.69 and 0.70 mg L−1 respectively. The minimum and maximum contents of fluoride ions in urban regions were 0.50 and 0.99 mg L−1 respectively, while the minimum and maximum content of fluoride ions in rural regions were 0.18 and 2.31 mg L−1 respectively. The variations of fluoride levels in drinking water of rural areas were more than urban areas. Based on results from Table 2, the population of 17 of 33 villages (51%) receive fluoride concentrations less than the limit recommended by WHO of 0.5 mg L−1, while the population of 4 of 33 villages (12%) receive fluoride concentrations higher than the maximum allowable concentration of fluoride in drinking water recommended by the WHO of 1.5 mg L−1. Moreover, distribution of fluoride levels in two seasons is presented in Figure 2 and 3, so that the highest concentrations of fluoride in drinking water were in the center and south locations of the studied area.

Table 2

Fluoride concentrations (mg L−1; mean and SD) in drinking water and frequency of hypertension in Khaf County, Eastern Iran

StationsLatitudeLongitudeMeanSDHypertension
Urban areas 
 Khaf 34.58687672 60.16021906 0.50 0.233 463 
 Nashtifan 34.47256712 60.20494397 0.54 0.339 451 
 Salami 34.74197028 59.98430501 0.54 0.091 475 
 Sangan 34.40604183 60.25967174 0.99 0.466 602 
 Qasemabad 34.35702395 59.85749109 0.91 0.014 400 
 Overall   0.69 0.229  
Rural areas 
 Sayed Abad 34.84537916 59.78127279 0.36 0.304 74 
 Sijavand 34.86352036 59.82868311 0.54 0.155 92 
 Chaman Abad 34.8323684 59.80180461 0.41 0.240 114 
 Mazrehshaikh 34.87756192 59.91496106 0.29 0.162 82 
 Sadeh 34.83145386 59.8949104 0.32 0.162 385 
 Khalil Abad 34.7886955 59.85509389 0.24 0.169 45 
 Hazarkhosheh 34.74454964 59.87032228 0.48 0.021 45 
 Ahmad Abad 34.77200424 59.90665737 0.35 0.141 140 
 Khair Abad 34.84628966 60.01281208 0.51 0.113 95 
 Hassan Abad 34.72589944 59.89459892 0.45 0.063 89 
 Chahar Deh 34.69690958 60.03558714 0.31 0.070 105 
 Razdab 34.81018533 60.12416025 0.27 0.219 73 
 Fayandar 34.66037219 60.0998979 0.46 0.077 70 
 Mahabad 34.58204424 60.06729215 2.31 0.487 12 
 Barakuh 34.68505963 60.17888137 0.58 0.205 35 
 Khargerd 34.54038693 60.18422183 0.56 0.056 116 
 Tizab 34.5160017 60.17971262 0.44 0.148 108 
 Barab 34.42904011 60.23412234 0.69 0.120 303 
 Nyaz Abad 34.23008266 60.24847463 1.40 0.148 83 
 Goryab 34.42357785 60.33274451 0.38 0.141 
 Dardoi 34.53296125 60.44891312 0.34 0.148 
 Zuzan 34.34673462 59.87040028 1.98 0.007 212 
 Ebrahimi 34.28437761 59.87041319 1.56 0.268 66 
 Baghbakhshi 34.45336889 59.52814272 0.65 0.148 80 
 Chahgachi 34.24257021 59.5531057 0.70 0.148 68 
 Kalshor (Abe-shirin) 34.02254292 59.79554483 0.29 0.070 57 
 Kalshor (Abe-shor) 34.02310912 59.79846006 0.68 0.049 44 
 Bonyabad (Abe-shor) 34.08077271 59.87445184 0.91 0.388 68 
 Bonyabad (Abe-shirin) 34.08007577v 59.87634548 0.18 0.084 17 
 Arg (Abe-shor) 34.1553623 59.91693782 1.85 0.155 10 
 Aliabad 34.21794791 59.96414062 1.32 0.021 17 
 Bias Abad 34.30870317 60.04716355 0.44 0.219 80 
 Baghcheh 34.43245196 60.10808853 1.04 0.410 
 Overall   0.70 0.161  
StationsLatitudeLongitudeMeanSDHypertension
Urban areas 
 Khaf 34.58687672 60.16021906 0.50 0.233 463 
 Nashtifan 34.47256712 60.20494397 0.54 0.339 451 
 Salami 34.74197028 59.98430501 0.54 0.091 475 
 Sangan 34.40604183 60.25967174 0.99 0.466 602 
 Qasemabad 34.35702395 59.85749109 0.91 0.014 400 
 Overall   0.69 0.229  
Rural areas 
 Sayed Abad 34.84537916 59.78127279 0.36 0.304 74 
 Sijavand 34.86352036 59.82868311 0.54 0.155 92 
 Chaman Abad 34.8323684 59.80180461 0.41 0.240 114 
 Mazrehshaikh 34.87756192 59.91496106 0.29 0.162 82 
 Sadeh 34.83145386 59.8949104 0.32 0.162 385 
 Khalil Abad 34.7886955 59.85509389 0.24 0.169 45 
 Hazarkhosheh 34.74454964 59.87032228 0.48 0.021 45 
 Ahmad Abad 34.77200424 59.90665737 0.35 0.141 140 
 Khair Abad 34.84628966 60.01281208 0.51 0.113 95 
 Hassan Abad 34.72589944 59.89459892 0.45 0.063 89 
 Chahar Deh 34.69690958 60.03558714 0.31 0.070 105 
 Razdab 34.81018533 60.12416025 0.27 0.219 73 
 Fayandar 34.66037219 60.0998979 0.46 0.077 70 
 Mahabad 34.58204424 60.06729215 2.31 0.487 12 
 Barakuh 34.68505963 60.17888137 0.58 0.205 35 
 Khargerd 34.54038693 60.18422183 0.56 0.056 116 
 Tizab 34.5160017 60.17971262 0.44 0.148 108 
 Barab 34.42904011 60.23412234 0.69 0.120 303 
 Nyaz Abad 34.23008266 60.24847463 1.40 0.148 83 
 Goryab 34.42357785 60.33274451 0.38 0.141 
 Dardoi 34.53296125 60.44891312 0.34 0.148 
 Zuzan 34.34673462 59.87040028 1.98 0.007 212 
 Ebrahimi 34.28437761 59.87041319 1.56 0.268 66 
 Baghbakhshi 34.45336889 59.52814272 0.65 0.148 80 
 Chahgachi 34.24257021 59.5531057 0.70 0.148 68 
 Kalshor (Abe-shirin) 34.02254292 59.79554483 0.29 0.070 57 
 Kalshor (Abe-shor) 34.02310912 59.79846006 0.68 0.049 44 
 Bonyabad (Abe-shor) 34.08077271 59.87445184 0.91 0.388 68 
 Bonyabad (Abe-shirin) 34.08007577v 59.87634548 0.18 0.084 17 
 Arg (Abe-shor) 34.1553623 59.91693782 1.85 0.155 10 
 Aliabad 34.21794791 59.96414062 1.32 0.021 17 
 Bias Abad 34.30870317 60.04716355 0.44 0.219 80 
 Baghcheh 34.43245196 60.10808853 1.04 0.410 
 Overall   0.70 0.161  
Figure 2

Distribution of fluoride concentrations in spring in Khaf County, Eastern Iran.

Figure 2

Distribution of fluoride concentrations in spring in Khaf County, Eastern Iran.

Figure 3

Distribution of fluoride concentrations in summer in Khaf County, Eastern Iran.

Figure 3

Distribution of fluoride concentrations in summer in Khaf County, Eastern Iran.

Calculation of chronic daily intake of fluoride

Mean value of EDIing for children, teens and adults in urban areas was 0.025, 0.011, and 0.002 mg kg−1 day−1 respectively, and for rural areas, the mean value of EDIing for children, teens and adults was 0.026, 0.011, 0.002 mg kg−1 day−1 respectively (Table 3). Furthermore, the mean value of the EDIderm for fluoride in urban samples for children, teens and adults was 7.964 × 10−6 (6.218 × 10−6 to 1.129 × 10−5), 0.0055 (0.0040 to 0.0078), and 0.0013 (0.0009 to 0.0019) mg kg−1 day−1 respectively (Table 3), and for rural areas, the mean value of the EDIderm for fluoride in rural samples for children, teens and adults was 8.087 × 10−6 (8.044 × 10−6 to 2.641 × 10−5), 0.0056 (0.0019 to 0.0183), and 0.0013 (0.0003 to 0.0045) mg kg−1 day−1 respectively.

Table 3

EDI (mg kg−1 day−1) for different age groups in the studied areas

StationsEDIing
EDIderm
Urban areasChildrenTeensAdultsChildrenTeensAdults
Khaf 0.0186 0.0085 0.0020 5.8E-06 0.0040 0.0009 
Nashtifan 0.0199 0.0090 0.0021 6.2E-06 0.0042 0.0010 
Salami 0.0201 0.0091 0.0022 6.2E-06 0.0043 0.0010 
Sangan 0.0365 0.0166 0.0040 1.1E-05 0.0078 0.0019 
Qasemabad 0.0336 0.0153 0.0036 1.0E-05 0.0072 0.0017 
Overall 0.0257 0.0117 0.0028 8.0E-06 0.0055 0.0013 
Rural areas 
 Sayed Abad 0.0134 0.0061 0.0014 4.2E-06 0.0028 0.0007 
 Sijavand 0.0199 0.0090 0.0021 6.2E-06 0.0042 0.0010 
 Chaman Abad 0.0151 0.0069 0.0016 4.7E-06 0.0032 0.0008 
 Mazrehshaikh 0.0109 0.0049 0.0011 3.4E-06 0.0023 0.0005 
 Sadeh 0.0120 0.0054 0.0013 3.7E-06 0.0025 0.0006 
 Khalil Abad 0.0088 0.0040 0.0009 2.7E-06 0.0019 0.0004 
 Hazarkhosheh 0.0179 0.0081 0.0019 5.5E-06 0.0038 0.0009 
 Ahmad Abad 0.0129 0.0058 0.0014 4.0E-06 0.0027 0.0006 
 Khair Abad 0.0188 0.0085 0.0020 5.8E-06 0.0040 0.0010 
 Hassan Abad 0.0168 0.0076 0.0018 5.2E-06 0.0036 0.0008 
 Chahar Deh 0.0114 0.0052 0.0012 3.5E-06 0.0024 0.0006 
 Razdab 0.0101 0.0046 0.0011 3.1E-06 0.0021 0.0005 
 Fayandar 0.0171 0.0078 0.0018 5.3E-06 0.0036 0.0009 
 Mahabad 0.0855 0.0390 0.0093 2.6E-05 0.0183 0.0045 
 Barakuh 0.0216 0.0098 0.0023 6.7E-06 0.0046 0.0012 
 Khargerd 0.0206 0.0094 0.0022 6.4E-06 0.0044 0.0010 
 Tizab 0.0164 0.0074 0.0018 5.1E-06 0.0035 0.0008 
 Barab 0.0256 0.0117 0.0028 7.9E-06 0.0055 0.0013 
 Nyaz Abad 0.0519 0.0236 0.0056 1.6E-05 0.0111 0.0027 
 Goryab 0.0140 0.0064 0.0015 4.3E-06 0.0030 0.0007 
 Dardoi 0.0127 0.0058 0.0013 3.9E-06 0.0027 0.0006 
 Zuzan 0.0733 0.0334 0.0080 2.3E-05 0.0157 0.0038 
 Ebrahimi 0.0576 0.0262 0.0063 1.8E-05 0.0123 0.0030 
 Baghbakhshi 0.0242 0.0110 0.0026 7.5E-06 0.0052 0.0012 
 Chahgachi 0.0260 0.0118 0.0028 8.0E-06 0.0055 0.0013 
 Kalshor (Abe-shirin) 0.0107 0.0048 0.0011 3.3E-06 0.0023 0.0005 
 Kalshor (Abe-shor) 0.0253 0.0115 0.0027 7.8E-06 0.0054 0.0013 
 Bonyabad (Abe-shor) 0.0338 0.0154 0.0037 1.0E-05 0.0072 0.0017 
 Bonyabad (Abe-shirin) 0.0066 0.0030 0.0007 2.1E-06 0.0014 0.0003 
 Arg (Abe-shor) 0.0683 0.0311 0.0074 2.1E-05 0.0146 0.0036 
 Aliabad 0.0489 0.0223 0.0053 1.5E-05 0.0105 0.0025 
 Bias Abad 0.0164 0.0074 0.00180 5.1E-06 0.0035 0.0008 
 Baghcheh 0.0384 0.0175 0.0042 1.2E-05 0.0082 0.0020 
 Overall 0.0261 0.0119 0.0028 8.2E-06 0.0056 0.0013 
StationsEDIing
EDIderm
Urban areasChildrenTeensAdultsChildrenTeensAdults
Khaf 0.0186 0.0085 0.0020 5.8E-06 0.0040 0.0009 
Nashtifan 0.0199 0.0090 0.0021 6.2E-06 0.0042 0.0010 
Salami 0.0201 0.0091 0.0022 6.2E-06 0.0043 0.0010 
Sangan 0.0365 0.0166 0.0040 1.1E-05 0.0078 0.0019 
Qasemabad 0.0336 0.0153 0.0036 1.0E-05 0.0072 0.0017 
Overall 0.0257 0.0117 0.0028 8.0E-06 0.0055 0.0013 
Rural areas 
 Sayed Abad 0.0134 0.0061 0.0014 4.2E-06 0.0028 0.0007 
 Sijavand 0.0199 0.0090 0.0021 6.2E-06 0.0042 0.0010 
 Chaman Abad 0.0151 0.0069 0.0016 4.7E-06 0.0032 0.0008 
 Mazrehshaikh 0.0109 0.0049 0.0011 3.4E-06 0.0023 0.0005 
 Sadeh 0.0120 0.0054 0.0013 3.7E-06 0.0025 0.0006 
 Khalil Abad 0.0088 0.0040 0.0009 2.7E-06 0.0019 0.0004 
 Hazarkhosheh 0.0179 0.0081 0.0019 5.5E-06 0.0038 0.0009 
 Ahmad Abad 0.0129 0.0058 0.0014 4.0E-06 0.0027 0.0006 
 Khair Abad 0.0188 0.0085 0.0020 5.8E-06 0.0040 0.0010 
 Hassan Abad 0.0168 0.0076 0.0018 5.2E-06 0.0036 0.0008 
 Chahar Deh 0.0114 0.0052 0.0012 3.5E-06 0.0024 0.0006 
 Razdab 0.0101 0.0046 0.0011 3.1E-06 0.0021 0.0005 
 Fayandar 0.0171 0.0078 0.0018 5.3E-06 0.0036 0.0009 
 Mahabad 0.0855 0.0390 0.0093 2.6E-05 0.0183 0.0045 
 Barakuh 0.0216 0.0098 0.0023 6.7E-06 0.0046 0.0012 
 Khargerd 0.0206 0.0094 0.0022 6.4E-06 0.0044 0.0010 
 Tizab 0.0164 0.0074 0.0018 5.1E-06 0.0035 0.0008 
 Barab 0.0256 0.0117 0.0028 7.9E-06 0.0055 0.0013 
 Nyaz Abad 0.0519 0.0236 0.0056 1.6E-05 0.0111 0.0027 
 Goryab 0.0140 0.0064 0.0015 4.3E-06 0.0030 0.0007 
 Dardoi 0.0127 0.0058 0.0013 3.9E-06 0.0027 0.0006 
 Zuzan 0.0733 0.0334 0.0080 2.3E-05 0.0157 0.0038 
 Ebrahimi 0.0576 0.0262 0.0063 1.8E-05 0.0123 0.0030 
 Baghbakhshi 0.0242 0.0110 0.0026 7.5E-06 0.0052 0.0012 
 Chahgachi 0.0260 0.0118 0.0028 8.0E-06 0.0055 0.0013 
 Kalshor (Abe-shirin) 0.0107 0.0048 0.0011 3.3E-06 0.0023 0.0005 
 Kalshor (Abe-shor) 0.0253 0.0115 0.0027 7.8E-06 0.0054 0.0013 
 Bonyabad (Abe-shor) 0.0338 0.0154 0.0037 1.0E-05 0.0072 0.0017 
 Bonyabad (Abe-shirin) 0.0066 0.0030 0.0007 2.1E-06 0.0014 0.0003 
 Arg (Abe-shor) 0.0683 0.0311 0.0074 2.1E-05 0.0146 0.0036 
 Aliabad 0.0489 0.0223 0.0053 1.5E-05 0.0105 0.0025 
 Bias Abad 0.0164 0.0074 0.00180 5.1E-06 0.0035 0.0008 
 Baghcheh 0.0384 0.0175 0.0042 1.2E-05 0.0082 0.0020 
 Overall 0.0261 0.0119 0.0028 8.2E-06 0.0056 0.0013 

Sensitivity analyses

Findings showed that the HQ value for consuming water containing fluoride for drinking was higher than the HQ level for dermal contact (Figure 4). Moreover, the most important variables affecting the value of HQing in the three age groups are drinking water ingestion rate (IRw) and fluoride concentration in water (CW), while concentration of fluoride in water and the fraction of skin in contact with water (F) are the most important variables in the value of HQderm in dermal contact. In the three age groups, IRw was the most important variable affecting HQoverall. Figure 5 shows the results of the sensitivity analysis to assess the non-carcinogenic risk for the three age groups exposed to fluoride. In urban areas, IRw was the most important variable affecting the amount of health risk in the three age groups of children, teens, and adults. After IRw, fluoride concentration in drinking water (C) and exposure frequency (EF) were the most effective variable on the value of health risk assessment in cities and rural areas.

Figure 4

Sensitivity analysis based on the type of dermal contact and drinking water ingestion for different age groups.

Figure 4

Sensitivity analysis based on the type of dermal contact and drinking water ingestion for different age groups.

Figure 5

Sensitivity analysis results for three age groups of children, teens and adults in studied regions.

Figure 5

Sensitivity analysis results for three age groups of children, teens and adults in studied regions.

DISCUSSION

The fluoride concentrations in drinking water of Khaf County ranged from 0.50 mg L−1–0.99 mg L−1 in urban areas whereas the ranges in rural region were 0.18 mg L−1–2.31 mg L−1. Although the fluoride levels determined in some rural areas were outside the range of that recommended by WHO, in temperate regions, where water intake is low, a fluoride level up to 1.5 mg L−1 is acceptable (World Health Organization 2011). Populations of 4 villages in the studied area were exposed to fluoride content higher than the WHO's maximum allowable concentration. On the other hand, it is illustrated that a higher concentration of fluoride in drinking water than 0.7 mg L−1 can pose some adverse health effects such as fluorosis, so that fluorosis prevalence was reported in different provinces of Iran (Fallahzadeh et al. 2018; Yousefi et al. 2018; Keramati et al. 2019). Moreover, some studies done in Saudi Arabia and Mexico also reported the presence of fluoride levels in drinking water exceeding the permissible limits (Hurtado et al. 2000; Brahman et al. 2014). Almost 200 million people from 25 countries are at risk of health problems for high concentration of fluoride in their drinking water (Ayoob & Gupta 2006; Amouei et al. 2012; Huang et al. 2017). There is a pressing need for de-fluoridation activities to be carried out in high-risk regions whose drinking water contains higher fluoride concentration (Chidambaram et al. 2013; Habuda-Stani et al. 2014).

Hard tissue deformities like dental and skeletal fluorosis are seen in populations exposed to high levels of fluoride in drinking water (Fordyce 2011). Higher concentrations cause damage to the soft tissues like liver, kidney, lung, and testis and may also induce skeletal cancer and neurotoxicological effects (Bassin et al. 2006; Barbier et al. 2010; Zhang et al. 2016). Fluoride levels of up to 1.5 mg L−1 in drinking water are acceptable for maximum benefits and minimum risks, which is also recommended by WHO, other organizations in other countries, such as the European Union (DECLG), Canada (Ottawa), Australia (NHMRC), Ozsvath and India (BIS) (Ozsvath 2009; European Union 2014; Government of Canada 2010, World Health Organization 2011; NHMRC 2011; IS10500 B 2012). The US Public Health Service has set the optimum concentration of fluoride in drinking water at 0.7 mg L−1 (Kohn et al. 2001).

Fluoride levels in drinking water of 17 villages in Khaf County were less than the limit recommended by WHO. Hence, it was observed that different areas within the same county demonstrated differing levels of fluoride in drinking water. When the fluoride level in drinking water is less than 0.5 mg L−1, the risk of dental caries increases (Dissanayake 1991; Ozsvath 2009). The low level of fluoride in drinking water observed in these regions was similar to those observed in various other studies (Huang et al. 2017; Ram et al. 2017). The villages with fluoride content less than 0.5 mg L−1 may require fluoridation programs, health education initiatives, fluoride supplements or any other combined strategies for prevention of dental caries in their population. Another recommendation is to establish a drinking water supply from deep wells with adequate fluoride content in that region with suitable conditions. Prevention of dental caries and strengthening of bones is observed when a population is exposed to low concentration of fluoride (0.5–1.5 mg L−1) in drinking water (Dissanayake 1991; Ozsvath 2009).

In our study, we also observed that children exposed to high levels of fluoride in drinking water demonstrated dental fluorosis. In similar studies, Huang et al. (2017), Guissouma et al. (2017), and Fallahzadeh et al. (2018) reported that the children age group was the population more at potential health risk of fluoride than other age groups (Guissouma et al. 2017; Huang et al. 2017; Fallahzadeh et al. 2018). The chronic daily intake of fluoride (daily exposure) was calculated in this study for three different age groups and both in rural as well as urban areas. The mean value of both EDIing and EDIderm was more in the children group compared to teens and adults, both in rural as well as urban areas. Signifying higher daily exposure of fluoride content in the children's age-group, this may have higher risk of adverse effects in them. Humans could be exposed to fluoride in drinking water through oral intake, dermal absorption, and inhalation. It was observed that health risk values (HQ) were higher for exposure to fluoride through consumption of water containing fluoride (HQing) than for exposure through dermal contact (HQderm). Out of all the variables considered for this analysis, the most important variables affecting the value of HQing in all the three age groups were drinking water ingestion rate (IRw) and fluoride concentration in water (CW), while concentration of fluoride in water and the fraction of skin in contact with water (F) were the most important variables in the value of HQderm in dermal contact. These observations were in accordance with the previous studies done in different populations in the same as well as different countries. For more accurate health risk assessment of fluoride exposure in further studies, there should be more focus on these parameters identified by sensitivity analysis (Huang et al. 2017; Fallahzadeh et al. 2018).

In our study, the overall HQ value was less than 1, indicating that there was negligible risk of non-carcinogenic effect of exposure to fluoride for the population of Khaf County. Similarly, in a study done by Zhang et al. (2016), the calculated national mean non-carcinogenic risk for Chinese residents were lower than 1, thus indicating the absence of potential health effects at the national level. However, the HQs of fluoride were >1 in some areas, which may pose possible health risks to local residents (Zhang et al. 2016). Among the three age groups assessed in this study, children were at the highest non-carcinogenic risk possibly because children have the lowest BW, and other exposure parameters are similar to those of teens and adults. These findings are consistent with the study done by Huang et al. (2017). In a study conducted by Guissouma et al. (2017), it was observed that consumers of drinking water in areas where the HQ is higher than the guidelines suffer from dental fluorosis (Guissouma et al. 2017). The fluoride level in drinking water is not the recommended standard for the control of dental caries; on the basis of the socioeconomic and climatic conditions and the dietary and oral hygiene habits of their population, each country should determine the concentrations of fluoride in drinking water (Khan et al. 2004). Although fluoride in drinking water is commonly considered the greatest contributor to daily intake, other fluoride sources, including beverages, foodstuffs and fluoride supplements, may also significantly contribute to daily fluoride intake (Erdal & Buchanan 2004; Li et al. 2009).

Hence, in this study, the non-carcinogenic risk due to exposure to fluoride could be underestimated because only the drinking water and dermal absorption exposure pathway was considered. Additional data related to fluoride exposure should be collected from each exposure pathway, and by considering additional resources and time, accurate and precise estimates should be further investigated. In future studies, to obtain accurate data on health risk estimates of fluoride exposure on the population of Khaf County, fluoride exposure via inhalation should be investigated. Measures should be implemented to strengthen the timely monitoring of fluoride levels in drinking water in Khaf County.

CONCLUSION

The findings of this study indicated that the fluoride levels in drinking water in rural and urban areas was 0.70 and 0.69 mg L−1 respectively, although fluoride concentrations in rural areas showed more variation than urban areas. Moreover, our results showed that the HQ value for consuming water containing fluoride for drinking was higher than the HQ level for dermal contact. Sensitivity analysis illustrated that the drinking water ingestion rate and fluoride concentration in water were the most important variables affecting the value of HQing, while the concentration of fluoride in water and the fraction of skin in contact with water were the most important variables in the value of HQderm in dermal contact in three age groups. As a result, the overall HQ in the studied areas was less than 1, indicating that there was negligible risk of non-carcinogenic effect of exposure to fluoride.

ACKNOWLEDGEMENTS

The authors of this study gratefully acknowledge the Research Council of Birjand University of Medical Sciences (Grant Number: 97/125) for the financial support.

REFERENCES

REFERENCES
Amouei
A.
,
Mahvi
A.
,
Mohammadi
A.
,
Asgharnia
H.
,
Fallah
S.
&
Khafajeh
A.
2012
Fluoride concentration in potable groundwater in rural areas of Khaf city, Razavi Khorasan province, Northeastern Iran
.
The International Journal of Occupational and Environmental Medicine
3
(
4
),
201
203
.
Asgari
G.
,
Roshani
B.
&
Ghanizadeh
G.
2012
The investigation of kinetic and isotherm of fluoride adsorption onto functionalize pumice stone
.
Journal of Hazardous Materials
217
,
123
132
.
Ayoob
S.
&
Gupta
A. K.
2006
Fluoride in drinking water: a review on the status and stress effects
.
Critical Reviews in Environmental Science and Technology
36
,
433
487
.
Barbier
O.
,
Arreola-Mendoza
L.
&
Del razo
L. M.
2010
Molecular mechanisms of fluoride toxicity
.
Chemico-biological Interactions
188
,
319
333
.
Bassin
E. B.
,
Wypij
D.
,
Davis
R. B.
&
Mittleman
M. A.
2006
Age-specific fluoride exposure in drinking water and osteosarcoma (United States)
.
Cancer Causes & Control
17
,
421
428
.
Brahman
K. D.
,
Kazi
T. G.
,
Baig
J. A.
,
Afridi
H. I.
,
Arain
S. S.
,
Khan
A.
&
Arain
M. B.
2014
Fluoride and arsenic exposure through water and grain crops in Nagarparkar, Pakistan
.
Chemosphere
100
,
182
189
.
Chen
N.
,
Zhang
Z.
,
Feng
C.
,
Sugiura
N.
,
Li
M.
&
Chen
R.
2010
Fluoride removal from water by granular ceramic adsorption
.
Journal of Colloid and Interface Science
348
,
579
584
.
Chidambaram
S.
,
Manikandan
S.
,
Ramanathan
A.
,
Prasanna
M.
,
Thivya
C.
,
Karmegam
U.
,
Thilagavathi
R.
&
Rajkumar
K.
2013
A study on the defluoridation in water by using natural soil
.
Applied Water Science
3
,
741
751
.
Choi
A. L.
,
Sun
G.
,
Zhang
Y.
&
Grandjean
P.
2012
Developmental fluoride neurotoxicity: a systematic review and meta-analysis
.
Environmental Health Perspectives
120
,
1362
1368
.
Choi
A. L.
,
Zhang
Y.
,
Sun
G.
,
Bellinger
D. C.
,
Wang
K.
,
Yang
X. J.
,
Li
J. S.
,
Zheng
Q.
,
Fu
Y.
&
Grandjean
P.
2015
Association of lifetime exposure to fluoride and cognitive functions in Chinese children: a pilot study
.
Neurotoxicology and Teratology
47
,
96
101
.
Dissanayake
C.
1991
The fluoride problem in the ground water of Sri Lanka – environmental management and health
.
International Journal of Environmental Studies
38
,
137
155
.
European Union
2014
European Union (Drinking Water) Regulations, SI No. 122. European Union, Brussels, Belgium
.
Fakhri
Y.
,
Mohseni-Bandpei
A.
,
Oliveri Conti
G.
,
Ferrante
M.
,
Cristaldi
A.
,
Jeihooni
A.
,
Karimi Dehkordi
M.
,
Alinejad
A.
,
Rasoulzadeh
H.
,
Mohseni
S.
,
Sarkhosh
M.
,
Keramati
H.
,
Moradi
B.
,
Amanidaz
N.
&
Baninameh
Z.
2018
Systematic review and health risk assessment of arsenic and lead in the fished shrimps from the Persian gulf
.
Food and Chemical Toxicology
113
,
278
286
.
Fallahzadeh
R. A.
,
Miri
M.
,
Taghavi
M.
,
Gholizadeh
A.
,
Anbarani
R.
,
Hosseini-Bandegharaei
A.
,
Ferrante
M.
&
Conti
G. O.
2018
Spatial variation and probabilistic risk assessment of exposure to fluoride in drinking water
.
Food and Chemical Toxicology
113
,
314
321
.
Fordyce
F. M.
2011
Fluorine: human health risks. In: Encyclopedia of Environmental Health (J. O. Nriagu, ed.), Vol. 2, pp. 776–785. Elsevier, New York, NY
.
Ghaderpoori
M.
,
Paydar
M.
,
Zarei
A.
,
Alidadi
H.
,
Najafpoor
A. A.
,
Gohary
A. H.
&
Shams
M.
2018
Health risk assessment of fluoride in water distribution network of Mashhad, Iran
.
Human and Ecological Risk Assessment: An International Journal
25
(
4
),
1
12
.
Government of Canada
2010
Guidelines for Canadian Drinking Water Quality: Guideline Technical Document — Fluoride
.
Federal-Provincial-Territorial Committee on Drinking Water. Government of Canada, Ottawa, ON, Canada
.
Grimaldo
M.
,
Borjaaburto
V. H.
,
Ramirez
A. L.
,
Ponce
M.
,
Rosas
M.
&
Diazbarriga
F.
1995
Endemic fluorosis in San-Luis-Potosi, Mexico. 1. Identification of risk-factors associated with human exposure to fluoride
.
Environmental Research
68
,
25
30
.
Guissouma
W.
,
Hakami
O.
,
Al-Rajab
A. J.
&
Tarhouni
J.
2017
Risk assessment of fluoride exposure in drinking water of Tunisia
.
Chemosphere
177
,
102
108
.
Habuda-Stani
M.
,
Ravanèi
M.
&
Flanagan
A.
2014
A review on adsorption of fluoride from aqueous solution
.
Materials
7
,
6317
6366
.
Hurtado
R.
,
Gardea-Torresdey
J.
&
Tiemann
K.
2000
Fluoride occurrence in tap water at ‘Los Altos de Jalisco’, in the central Mexico region
. In:
Proceedings of the 2000 Conference on Hazardous Waste Research: Environmental Changes and Solutions to Resource Development, Production, and use
, pp.
23
25
.
IS10500, B
2012
Indian Standard Drinking Water–Specification (Second Revision)
.
Bureau of Indian Standards (BIS)
,
New Delhi
.
Kazi
T. G.
,
Brahman
K. D.
,
Baig
J. A.
&
Afridi
H. I.
2018a
A new efficient indigenous material for simultaneous removal of fluoride and inorganic arsenic species from groundwater
.
Journal of Hazardous Materials
357
(
5
),
159
167
.
Kazi
T. G.
,
Brahman
K. D.
,
Afridi
H. I.
,
Shah
F.
&
Arain
M. B.
2018b
Effects of high fluoride content in livestock drinking water on milk samples of different cattle in endemic area of Pakistan: risk assessment for children
.
Environmental Science and Pollution Research
25
,
12909
12914
.
Keramati
H.
,
Miri
A.
,
Baghaei
M.
,
Rahimizadeh
A.
,
Ghorbani
R.
,
Fakhri
Y.
,
Bay
A.
,
Moradi
M.
,
Bahmani
Z.
,
Ghaderpoori
M.
&
Mousavi Khaneghah
A.
2019
Fluoride in Iranian drinking water resources: a systematic review, meta-analysis and non-carcinogenic risk assessment
.
Biological Trace Element Research
188
,
261
273
.
Keshavarzi
A.
,
Sharifzadeh
M.
,
Haghighi
A. K.
,
Amin
S.
,
Keshtkar
S.
&
Bamdad
A.
2006
Rural domestic water consumption behavior: a case study in Ramjerd area, Fars province, IR Iran
.
Water Research
40
,
1173
1178
.
Khan
A. A.
,
Whelton
H.
&
O'Mullane
D.
2004
Is the fluoride level in drinking water a gold standard for the control of dental caries?
International Dental Journal
54
,
256
260
.
Kir
E.
,
Oruc
H.
,
Kir
I.
&
Sardohan-Koseoglu
T.
2016
Removal of fluoride from aqueous solution by natural and acid-activated diatomite and ignimbrite materials
.
Desalination and Water Treatment
57
,
21944
21956
.
Kohn
W. G.
,
Maas
W. R.
,
Malvitz
D. M.
,
Presson
S. M.
&
Shaddix
K. K.
2001
Recommendations for using fluoride to prevent and control dental caries in the United States
.
Li
H.-R.
,
Liu
Q.-B.
,
Wang
W.-Y.
,
Yang
L.-S.
,
Li
Y.-H.
,
Feng
F.-J.
,
Zhao
X.-Y.
,
Hou
K.
&
Wang
G.
2009
Fluoride in drinking water, brick tea infusion and human urine in two counties in Inner Mongolia, China
.
Journal of Hazardous Materials
167
,
892
895
.
Madani
K.
2014
Water management in Iran: what is causing the looming crisis?
Journal of Environmental Studies and Sciences
4
,
315
328
.
Maheshwari
R.
2006
Fluoride in drinking water and its removal
.
Journal of Hazardous Materials
137
,
456
463
.
Miretzky
P.
&
Cirelli
A. F.
2011
Fluoride removal from water by chitosan derivatives and composites: a review
.
Journal of Fluorine Chemistry
132
,
231
240
.
National Research Council
2007
Fluoride in Drinking Water: A Scientific Review of EPA's Standards
.
National Academies Press
,
Washington, DC
.
Nhmrc
N.
2011
Australian Drinking Water Guidelines Paper 6 National Water Quality Management Strategy
.
National Health and Medical Research Council, National Resource Management Ministerial Council, Commonwealth of Australia
,
Canberra
, pp.
7
5
.
Ozsvath
D. L.
2009
Fluoride and environmental health: a review
.
Reviews in Environmental Science and Bio/Technology
8
,
59
79
.
Qasemi
M.
,
Afsharnia
M.
,
Zarei
A.
,
Farhang
M.
&
Allahdadi
M.
2018
Non-carcinogenic risk assessment to human health due to intake of fluoride in the groundwater in rural areas of Gonabad and Bajestan, Iran: a case study
.
Human and Ecological Risk Assessment: An International Journal
25
(
5
),
1
12
.
Ram
S. M.
,
Thakkar
V. P.
&
Machale
P.
2017
Determination of fluoride level in drinking water from water samples in Navi Mumbai, Maharashtra
.
Journal of Indian Association of Public Health Dentistry
15
,
395
.
Soltani
M.
,
Laux
P.
,
Kunstmann
H.
,
Stan
K.
,
Sohrabi
M.
,
Molanejad
M.
,
Sabziparvar
A.
,
Saadatabadi
A. R.
,
Ranjbar
F.
&
Rousta
I.
2016
Assessment of climate variations in temperature and precipitation extreme events over Iran
.
Theoretical and Applied Climatology
126
,
775
795
.
USEPA
2004
Risk Assessment Guidance for Superfund. Volume I: Human Health Evaluation Manual (Part A)
.
EPA
,
Washington, DC
.
USPHS
1963
Public Health Service drinking water standards. In: SERVICE, U. S. P. H. (ed.). US Dept. of Health, Education, and Welfare, Public Health Service [for sale by the Superintendent of Documents, US Govt. Print. Off
.
WHO
2004
Guidelines for drinking-water quality: recommendations
.
World Health Organization
,
Washington, DC, USA
.
World Health Organization
2011
Guidelines for Drinking-Water Quality
. 4th edn.
World Health Organization
,
Geneva, Switzerland
.
Xu
X.
,
Li
Q.
,
Cui
H.
,
Pang
J.
,
Sun
L.
,
An
H.
&
Zhai
J.
2011
Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres
.
Desalination
272
,
233
239
.
Yousefi
M.
,
Yaseri
M.
,
Nabizadeh
R.
,
Hooshmand
E.
,
Jalilzadeh
M.
,
Mahvi
A. H.
&
Mohammadi
A. A.
2018
Association of hypertension, body mass index, and waist circumference with fluoride intake; water drinking in residents of fluoride endemic areas, Iran
.
Biological Trace Element research
185
(
2
),
1
7
.
Zhang
S.
,
Niu
Q.
,
Gao
H.
,
Ma
R.
,
Lei
R.
,
Zhang
C.
,
Xia
T.
,
Li
P.
,
Xu
C.
&
Wang
C.
2016
Excessive apoptosis and defective autophagy contribute to developmental testicular toxicity induced by fluoride
.
Environmental Pollution
212
,
97
104
.