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Fluoride Concentration in Tap Water from Different Regions in Thailand

Fluoride supplementation in drinking tap water is one of the well-known effective methods for dental caries prevention. However, overexposure to fluoride following excessive fluoride intake from drinking water leads to dental fluorosis. Therefore, the assessment of daily fluoride consumption is required to calculate the optimal fluoride intake. The present study investigated the fluoride concentration in tap water collected from different areas in Thailand. A total of 27 locations were selected. Three samples of tap water (500 mL each) were independently collected from one location. Each sample in the same location was collected from the same faucet of tap water and stored in different containers. The samples were collected by dental students or dentists who worked in the selected areas from March 2020 to June 2020. Briefly, the faucet was cleaned with the tap water and the water was run from the faucet for 1-2 mins. Then, water was collected in 500 mL bottles and immediately capped. Samples were then stored at room temperature in tightly sealed bottles until analysis. Findings showed that most samples contained fluoride at a concentration lower than 0.7 mg/mL. Further, the water pH was in the range of 6.81-8.37. These levels were lower than the cut-offs established by the World Health Organization (WHO) for maximum levels of fluoride and pH in drinking water. In conclusion, the present study demonstrated that fluoride levels in tap water from different regions in Thailand are lower than those recommended by WHO for fluoride levels in drinking water.


INTRODUCTION
Fluoride is a well-known chemical element for dental caries prevention since it enhances enamel remineralization, attenuates demineralization, and contains antibacterial activity 1 . Professional use of fluoride includes oral fluoride systemic supplementation and fluoride varnish applications 1,2 . In addition, topical fluoridated products such as toothpaste and mouthwash are recommended for home use to prevent caries [1][2][3] . In addition, fluoridated water in local communities is another approach utilized to reduce the risk and incidence of dental caries in many countries 1,4 . The addition of fluoride to the water supply has been reported as an effective protocol to prevent dental caries 5 . Caries incidence in communities that are supplied fluoridated water is lower than the others with no fluoridation 6,7 . In this regard, a study indicated that the caries prevention effect of fluoridated water was more prominent in primary dentition 6 . In permanent dentition, water fluoridation can also reduce caries prevalence. However, the systematic review and meta-analysis indicated that water fluoridation had a higher impact on caries prevention in the primary dentition 8 . The lack of water fluoridation is associated with the increase in severity and prevalence of dental caries 7 . The majority of dentists and medical practitioners perceive and agree that community water fluoridation may benefit caries prevention 9 . However, the perception of whether drinking water is properly fluoridated to prevent caries remains a concern 9 . A report demonstrated that only 33% of parents in Qatar allowed their children to consume fluoridated tap water even though they knew the benefit of fluoride in the prevention of tooth decay 10 . This could be due to the misunderstanding and unproven concerns concerning water fluoridation 10 . In Thailand, fluoride in the water is naturally derived and not through public water fluoridation approaches 11 . A milk-fluoridation public policy program was implemented in Thai schools since 2000 for 3-to 12-yearold children and this program was proved to be a costeffective intervention for dental caries prevention 12 . However, such a milk-fluoridation program was discontinued.
Despite the positive influence of fluoride on tooth decay, it has been demonstrated that excessive fluoride intake leads to a marked increase in dental fluorosis 13 . Optimization of fluoride concentration in community tap water could control the prevalence rates of both tooth decay and dental fluorosis 14 . In this regard, the decrease of fluoride concentration from 0.7 ppm to 0.5 ppm could decrease the incidence of dental fluorosis while effectively preventing dental caries in investigated target groups 14 . Risk factors for dental fluorosis include early weaning, dietary fluoride supplementation, fluoridated toothpaste swallowing, fluoridated milk, and soft-drink consumption as well as community water fluoridation 13 . It has been shown that dental fluorosis risk is associated with fluoride intake during the age of 2-8 years old 15 . In vitro studies have shown that ameloblasts are sensitive to fluoride. In terms of cellular mechanisms, fluoride-induced apoptosis and oxidative stress reduced the expression of a gene associated with amelogenesis imperfecta (Fam83h) in ameloblasts [16][17][18] . Further, fluoride bound to enamel matrix protein can result in compromised crystal growth 19 . Fluoride exposure during secretory and maturation stages caused more severe enamel defects 20 . Hence, the control of fluoride consumption and modified fluoride exposure could be a crucial approach to manipulate the risk of both dental caries and fluorosis. In Thailand, urban communities are supplied with municipal water. Only a few rural communities use underground and rainwater. The pH of water affects tooth structure and morphology. A pH lower than 5.5 and 6.8 results in the erosion of enamel and root dentin, respectively.
Nowadays, tap water is a common source for household consumption even in urban areas. The present study investigated the fluoride concentration and pH in tap water collected from different areas in Thailand. The information from this study could be employed by dental personnel to safely calculate the optimal fluoride supplementation for high-caries risk individuals in those local areas.

MATERIALS AND METHODS
Municipal tap water was collected from central, northern, northeastern, southern, and eastern regions of Thailand ( Fig. 1). A total of 27 locations were selected. Three samples of tap water (500 mL each) were independently collected from one location. Each sample in the same location was collected from the same faucet of tap water and stored in different containers. The samples were collected by dental students or dentists who worked in the selected areas from March 2020 to June 2020. Briefly, the faucet was cleaned with the tap water and the water was run from the faucet for 1-2 mins. Then, water was collected in 500 mL bottles and immediately capped 21 . Samples were then stored at room temperature in tightly sealed bottles until analysis 22 .
Fluoride content was examined by a fluoride ion selective electrode (Versa Star, USA) 23 . Fluoride standard solution was employed as a reference for calibration. TISAB III solution was mixed with the sample at a 1:10 ratio. pH measurement was examined using pH meter 24 . A standard solution was employed for pH calibration. Each sample was evaluated three times by fluoride ion selective electrode and pH meter. The elec- trodes were rinsed three times with deionized water and dried before and after each measurement. The fluoride concentration was averaged from three independent samples collected at the same location.
Descriptive analysis of the data was conducted using Prism 8 (GraphPad Software, CA, USA). Average values were calculated from triplicate measurements for each sample. Then, the mean and standard deviation were calculated for each location.

RESULTS
In all collected tap water samples, fluoride concentrations were between 0.043 to 0.671 mg/L and pH values ranged from 6.81 to 8.37 (Table 1). There was no relation between pH values and fluoride concentration in the examined samples. Each sample was measured three times, and the difference in measured fluoride and pH values ranged from 0-0.012 and 0-0.04, respectively. Fluoride concentration in tap water measured in different regions did not have any marked difference. Two locations in Saraburi province (Sao Hai district and Wang Muang district) exhibited marginal fluoride levels close to guidelines established by Thailand's public policies. Wang Muang district and Sao Hai district in the Saraburi province had 0.6710.0002 and 0.6350.005 mg/L of fluoride, respectively.

DISCUSSIONS
In their guidelines, the WHO suggested a fluoride intake from drinking water at the concentration of 1.5 mg/L [25][26][27] . This value was based on the assumption that individuals have on average a 60 kg body weight and a 2L/day water intake [25][26][27] . High fluoride intake (with levels higher than 1.5 mg/L) appears to increase the risk of dental fluorosis. For countries located in tropical regions, the 0.8 mg/L fluoride concentration was set in the WHO guideline due to the potentially high volume of water consumption was seen in tropical climates 26,27 . However, Thailand's guideline is set at 0.7 mg/L for water fluoride intake daily (assuming 2L/day for a 60 kg weight person) 25,28 .
For qualified drinking water, WHO guidelines indicate that the pH should range between 6.5 and 9.5 29 . The pH of drinking water below or above the levels had corrosive effects on human tissues. All samples in the present study showed a pH within the range of those recommended by WHO for drinking water.
Among the different community locations, tap water at Wang Muang district and Sao Hai district in the Saraburi province exhibited relatively moderate fluoride levels. Wang Muang district and Sao Hai district had 0.6710.0002 and 0.6350.005 mg/L of fluoride, respectively. These concentrations of fluoride were near those cut-offs suggested in Thailand's public policy guidelines. The raw water source was not identified in this study and the samples were collected only at a onetime point from one water outlet. Hence, further extensive water collections and investigations should be performed to confirm the fluoride levels in those locations. These would be deemed necessary to study for example the source identification of raw water and seasonal effects on fluoride concentration. Tap water from the other sources in the present study had fluoride concentrations much lower compared to the cut-offs suggested in Thailand's public policy.
Despite those aforementioned locations, the remainning water samples from other regions and districts of Thailand had fluoride levels in tap water within the normal range (markedly below 0.7 mg/L). Fluoride concentrations found in this study were comparable to those published previously [30][31][32] . A study in Morocco showed the mean fluoride value was 0.94 mg/L in tap water 30 . Fluoride concentration was detected at a concentration lower than 0.81 mg/L in the water supply of the tropical region of Piaui State in Brazil 31 . The mean fluoride levels in tap water from Mexico City and Veracruz was 0.20 mg/L 32 . The fluoride in municipal water supply in other Asian developing regions such as Nepal was on average 0.09 mg/L 33 . This concentration was lower than those detected in natural water resources 33 . Taken together, the tap water in several regions and districts in Thailand exhibited safe values of fluoride in tap water that minimize the risk for dental and/or skeletal fluorosis.
One limitation of the present study is that source information was lacking. Tap water was collected from faucets without the data of water resources i.e. underground water, rainwater, or surface water, etc. The depth of the water wells was also not collected. Thus, the interpretation of our results must be carefully performed. Despite these limitations, there was no evidence of high fluoride content in the examined samples that surpassed cut-offs established by policy guidelines in Thailand.
In the northern part of Thailand, high fluoride concentration was reported in the drinking water resource of Chiang Mai and Lamphun province 34 . Tap water from village waterworks in the northern part of Thailand utilized underground water as a water resource. This tap water had a high fluoride concentration (5.94  0.29 mg/L) compared with tap water from private wells (0.7  0.05 mg/L) 25 . In urban communities in Thailand, people seldomly consume water directly from the tap. Thai residents usually employ tap water treatment tech-niques prior to drinking (for example by boiling or via filtration and/or purification systems). However, Thai individuals usually use raw tap water for cooking. To note that boiling alone does not affect the levels of fluoride in water, even if fluoride concentrations are increased due to the reduction of water volume 35 . However, suspended brushite and calcite in fluoridating water followed by a boiling step was proved to effecttively reduce the fluoride concentration in tap water 36 . Interestingly, this method could be a simple and relatively low-cost technique for defluoridation of drinking water. In communities that exhibit high fluoride content in groundwater, a study showed that the implementation of reverse osmosis in tap water can be an affordable approach to reduce fluoride consumption from cooking and drinking with tap water 25 , minimizing the risk for dental fluorosis. However, water treatment prior to consumption is not available in certain rural areas in Thailand. Therefore, the risk of high fluoride water intake on oral health must be communicated to locals by dental practitioners and authorities.
Despite the concern of overconsumption of fluoride from tap water in some areas, one must acknowledge that an optimal fluoride concentration in tap water is an effective procedure to control dental caries at a community level 1,5,14 . Hence, a survey of fluoride concentration in different water sources used for consumption (including tap water and bottled water) should be widely and regularly implemented. A study done in Mexico reported that fluoride concentrations in tap water but not in bottled beverages are not correlated with the severity of dental fluorosis 37 . Another study from the Gaza Strip demonstrated that the main source of total daily fluoride consumption in children is primarily eating foods followed by drinking beverages 38 . Hence, other sources of fluoride have to be taken into consideration to determine the total daily intake at each community level.
In conclusion, the present study demonstrated that fluoride levels in tap water from a different region in Thailand were lower (<0.7 mg/L) than those recommended by WHO for fluoride levels in drinking water. This information could be useful to calculate the optimal fluoride supplementation for those individuals in need, particularly the ones with high caries risk. Future investigations should be performed to establish standard protocols and public health policies to regularly quantify the fluoride levels in tap water and their contribution to the total daily fluoride intake. These protocols and policies would allow oral health care practitioners to consistently and safely calculate the optimal fluoride supplementation for each local community, particularly for individuals with high caries risk.