Dental caries is a representative oral disease and refers to a phenomenon in which acid from dental biofilm erodes the enamel and dentin of the teeth, creating lesions on the teeth. Dental caries is a chronic and widespread disease, and once dental tissue is damaged, it is difficult to repair, so if left untreated, dental caries can even lead to tooth loss. Therefore, governments around the world are promoting public oral health projects to prevent dental caries [1].

Fluoride, one of the most effective substances in preventing dental caries, changes hydroxyapatite into a more dense fluorapatite structure by substituting fluoride ions on the surface of hemorrhoids, increasing acid resistance and achieving a dental caries prevention effect. There are two ways to use fluoride: by applying it topically or administering it systemically. A systemic administration method that allows frequent exposure to relatively low concentrations of fluoride is suitable for public oral health projects. Systemic administration methods include tap water fluoride concentration adjustment project, table salt fluoride method, wheat flour fluoride method, milk fluoride method, and fluoride tablets or fluoride syrup method [1]. However, taking large amounts of fluoride every day can cause dental fluorosis, where white or brown spots appear on the surface of teeth [2]. Therefore, before implementing a public oral health project using fluoride, the appropriate fluoride concentration must be determined and regulated according to the characteristics of the region.

Laos (Lao People’s Democratic Republic) is a socialist country located inland on the Indochina Peninsula, bordering Vietnam, Thailand, Myanmar, Cambodia, and China. It has an area of 239,955 km² and consists of a total of 17 provinces, with the capital being Vientiane. As of 2018, there are about 690,000 people living there, and the per capita gross domestic product (GDP) is about $2,600. Although it is one of the lowest economies in the world, it is a developing country with an economic growth rate that ranks 19th [3]. Laos is achieving social and economic development, but with the growth of GDP, annual sugar consumption per person continues to increase, increasing the risk of caries [3]. In contrast, public oral health projects are not being implemented well, so there is a need to revitalize projects nationwide. The World Health Organization (WHO) proposes the tap water fluoride concentration adjustment project as a representative public oral health promotion project. The tap water fluoride concentration adjustment project is a project to add an appropriate amount of fluoride to central water facilities and supply it to local residents [4]. However, this may not be a business method suitable for Laos. In Laos, groundwater is a major water resource, and residents, regardless of urban or rural areas, dig wells in their homes to receive groundwater as domestic water [4]. On the other hand, the central supply system for water facilities is not well developed, so to properly supply fluoride, basic water facilities must be maintained, so other methods must be applied to take fluoride without using the central water facility.

A suitable alternative is the school water fluoride concentration adjustment project. The school water fluoride concentration adjustment project is to adjust the school water fluoride concentration, and in order to implement this, it is necessary to know how much fluoride is contained in the groundwater used by the school. To prevent the side effects of fluoride and ensure the dental caries prevention effect, the fluoride concentration must be adjusted appropriately. Accordingly, this study measures the fluoride concentration of groundwater in elementary, middle, and high schools in five major regions of Laos and uses it as basic data for the school water fluoride concentration adjustment project in Laos.

1. Research subject

This study targeted groundwater in Vientiane, Luang Prabang, Luang Namtha, Savannakhet, and Pakse provinces in Laos. After dividing it into urban and rural areas, each region was divided into urban and rural areas. Fluoride concentrations were measured in groundwater from elementary schools (5-year course) and middle and high schools (7-year course) [5-7]. The capital of each Laos, two northern regions, and two southern regions centered around it were selected as test regions. The location can be checked on the map below (Figure 1).

Figure 1. Map of Laos.

2. Methods

In the winter of 2020, groundwater used as drinking water in elementary, middle, and high schools in urban and rural areas of Vientiane, Luang Prabang, Luang Namtha, Savannakhet, and Pakse provinces in five regions of Laos was collected and converted into bottled water with a fluoride concentration of 0.000 ppm. It was sorted and stored in 350 ml PET bottles.

The collected groundwater was measured using Photometers w/Cal Check: HI 96729 (HANNA Instruments, Seoul, Korea). The machine measures fluorine concentration by measuring the intensity of each wavelength of the sample with a spectrophotometer and calculating chromaticity coordinates. EPA method 340.1 and SPADNS method were applied, and the measurement range is 0.000 ppm to 2.000 ppm. There is a possibility that extremely small fluorine concentrations may not be detected (Figure 2).

Figure 2. Fluoride concentration measuring machine.

A standard sample with a fluorine concentration of 0.000 ppm was prepared in advance, and after adjusting the zero point, the fluorine concentration of a solution obtained by diluting 2 ml of the sample included in the measurement kit and 10 ml of groundwater was measured. In order to reduce the error range, the above process was repeated 5 times each, and the results were obtained by averaging the measurements.

1. Fluoride concentration in groundwater of elementary, middle, and high schools in 5 regions of Laos

The average values of fluoride concentration in urban and rural elementary schools and middle and high schools in five regions in Laos are shown in Table 1 and Table 2.

Table 1 . Average including zero values of fluoride concentrations of ground-water in primary and secondary schools in five areas of Laos

	Vientiane	Luang Prabang	Luang Namtha	Savannakhet	Pakse	Overall
Urban-primary	0.050	0.050	0.060	0.160	0.090
Urban-secondary	0.000	0.020	0.020	0.030	0.220
Rural-primary	0.000	0.030	0.100	0.010	0.410
Rural-secondary	0.000	0.010	0.130	0.000	0.110
Mean±standard deviation	0.013±0.025	0.028±0.017	0.078±0.048	0.050±0.074	0.208±0.147	0.075±0.099

Table 2 . Average not including zero values of fluoride concentrations of ground-water in primary and secondary schools in five areas of Laos

	Vientiane	Luang Prabang	Luang Namtha	Savannakhet	Pakse	Overall
Urban-primary	0.050	0.050	0.060	0.160	0.090
Urban-secondary	-	0.020	0.020	0.030	0.220
Rural-primary	-	0.030	0.100	0.010	0.410
Rural-secondary	-	0.010	0.130	-	0.110
Mean±standard deviation	0.050	0.028±0.017	0.078±0.048	0.067±0.081	0.208±0.147	0.089±0.103

Table 1 shows the average value when the value 0 is included, and Table 2 shows the average value when the value 0 is not included. The average value including the value of 0 was Pakse, which showed the highest fluoride concentration at 0.208 ppm, and Vientiane showed the lowest concentration at 0.013 ppm. Across the entire region, fluoride concentrations were most frequently in the range of 0.000 ppm or more and less than 0.050 ppm (Table 3).

Table 3 . A frequency table of fluoride concentrations in Laos (unit: ppm)

Fluoride concentration (ppm)	Number of region
0.000	4
More than 0.000 and less than 0.050	6
More than 0.050 and less than 0.100	4
More than 0.100 and less than 0.150	3
0.150 or more	3

The five regions of Laos that are the subject of investigation in this study correspond to major cities in Laos. The level of caries experience among most age groups of students in the capital city of Vientiane Province was found to be much higher than in other regions [5-7]. As of 2019, the caries experience index for 12-year-old children and permanent teeth was 3.30, and for 15-year-old children, it was 4.99, which was higher than the 2009 value, showing an increase in dental caries experience. This is higher than the average caries experience permanent tooth index of 2.10 for all 12-year-olds in Laos as of 2010 surveyed by WHO [6]. As a result of an oral health survey targeting students in Luang Prabang province, the caries experience permanent tooth index of 12-year-old children was lower than before, but overall, most age groups had a high caries experience, so the dental caries prevention and treatment projects were carried out in parallel. It has been reported that this is necessary and the permanent caries rate for most students in Luang Namtha Province was found to be very high at 95-98%, and the caries retention rate and caries fatality rate were also very high, indicating continued oral health promotion projects. Interest was urged [6]. The 2019 Student Oral Health Survey in Savannakhet Province also showed similar results to other regions, and the study in Pakse Province has not yet been clearly reported [5,6]. However, looking at the results of other regions, it is judged that the overall oral health index is still low, so it is necessary to revitalize prevention projects in parallel with treatment projects nationwide.

Since Laos is a socialist country, an oral health care system is being implemented under strict government leadership, but medical services are not sufficiently provided due to lack of government finances. Except for the SMART (Simplified and Modified Atraumatic Restoration Treatment) program and the salt fluoride project, oral health promotion projects for the public are not very actively underway. The SMART program is a simpler version of Thailand’s ART (Atraumatic Restoration Treatment) program. It removes caries using only manual instruments and then fills and covers them with glass ionomer cement [8]. There are approximately 500 dentists in Laos, and the number of patients per capita is known to be approximately 14,000. Because there is only one dental university in the country, it is difficult to focus on individual oral health promotion due to a shortage of dental professionals [9,10]. Although we are cooperating with Korea to improve the quality of medical services by strengthening the oral health system through exchanges with organizations such as the Korea Foundation for International Healthcare (KOFIH), oral health in Laos is still lacking. Medical care requires continuous development, and new projects must be designed and implemented [11].

The water fluoride concentration adjustment project, a representative public oral health project, started in the United States in 1945 and is being implemented in many countries. In order to prevent the side effects of fluoride, WHO declared the appropriate fluoride concentration to be 1.000 ppm when implementing the tap water fluoride concentration adjustment project and mentioned that the appropriate fluoride concentration should be applied differently considering regional climate and fluoride dose through other foods [1,4]. The US Public Health Service (USPHS) proposed a new recommended fluoride concentration range independent of the WHO’s standards, and the recently updated recommended range is 0.700 ppm [12]. Countries other than the United States that have implemented the project include Canada, Australia, Hong Kong, the United Kingdom, and Israel, and Ireland and Singapore are mandating the addition of fluoride to tap water in all regions [1,4]. Most of the countries implementing the tap water fluoride concentration adjustment project are developed countries and can provide smooth water supply. However, countries where it is difficult to implement the project due to limitations in the central water supply system are instead supplementing the insufficient fluoride dose through salt fluoridation and milk fluoridation [13].

Among them, salt fluoridation is a project that adds 0.250 ppm of fluoride to commercially available salt to ensure that fluoride is taken on a daily basis. The salt fluoridation program was first implemented in Switzerland in 1955 and is mainly implemented in European countries such as Germany, France, Hungary, the Czech Republic, Austria, Spain, and Slovakia. It has different advantages from the water fluoridation program in areas where water fluoridation program is difficult to implement. Because it can be used as an alternative, it is widely practiced in developing countries. In the case of salt, unlike water, it does not require special facilities for supply, so it can provide a relatively wide range of benefits to the lower social and economic classes, making it easy to apply in developing countries. The salt fluoridation program is being successfully implemented in developing countries in South America, such as Jamaica, Mexico, Colombia, Costa Rica, and Uruguay [14,15]. Laos is also implementing a salt fluoridation program with the help of WHO, but there has been no report on whether it will be implemented nationwide.

The School Water Fluoride Concentration Adjustment Project is a project to dilute fluoride in school wells to supply fluoride to children in areas where central water supply facilities are inadequate. Since fluoride is not taken from the period of tooth formation, it does not have the best caries prevention effect, but the great advantage is that recipients can receive the benefits without any effort [16]. School-age children usually spend 20-25% of their day at school. At this time, children consume an amount of school water equal to the proportion of time they spend at school. Therefore, it is appropriate to set the fluoride concentration in the school water fluoride concentration adjustment project to be 4.5 times higher than the appropriate concentration in the fluoride concentration adjustment project implemented in the community. There is a case in which the effects and side effects were tested by consuming fluoridated water at a concentration of 7 times, but the side effects occurred in only 0.4% of the test subjects, so it can be considered that there were almost no side effects, and the caries prevention effect was not significantly different from the concentration of 4.5 times [17,18]. The school water fluoride concentration adjustment project allows many children to benefit from the dental caries prevention effect of fluoride with a minimum of manpower, equipment, and funds. This can be advantageous in Laos, where it is difficult to make large financial investments in public oral health projects.

The prevalence of caries among children in Laos is still high, and to solve this problem, it is necessary to implement not only treatment projects but also preventive projects that include taking fluoride. Considering the current situation in Laos, it is judged to be difficult to revitalize the tap water fluoride concentration adjustment project. It would be a good idea to expand the already-in-progress salt fluoride project nationwide, but there is a need to additionally implement a school oral health project to prevent caries in children who consume a lot of sugar. Accordingly, this study measured the groundwater fluoride concentration of each school and found that the groundwater fluoride concentration of most urban and rural schools was less than 1.000 ppm. Although the fluoride concentration in Pakse Province was found to be more than 0.100 ppm higher than in other regions, it is difficult to see this as being meaningful. Therefore, it is believed that the school water fluoride concentration adjustment project should be activated to enable children to take additional fluoride by administering fluoride to the groundwater in schools in each region to increase the concentration to 4.5 times the appropriate fluoride concentration of drinking water, which is 0.700 ppm.

The authors measured the groundwater fluoride concentration of urban and rural elementary and middle and high schools in five regions of Laos: Vientiane, Luang Prabang, Luang Namtha, Savannakhet, and Pakse, and obtained the following results.

1. The highest fluoride concentration was 0.410 ppm in rural elementary schools in Pakse Province, and 0.000 ppm in most of Vientiane Province.

2. The average fluoride concentration, including the 0 value, was 0.075 ppm, and the average fluoride concentration excluding the 0 value was 0.089 ppm.

3. The most frequent fluoride concentration was in the range of 0.000 ppm to 0.050 ppm.

The groundwater fluoride concentration in all five regions of Laos was found to be below the appropriate drinking water fluoride concentration of 0.700 ppm.

No potential conflict of interest relevant to this article was reported.