by GL. Tayanin*, Faculty of Odontology, Malmö University, Sweden

Introduction
The concentration of fluoride in drinking waters is one of the factors strongly influencing the dental caries situation globally. Data on fluoride sources in Lao People's Democratic Republic is only available to a limited extent (1). The aim of this study was to collect drinking water samples from some different parts of Laos using a standardised method for fluoride determination and eventual pilot fluoride mapping of some provinces of the country. This information would be very useful in the planning and implementation of oral health primary prevention programmes in the future.

Materials and Method
Drinking water samples were collected from different parts of the country in July 1999.

Sample Collection:

Samples were collected from 50 different samples of drinking water, representing big and small cities and villages. Drinking water from each source (see below) such as bottled, tap, hand pump, springs, stream, lake and rivers were collected in clean and dry 60 ml polypropylene containers. Each sample was labelled giving the time, date, site and source. When collecting tap water, the tap was allowed to run for a few minutes so as to collect an evenly distributed sample of water.

The starting point of the water sample collection was the capital Vientaine which belongs to the Vientaine Municipality. Water samples were collected from tap, bottled water, ice cubes and the river (Table 1). From here moving north, samples were collected from the Vientaine Province, then from the mountainous province of Xiengkhouang (2000 m above sea level), then going northwestwards from Luang Prabang province (500-1000 m above sea level) and Oudomxay province (1000 m above sea level) and returning to the capital. 

The samples were analysed for fluoride concentration using standard methods in the Department of Cariology, Faculty of Dental Sciences, University of Malmö Sweden.

Analysis of fluoride in water samples:

After collection the samples were stored in a refridgerator. The F- concentration was analysed two weeks later by using an ion-specific electrode and difference electrode, Fluoride Electrode Instrument (Orion).

• Prepared standard solution ranging between 1.0 ppm - 0.1 ppm were chosen presuming that the unknown sample concentration of fluoride may fall in between these two standards.
• TISAB (Total Ionic Strength Adjustment Buffer), was added to the standard fluoride solutions to adjust the pH of the sample and to break up complexes.
• The instrument was calibrated with the two standard solution.
• To each water sample TISAB III was added in correct ratio.
• To determine the F- concentration, the electrode was left to stand in the solution for 3 minutes.
• After 3 minutes the F- concentration in the water sample was read directly from the digital display meter.

For each sample, two fluoride readings were taken. 43 (86%) of the 50 samples showed no difference betweeen the two measurements. 0.01 difference was seen in 3 (6%) of the samples, 0.02 and 0.03 differences were observed in 1 (2%) and 3 (6%) samples respectively. Only the first reading was included in the study.

Results
The main drinking water source is the Mekong river in Vientiane municipality. Usually water is boiled before drinking in Laos. Highest concentration of fluoride was recorded in the Luang Prabang province, in the water sample from a hand pump which was 0.512 F- ppm. The lowest level of 0.013 F- ppm was observed from a stream in the Nakanthung village, Vientiane province, and from bottled water in Moung Xay district in Oudomxay province.

Fluoride concentrations (ppm) in various water sources in different provinces in Laos, July 1999

Province: Vientiane Municipality

Province: Vientiane

Province: Xiengkhouang

Province: Luang Prabang

Province: Oudomxay

Fluoride Map : showing the fluoride concentrations observed in the water samples taken from some provinces in Laos.

Conclusion
The WHO in 1994 has recommended fluoride levels of 0.5-1.0 mg/l (0.5-1.0 F- ppm) in the drinking water for optimal cariostatic effect, the lower limits for hot climate (as people drink more water) and upper limit for cold climates (2). The concentrations of fluorides in the drinking water observed in this study are below this optimal level. The study tells that one important factor contributing to caries resistance is missing. Should caries-inducing factors increase in Laos, for example the consumption of sucrose and other caries inducing carbohydrates, the need for effective oral health primary preventive programmes is great, greater than if fluoride had been present.

References
Wong Hee Deong, Implementation of the oral health component of the health-promoting school project, mission report, WPRO/WHO, 1997.
Fluorides and oral health, report of a WHO Expert Committee on Oral Health Status and Fluoride Use, 1994, WHO/Geneva.

Acknowledgements
Thanks are due to Dr Khamhoung Prommavongsa, Chief of Dental Service, Mahosot Hospital, Vientiane, for valuable discussions on the oral health situation in Laos.

We also wish to thank profusely Dr Somphone Doangchannalad, Chief of Dental Service, Luang Prabang, Dr Sinsay, Chief of Dental Service Xiengkhouang, Mr Norm Chief of Dental Service, Oudomxay, for their generous and enthusiastic contribuions which enabled us to gain insight on practising dentistry in Laos.

* This project was performed by GL. Tayanin as a Student Research Project at the Faculty of Odontology, Malmö University, Sweden under the guidance of Prof. D. Bratthall and Dr JR. Stjernswärd. Mrs E. Thornqvist was supervising the fluorie determinations in the laboratory.