Water is required to sustain life on the planet 24 , and access to clean water is one of the fundamental rights described in Article 3 of the Universal Declaration of Human Rights 23 . In Brazil, a portion of the population has been denied this right, since 17% of Brazilian cities do not have municipal water systems 11 .
An inverse relationship between the fluoride levels in drinking water and the incidence of dental caries was first observed in the United States at the beginning of the last century 14 . This observation, in association with later studies, resulted in the addition of fluoride to public water supplies to prevent dental caries, a strategy that is now recommended for all citizens 22 .
The first implementation of a water fluoridation system occurred in Grand Rapids, Michigan, USA, in 1942. Widespread adoption of fluoridation potentially reduces the incidence of caries by 50%, and for this reason fluoridation has been used in several countries 17 .
In 1953, Baixo Gandu City in the state of Espírito Santo became the first Brazilian city to employ water supply fluoridation 19 . Some years later, Federal Law number 6050 (May 24th, 1974) required the addition of fluoride to public water supplies in cities with water treatment systems 1 . This measure is considered one of the 10 most important public health advances in the 20th century 5 because it established a comprehensive, economical, and effective means of continuously providing a health benefit to a large population at minimal risk.
Periodic analyses of fluoride levels in water supplies originated with the goal of providing uniform fluoride levels independent of natural concentrations or water supply companies. The external control consists in the principle that if any service offers risk or represents a protection factor for public health, the control regarding producer, production process, distribution and consumption, should be controlled by State Institutions. It has evolved into a way of improving the quality and consistency of fluoridation, and has been instituted in many different regions of Brazil 12,16,18 .
The existence of mechanisms that enable the control of naturally-occurring and supplemental fluoride levels in water maximizes the benefits of preventing dental caries while minimizing the risk of dental fluorosis 6,8,13 .
Previous studies 20,21 have measured the fluoride levels in public water supplies and classified samples into “appropriate” and “inappropriate” categories. However, this dichotomous classification limits the interpretation of the water analysis results.
We classified fluoride levels in samples from public water supplies in 40 cities, according to the recently proposed risk/benefit criteria 6 , and investigated the consistency of fluoride concentrations in the samples over a 7-year period.
MATERIAL AND METHODS
Type and period of study
A longitudinal study was conducted using sampling sites in 40 cities located in São Paulo State, Brazil. To conduct the study, formal and personal contacts were made with the secretaries of health, oral health coordinators and those responsible for the public water supply of each municipality. The trial included all cities (n=40) belonging to the Regional Health Department II (RHDII). Samples were collected monthly from public water distribution systems from November 2004 to October 2011, yielding 84 months of analysis.
Identification of collection sites
Sample collection sites were selected according to the number of water sources or treatment plants in each city. Each source was assigned three collection sites 21 . Maps of the distribution system were obtained from the water treatment authority and used to randomize the collection sites, including all sources of treated water.
Collection of water samples
Polyethylene bottles (50 mL) were rinsed with deionized water and identified with the city name, collection site, and date. At the collection site, the bottles were rinsed three times with the water comprised of the sample. Samples were collected on the same day during the first week of each month and analyzed in the laboratory of NEPESCO (Center for Research in Public Health) at the Universidade Estadual Paulista - UNESP.
Analysis of fluoride levels
The fluoride content of the samples was determined potentiometrically using an ion analyzer (Orion EA940) equipped with a fluoride ion-specific electrode (Orion 9609BN). A calibration curve was constructed over the expected concentration range of the samples using standards containing 0.1, 0.2, 0.4, 0.8, and 1.6 mg F/L and prepared using NaF. The standards were prepared in triplicate to minimize the effect of measurement errors. To avoid interference from other ions, an equal volume (1 mL) of TISAB II® solution was added to each sample (Orion, USA). Duplicate measurements of each sample were recorded using the Excel software (Microsoft Office 2010), and values of mV were converted to mg F/L using a standard curve [correlation coefficient (r) 0.999]. When the difference between the duplicate readings was larger than 1 mV, a third reading was obtained for confirmation of the data.
Considering that the average maximum annual temperatures in the cities participating in this study were between 26.3°C and 32.5°C, the fluoride level offering the best risk-benefit combination was 0.55-0.84 mg F/L (Table 1) 6 .
|City Artificial Fluoride||Classification of fluoride levels|
|0.00 - 0.44||0.45 - 0.54||0.55 - 0.84||0.85 - 1.14||1.15 - 1.44||≥1.45||Total|
|Bento de Abreu||13||5.2||44||17.7||176||70.7||15||6||1||0.4||0||0||249||100|
|Muruntinga do Sul||164||67.5||34||14||39||16||6||2.5||0||0||0||0||243||100|
|Santópolis do Aguapeí||4||1.8||2||0.9||219||96.1||3||1.3||0||0||0||0||228||100|
|Santo Antônio do Aracanguá||171||37.5||55||12.1||169||37.1||50||11||6||1.3||5||1.1||456||100|
Fluoride was not added to public water supplies in 7.5% (n=3) of the cities, and in other cities, ion aggregation occurred in at least one water source (Table 2).
|Cities||Collection sites||Fluoride||Water sources|
|Artificial||Natural||Shallow well||Deep well||River, stream or dam|
|Bento de Abreu||3||X||X|
|Murutinga do Sul||3||X||X|
|Santo Antônio do Aracanguá||6||X||X|
|Santópolis do Aguapeí||3||X||X||X|
A variety of water sources are used in the composition of the water supply system of the cities, among which can be observed 67.5% (n=27) from shallow wells, 25% (n=10) from deep wells, and 22.5% (n=9) from rivers, streams, or dams. Some cities had more than one source of water supply (Table 2). Between November 2004 and October 2011, 19,533 samples of water were analyzed in duplicate from 291 collection sites in 40 cities, totaling 39,066 analyses (Table 3). In some months, the collect of samples was not performed due to difficulties in accessing the location.
|Fluoride level (mgF/L)||Benefit||Risk|
|0.00 to 0.44||insignificant||insignificant|
|0.45 to 0.54||minimum||low|
|0.55 to 0.84||maximum||low|
|0.85 to 1.14||maximum||moderate|
|1.15 to 1.44||questionable||high|
|1.45 or more||harm||very high|
In 62% (n=24) of the cities, most of the samples were classified into the category that offer the best combination of risk-benefit (0.55 to 0.84 mg F/L). It was observed that 51.57% of the samples from cities with fluoridated water and 45.48% of the samples from cities with non-fluoridated water were included in this category.
Fluoride levels below 0.55 mg F/L were found in 30.13% of the water samples. Of this total, 30.53% were in cities providing fluoridated water. Among the non-fluoridated water samples, 35.42% had fluoride levels above 0.84 mg F/L.
Table 4 shows the variation of fluoride average concentrations (mgF/L) of the water samples, and the standard variation, according to the cities, during 7 years of analysis.
|Bento de Abreu||0.53||0.17||0.6||0.1||0.65||0.13||0.66||0.18||0.57||0.08||0.58||0.12||0.72||0.13||0.77||0.09|
|Muruntinga do Sul||0.62||0.18||0.37||0.12||0.49||0.19||0.49||0.2||0.33||0.11||0.38||0.19||0.32||0.21||0.23||0.08|
|Santópolis do Aguapeí||0.69||0.02||0.66||0.13||0.69||0.08||0.68||0.06||0.67||0.07||0.7||0.05||0.66||0.05||0.68||0.13|
|Sant. Ant. do Aracanguá||0.07||0.01||0.4||0.36||0.72||0.15||0.51||0.24||0.52||0.18||0.37||0.29||0.79||0.38||0.65||0.14|
Discussions of optimal fluoride levels in public water supplies are ongoing among researchers worldwide. The United States Department of Health and Human Services (HHS) has proposed a new recommendation for fluoride content in drinking water of 0.7 mg F/L, replacing the current guideline that specifies a level between 0.7 and 1.2 mg F/L 9 . In Brazil, legislation establishing the procedures and responsibilities relating to the control and surveillance of water quality for human consumption 2 has recently undergone public review 6 .
This study classified the water samples in accordance with recommendations published by the Collaboration Center for Oral Health Surveillance of the Health Ministry (CECOL/USP, 2011) 6 , taking into consideration both the benefits of fluoride in preventing dental caries and the risk of dental fluorosis. The document suggests three classifications based on the average maximum annual temperature of the locality, since the optimal fluoride concentration in drinking water is dependent on the level of water consumption. Higher levels are preferred in lower-temperature regions where water consumption is lower, whereas in higher temperature regions, the recommended fluoride levels are lower 10 . According to this classification, 51.36% of the 19,533 samples had fluoride levels within the optimum risk-benefit range.
Fluoride levels outside of the recommended parameters were observed during a previous research in the same region 20,21 . During a six-month analysis in 2005, Saliba, et al. 21 (2006) reported that approximately 62% of the water samples were outside of the recommended fluoride concentration range. Another 36-month study found that approximately 43% of the samples from cities with only a single source of water were outside of the recommended parameters 24 . Deficiencies in fluoridation were also described in studies performed in other regions 3,15 .
Some cities undertaking water fluoridation had a majority of water samples outside the optimum range of fluoride levels. Most of these cities employed more than one water supply, promoting interconnection and mixing of water from different sources in the distribution system and making it difficult to maintain fluoride levels within the recommended range 12 . However, none of the deviations persisted for extended periods at any sampling site during the 7-year study, minimizing undesirable effects 4 .
Among the three cities that did not add fluoride, only one had a majority of samples within the recommended range. In that city, the source of supply already contained the recommended level of fluoride. In one of the remaining cities, 53.78% of the water samples contained less than 0.55 mg F/L, while in the third city approximately 75% of the samples contained fluoride levels above 0.84 mg F/L, increasing the risk of dental fluorosis.
Only 8% of the water samples from cities adding fluoride exhibited fluoride levels above 0.84 mg F/L, and fluoride levels were above 1.44 mg F/L in only 1.46% of the samples.
In two of the three cities performing water fluoridation in which most of the samples contained elevated fluoride levels, the water sources were deep wells that are known to naturally contain higher fluoride levels. This highlights the need to examine water distribution networks and to develop dilution and mixing procedures to optimize the use of the natural fluoride present in deep-water wells and achieve the desired fluoride concentrations 7 .
There was some variability among the samples from different sites in the same city, demonstrating the importance of selecting collection sites according to the number and location of the water supply. In addition to sample collection bottles, a document was sent each month to the cities requesting notification of any changes to the water distribution network caused by the opening or closing of any water supply. In order to assist the cities in achieving optimum fluoride levels, the results of the analyses were sent monthly to the secretaries of health, oral health coordinators, and those responsible for the water supply in each municipality.
The effect of fluctuations in fluoride concentration on the risk/benefit analysis is dependent on the duration of the fluctuations. It is essential that the addition of fluoride be maintained without large fluctuations, since reduced levels provide minimal health benefits while wasting public resources and excessive levels promote undesirable effects 4 .
The findings of this study demonstrate the importance of longitudinal studies to assist in maintaining optimum fluoride levels in public water supplies.
Most samples from cities providing fluoridated water were within the concentration range providing the best combination of risks and benefits, reaffirming the safety of the method. High fluoride levels were observed in samples from deep wells. We suggest further studies to optimize the use of natural fluoride to reduce fluoridation costs and the risk of dental fluorosis.