1 Volume 24 2025 e256062 Original Research Braz J Oral Sci. 2025;24:e256062http://dx.doi.org/10.20396/bjos.v24i00.8676062 1 Department of Biosciences, Faculdade de Odontologia de Piracicaba (FOP), Universidade de Campinas (UNICAMP), Piracicaba, São Paulo, Brazil. 2 Department: Pathology and Dental Clinics, Federal University of Piauí, Teresina, Piauí, Brazil. Corresponding author: Marcoeli Silva de Moura Department of Pathology and Dental Clinics at the Federal University of Piauí Rua Danilo Romero, 1776 – Horto Florestal / 64052-510 Teresina - Piauí – Brazil Phone: +55 86 99987-9556 / Fax: 86 3223-9568 e-mail: marcoeli-moura@uol.com.br Editor: Dr. Altair A. Del Bel Cury Received: March 11, 2024 Accepted: August 14, 2024 Heterocontrol of water fluoridation in a municipality in northeast of Brazil after the privatization of the public supply service Gildenilson Oliveira Júnior1 , Letícia Caminha Aguiar Lopes2 , Cacilda Castelo Branco Lima2 , Marina de Deus Moura de Lima2 , Lúcia de Fatima Almeida de Deus Moura2 , Cinthia Pereira Machado Tabchoury1 , Marcoeli Silva de Moura2* Aim: The aim of the present study was to analyze fluoride levels in the public water supply of Teresina, Piauí, Brazil, following the change of the public supply service. Methods: This is a prospective descriptive laboratory study. The water samples were collected from 13 random locations over a period of 13 months. Three neighborhoods were randomly selected from each of the four zones (north, south, east and southeast) of the municipality (n=12) and one location close to the Water Treatment Plant (WTP) (n=1). The samples collect followed the VIGIFLUOR guide. Fluoride concentration analyses were performed in duplicate using a specific ion electrode (Orion 96–06; Thermo Scientific Orion., Boston, MA, USA) coupled to a VersaStar potentiometer (Thermo Scientific Orion), and the results were expressed in mg F/L (ppm F). Descriptive analysis and classification of samples were carried out according to the criteria of the Ministry of Health Collaborating Center for Oral Health Surveillance. Results: Out of the 169 samples analyzed, 158 samples (93.5%) were classified as inappropriate (ranging from 0.00 to 0.34 ppm F), and 11 samples were classified as appropriate (6.5%) (ranging from 0.45 to 0.74 ppm F). Conclusions: The majority of the water samples from the public supply in Teresina, particularly from the southeast and east zones, exhibited fluoride concentrations below the range considered optimal for controlling dental caries and minimizing the risk of fluorosis. Implementing heterocontrol is essential to maintain the quality of water distributed to the population. Keywords: Fluoridation. Fluorides. Public health. https://orcid.org/0000-0003-1909-0512 https://orcid.org/0000-0002-5345-6146 https://orcid.org/0000-0002-2977-6035 https://orcid.org/0000-0002-7641-6331 https://orcid.org/0000-0002-4112-1533 https://orcid.org/0000-0002-7660-5685 https://orcid.org/0000-0002-9044-9025 2 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 Introduction Fluoridation of public water supplies is an effective measure for controlling dental caries1,2, as recommended by the World Health Organization3. This practice has been shown to reduce the experience of the disease by 50% to 65% in populations exposed continuously from birth, where water is the sole source of fluoride4. Even among popu- lations using fluoridated toothpastes, there is a correlation between fluoridated water and reduced dental caries, indicating that fluoride in water remains a significant factor in controlling and preventing the disease in individuals with similar socioeconomic and demographic conditions5,6. Fluoridation in the municipality of Teresina, Piauí was implemented in 1978 by the state company Águas e Esgotos do Piauí S/A (AGESPISA), interrupted in 1986, and resumed in August 19977. Studies conducted in Teresina in the early 2000s confirmed both the presence7 and absence of fluoride8. Subsequent research on fluoride intake by children9-11 and the evaluation of dental caries and fluorosis in schoolchildren in the municipality5,6 confirmed the presence of fluoride in public water supply from 2012 to 2017. In August 2017, water supply, sewage collection, and treatment services were granted through a public-private partnership by the municipality and began to be man- aged by the company “Águas de Teresina”12. In Brazil, Law 6.050/1974 specifies that in locations with a Water Treatment Station (WTS), projects aimed at building or expanding public supply systems must include plans for water fluoridation13. Surveillance of this measure is necessary to ensure optimal concentrations of fluoride, which offer the maximum anti-caries benefit while minimizing the risk of dental fluorosis14. Heterocontrol is a surveillance principle that refers to the oversight of a public service by an institution other than the one imple- menting the measure15. Heterocontrol studies have identified deficiencies in the water fluoridation process, evidenced by the low or high concentrations of fluoride found in the analyzed samples8,16-29. Due to the lack of heterocontrol studies in Teresina after 2005 and the change in the company responsible for water supply, it was necessary to conduct an assessment of the fluoride concentration. This was done to determine whether the population continued to benefit from this public health measure. Therefore, the objective of this study was to perform heterocontrol of the fluoride concentration in the water supply distributed to the population. Methods Study location Teresina is a Brazilian municipality and the capital of the State of Piauí. It has an area of 1,391.293 km² and a population of 866,300 inhabitants, with a human development index of 0.75130. Approximately 6% of this population lives in the rural area of the city and does not benefit from the water supply system and con- sequent fluoridation6. The average maximum daily temperature is approximately 33.8ºC31. Teresina has two Water Treatment Plants (WTP). The larger plant is 3 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 responsible for treating and distributing fluoridated water to approximately 85% of the urban population. Study design A prospective descriptive laboratory study and analysis of the fluoride concentration in water were carried out. Sample collection The recommendations of the VIGIFLUOR Guide for Coverage and Monitoring of Water Fluoridation in Brazil for municipalities with more than 50,000 inhabi- tants were followed for the selection of sample collection points and the stan- dardization of this collection32. Three neighborhoods were randomly selected from each of the four zones (north, south, east and southeast) of the municipal- ity (n=12), and a school from each neighborhood was randomly selected, along with one location close to the Water Treatment Plant (WTP) (n=1), totaling 13 col- lection points. The samples were collected by school center employees, who were instructed by the research team on how to carry out these collections according to the VIGIFLUOR Guide32. The guide stipulates that water samples should be collected at public education or health units and that a collection point should be located near the WTP exit. The water samples were collected directly from the water meter or from the first tap immedi- ately after the water meter to ensure they reflect the fluoride concentration in the water distribution network in real time. To assess the continuity of exposure to fluo- ridated water, samples were collected once a month, on the same day and time, over a period of 13 months. During collection, the tap was opened for thirty seconds to eliminate any water that had stagnated in the pipes. The samples were placed in plas- tic bottles that were previously labeled and identified according to the area, neighbor- hood, and school32. The bottles were washed three times before collecting the water sample. The samples were then stored in a refrigerator until analysis in the laboratory. To ensure that all collection points were supplied by the WTP, the company “Águas de Teresina” was asked for the list of neighborhoods and their respective schools that were supplied by the WTP. Sample tests were carried out at the Biochemistry Laboratory of the Faculdade de Odontologia de Piracicaba (FOP – UNICAMP), Piracicaba, São Paulo. The count- er-tests were stored in a refrigerator in the Departamento de Patologia e Clínica Odontológica da Universidade Federal do Piauí (UFPI). Fluoride concentration analysis The fluoride concentration was determined using an ion-specific electrode for flu- oride (F-ISE) (Orion 96–06; Thermo Scientific Orion., Boston, MA, USA) coupled to a VersaStar potentiometer (Thermo Scientific Orion). The equipment was cali- brated with standard solutions containing fluoride concentrations from 0.05 to 1.6 µg F/mL. The water samples were prepared for analysis with F-ISE as described in VIGIFLUOR Guide32. Volumes of 1.0 mL of the samples were transferred to a 4 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 coded tube and buffered with 1.0 mL of TISAB II 50% (v/v). For each analysis, a calibration curve was made. Analyses were performed in duplicate, and the flu- oride concentrations in the samples were determined from the linear regression of the logarithm of fluoride concentrations of the standards with mV (millivolts) values using Excel spreadsheet® (Microsoft Corporation., Chicago, USA). The results were expressed in mg F/L (ppm F). Teresina has an average maximum temperature of approximately 33.8 degrees31 and an optimum fluoride content for the region should be between 0.45 and 0.74 ppm F32. Data analysis A descriptive analysis was carried out using Excel® (Microsoft Corporation., Chicago, USA), considering adequate concentrations to be between 0.45 and 0.74 ppm F, depending on the average local temperatures and the balance between the anti-caries benefit and the risk of fluorosis. This range is based on guidelines from the Collabo- rating Center of the Ministry of Health for Oral Health Surveillance of the University of São Paulo (CECOL/USP, 2011) (Table 1)33. Table 1. Fluoride concentration in water (µg F/mL) for locations where average annual maximum temperatures are above 32.5º C – Collaborating Center of the Ministry of Health on Oral Health Surveillance, 2011. Fluoride content in water (in ppm or mg F/mL) Benefit (Prevent tooth decay) Risk (Produce dental fluorosis) 0.00 to 0.34 Insignificant Insignificant 0.35 to 0.44 Minimum Low 0.45 to 0.74 (*) Maximum Low 0.75 to 0.84 Maximum Moderate 0.85 to 1.44 Questionable High 1.45 or more Harm Very high * The range of 0.45 to 0.74 mg F/L provides the best benefit-risk combination, according to the Collaborating Center of the Ministry of Health on Oral Health Surveillance, 2011. Results Of the 156 samples analyzed, 145 samples (93%) were classified as inappropriate (between 0.00 and 0.34 ppm F), and 11 samples (7%) as appropriate (between 0.45 and 0.74 ppm F). Four samples occurs in the first month of this study and seven of the 11 samples classified as appropriate occurred in the last three months of the heterocontrol. The mean and standard deviation of the fluoride content of the samples collected, per collection point over the 12 months, are presented in Table 2. In April 2023, a spot check was carried out to test fluoride levels again, as all sam- ples (n=13) were inadequate. The majority of the values presented were below the recommended optimum value (0.45 to 0.74 ppm F). Data on the concentration of 5 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 fluoride in the water supply by administrative zone are shown in Table 3, and the results show that only north and south zones presented samples at the optimal fluoride concentration. Table 2. Fluoride concentration (µg F/mL) of water samples from Teresina, Piauí from October 2021 to September 2022 and in April 2023 according to each collect point. Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Apr Mean SD 2021 2022 2023 1 0.42 0.35 0.31 0.36 0.18 0.23 0.33 0.24 0.24 0.54 0.33 0.50 0.27 0.33 0.10 2 0.22 0.15 0.17 0.13 0.10 0.14 0.19 0.20 0.23 0.31 0.22 0.37 0.20 0.20 0.07 3 0.51 0.33 0.43 0.24 0.22 0.22 0.39 0.41 0.27 0.49 0.52 0.06 0.20 0.33 0.14 4 0.50 0.33 0.31 0.25 0.23 0.17 0.44 0.38 0.38 0.52 0.43 0.45 0.36 0.36 0.10 5 0.55 0.44 0.41 0.19 0.19 0.18 0.19 0.25 0.21 0.36 0.41 0.43 0.29 0.32 0.12 6 0.37 0.26 0.26 0.20 0.20 0.18 0.27 0.31 0.31 0.38 0.31 0.35 0.34 0.29 0.06 7 0.24 0.26 0.25 0.18 0.12 0.14 0.28 0.25 0.31 0.33 0.37 0.34 0.28 0.26 0.07 8 0.32 0.19 0.21 0.23 0.33 0.07 0.24 0.19 0.26 0.34 0.28 0.40 0.20 0.25 0.08 9 0.09 0.12 0.16 0.16 0.06 0.20 0.19 0.24 0.25 0.16 0.22 0.22 0.20 0.17 0.05 10 0.14 0.14 013 0.11 0.08 0.12 0.13 0.18 0.18 0.16 0.22 0.21 0.16 0.15 0.03 11 0.33 0.12 0.18 0.11 0.11 0.10 0.17 0.17 0.18 0.17 0.18 0.30 0.17 0.17 0.06 12 0.20 0.12 0.13 0.10 0.12 0.08 0.21 0.18 0.21 0.23 0.22 0.26 0.18 0.17 0.05 13 0.54 0.35 0.31 0.27 0.25 0.28 0.21 0.33 0.21 0.36 0.48 0.26 0.16 0.31 0.10 * The numbers 1 to 13 in the first column refer to the identification of each collection point. Table 3. Fluoride concentration (mg F/mL) of water samples from Teresina, Piauí, from October 2021 to September 2022 and in April 2023, categorized by urban area. Zone Below optimal concentration (0.00 to 0.44 µg F/mL) Optimal concentration (between 0.45 and 0.74 µg F/mL) Above optimal concentration (0.75 µg F/mL or more) n % n % n % North 35 20.8 4 2.3 0 0.0 South 45 26.7 7 4.2 0 0.0 East 39 23.0 0 0.0 0 0.0 Southeast 39 23.0 0 0.0 0 0.0 Total 158 93.5 11 6.5 0 0.0 Discussion Analyses of the fluoride concentration in the water of Teresina, Brazil, over thirteen months, demonstrated that the majority of samples (93,5%) presented levels below 6 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 those recommended for the control of dental caries and minimal risk of develop- ing fluorosis. The averages per collection point were also below the optimal lev- els according to CECOL classification33. Only the North and South zones presented samples with fluoride concentrations within the recommended range during the period evaluated in this study. These results can be explained by their geographic proximity to the region with the largest WTP, which likely does not receive additional water from tubular wells that undergo only chlorination as treatment and are inte- grated into existing reservoirs throughout the city. However, a geographic analysis in the municipality, as well as an analysis of the location of these reservoirs, is nec- essary to substantiate this hypothesis. The dates obtained in this study are similar to those from other studies and demon- strated less coverage of fluoridation in the North, Northeast and Center-West regions7,8,20-23,28,34 comparing to South17,19,26,27,29,35-40 and Southeast16,18,24,25,41-49 regions. The first heterocontrol study conducted in Teresina demonstrated 37% of the sam- ples had fluoride concentration below the optimum range, 10% above and 53% within the optimal limits for the city7. Another study in the state of Piauí, conducted in three cities (Teresina, Floriano and Parnaíba) with a total of 576 samples ana- lyzed, found that only 4.3% (n=25) presented values within the range considered adequate for the city of Teresina8. In another Northeast regions as in Pernambuco, Brazil, in a 12-month heterocontrol conducted in the city of Guarabira, Pernambuco, where 83.3% of the analyzed samples had fluoride concentrations below the recom- mended level23. In the northern region, a heterocontrol study conducted in Manaus, Amazonas, between 2016 and 2018 showed that of the 2,874 samples collected, 49% had inadequate concentrations for the region50. These data show that, even with the mandatory fluoridation of water, there is no adjustment in the established fluoride concentration. The parameters used in this study to evaluate fluoride concentration were based on the CECOL-USP criteria33 (2011), which recommend that the evaluation of flu- oride content in water consider the preventive benefit for dental caries and the risk of developing dental fluorosis. For the city of Teresina, with average monthly and annual daily maximum temperatures above 32.5ºC31, the optimal benefit-risk combination is in the range of 0.45 to 0.74 mg F/L33. These values differ from the criteria established by Ordinance No. 635/197551, which specifies an optimal concentration of 0.6 to 0.8 mg F/L for the water supply in the national territory. However, this Ordinance only considers the average maximum daily temperatures, introducing a confounding factor due to the differing measurement intervals for fluoride content specified by Ordinance No. 63551 and those indicated by the CECOL-USP parameters. In 2015, the United States Public Health Service (PHS) updated and replaced its 1962 Drinking Water Standards related to community water fluoridation, recommending an optimal fluoride concentration of 0.7 mg/L. This concentration provides the best balance of protection from dental caries while limiting the risk of dental fluorosis. The earlier PHS recommendation for flu- oride concentrations was based on outdoor air temperature of geographic areas and ranged from 0.7 to 1.2 mg/L52. 7 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 Water fluoridation is widely recognized as a crucial method for controlling dental caries1, supported by the World Health Organization3 (WHO). In 2022, the Inter- national Association for Dental Research issued a position on water fluoridation, declaring that the method has a protective effect against tooth decay, is safe, effective, economical and reduces inequalities in oral health53. Fluoridation of pub- lic water supplies is the simplest collective way of maintaining a low and constant dose of fluoride in the oral cavity and there is no evidence that fluoride in water at optimal concentration causes any other systemic effects on humans other than dental fluorosis54. The interruption of water fluoridation triggers an increase in the prevalence of dental caries, equating the severity of the disease with that of groups exposed to low or no fluoride levels1. In EUA it was demonstrated that starting water fluoridation in public supply service decreased the occurrence of caries in children aged 4-17 by 30-50% and the interruption of water fluoridation increased caries experience by 18%55. A compilation of subsequent studies pro- vided evidence confirming that the economic benefit of community water fluori- dation exceeds the cost of the intervention. Furthermore, the cost-benefit ratio increases with the size of the community’s population56. Regarding the cost-benefit of fluoridated water in Brazil, it was demonstrated that the average cost per capita/year of implementing fluoridated water in the city of São Paulo was R$ 0.08 (US$ 0.03) in 2003, considering costs with initial installation capital, chemical product, system operation (system maintenance, electrical energy and human resources) and monitoring of fluoride levels57. In Ire- land, despite the population having access to other means of fluoride, it has been demonstrated that the implementation of water fluoridation remains a cost-effec- tive public health intervention for schoolchildren58. In New Zeland, the cost-effec- tiveness of community water fluoridation was found to be highly cost-effective, with the benefits in terms of averted suffering and dental care costs discernible for communities as small as 500 people. There is a nine times pay-off, with sav- ings over two decades59. Some limitations can be considered in this study. Although there was oversight of the water samples collection by school employees, the research team did not have in-person control during the collection process; Oversight occurred virtually through messages on the WhatsApp® application and by sending photos of the samples col- lected by employees to the research team members. In conclusion, the majority of samples collected from the public water supply showed fluoride concentrations below the range considered optimal for the munic- ipality of Teresina, Brazil. This was particularly evident in the Southeast and East zones, where no samples had fluoride concentrations within the optimal range for the control of tooth decay and minimal risk of developing fluorosis. It is necessary to identify flaws in the water supply for the population of Teresina, requiring joint action between the city’s Municipal Health Department and the concessionaire responsible for the water supply, Águas de Teresina, to coordinate actions to resolve this issue and ensure the benefit of fluoridation of public water supply for the population. The implementation of heterocontrol is essential for maintaining the quality of water distributed to the population. 8 Oliveira Júnior et al. Braz J Oral Sci. 2025;24:e256062 Acknowledgments We thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for granting a scholarship at the Programa Institucional de Bolsas de Iniciação Cientí- fica (PIBIC) to the first author. Conflict of Interest The authors have no conflict of interest to disclose. Data availability Datasets related to this article will be available upon request to the corresponding author. Author Contribution All authors have made substantive contribution to this study and/or manuscript. Gildenilson Oliveira Júnior: Conceived and designed the experiment, Performed the experiments, Analyzed the data, Wrote and reviewed it critically for import- ant intellectual content the paper. Letícia Caminha Aguiar Lopes: Analyzed the data, Wrote and reviewed it critically for important intellectual content the paper. Cacilda Castelo Branco Lima: Analyzed the data. Marina de Deus Moura de Lima: Analyzed the data, Wrote and reviewed it critically for important intellectual con- tent the paper. Lúcia de Fatima Almeida de Deus Moura: Analyzed the data, Wrote and reviewed it critically for important intellectual content the paper. Cinthia Pereira Machado Tabchoury: Conceived and designed the experiment, Performed the experiments, Analyzed the data, Wrote and reviewed it critically for important intellectual content the paper. Marcoeli Silva de Moura: Conceived and designed the experiment, Analyzed the data, Wrote and reviewed it critically for important intellectual content the paper. All authors approved final version to be published and agreement to be accountable for all aspects of the work in ensuring that ques- tions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. References 1. Iheozor-Ejiofor Z, Worthington HV, Walsh T, O’Malley L, Clarkson JE, Macey R, et al. 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