Although water fluoridation in Brazil has been mandatory since the 70s [9], in practice, millions of Brazilians do not have access to this important preventive measure. The epidemiological consequences of these acts were highlighted in last oral health survey carried out in Brazil [41], which showed that 54% of the 5-year-old children had experienced caries. The caries experience increases with age, reaching impressive 27.5 decayed, missing or filled teeth (DMF-T) among the elderly. In this context, water fluoridation remains an effective measure for caries prevention, considering that in regions highly covered by water fluoridation, like South and Southeast, the mean DMF-T (2.06 and 1.72, respectively) was considerably lower than that found in regions with low coverage of fluoridated water, such as North and Northeast (mean DMF-T?=?3.16 and 2.63, respectively).

The preventive effects of fluoride are more efficient when associated with water consumption because it is a vehicle of high ingestion and frequency, collective coverage, and low cost [6]. Several countries that perform water fluoridation observed a drop in caries indexes after implementing such measure [42, 43]. Every time fluoridated water or food cooked with it is ingested, a transient increase of fluoride concentrations in saliva and dental biofilm is observed (lasts about 1 to 2 h). Subsequently, fluoride is absorbed and distributed to the body. Part of the absorbed fluoride returns to the oral cavity through saliva. Thus, the frequent and continuous intake of fluoridated water maintains high fluoride levels in the oral cavity, to interfere with the processes of de- and remineralization [44].

Our results showed that less than half of the total water samples were within the standards established by law [16]. Although guaranteed by law, the surveillance of fluoridation in Brazilian public water supplies is deficient and may be attributed to factors such as local temperature, instability of the fluoride salt, or even to the difficulty regarding the functioning of the supply system [11].

Another result observed in the present meta-analysis was the high number of samples below the minimum concentration required to guarantee the benefits of fluorided water in caries control. The clinical effect of this variation will depend on the exposure duration. When fluoride is used in a community-based approach like fluoridated water, the maintenance of the optimal fluoride concentration is essential to guarantee its effect on caries control. If fluoridated water is constantly ingested in doses below the “optimal”, the retention of fluoride in oral fluids (saliva and biofilm fluid) will be low, compromising the preventive and therapeutic effect of this measure on caries development. The cities with the best indexes of fluoridation are in the southeast region and the worst indexes are in the northeast region. Such finding may reflect the different levels of social development of these regions, considering that a great portion of the states in the northeast region present the lowest human development index (HDI) of the country [45]. This result is supported by Daré et al. [46], in which the regions with the lowest HDI presented the worst results in the fluoridation of the water supply. Thus, the awareness for better training and control of the technical fluoridation team should be performed by government agencies to provide the whole population with ideal fluoridation levels in public water supplies.

The systemic chronic ingestion of fluoride is related to dental fluorosis [47]. Fluorosis is described as a symmetrical hypomineralization that affects tooth enamel. In the present study, 6.7% of the samples presented fluoride concentrations above the recommendation. Considering that the included studies assessed the fluoride concentration in water for at least 12 months, and in some cases the fluoride concentration was maintained elevated for consecutive months, the daily consumption of this water or food prepared with it may lead to the occurrence of dental fluorosis. However, the clinical effect of this fluorosis does not appear to be a concern, since the last Brazilian dental survey found a 16% prevalence of dental fluorosis in 12 year-old children, which was restricted to mild and very mild severity forms [48].

Besides drinking fluoridated water, these children also used fluoridated toothpaste daily. In fact, studies conducted in countries where the population is exposed to these two sources of fluoride, such as United States and Australia, show that dental caries affects a person’s quality of life more than fluorosis [49, 50]. This can be explained by the fact that the most common levels of fluorosis registered in these countries are mild and very mild, similarly to what is observed in Brazil.

Furthermore, early access to fluoride products, including the use of fluoridated toothpaste, use of mouthwash solutions and professional application before the age of three are also pointed out as a risk factors for dental fluorosis. In addition, other additional sources of fluoride such as mineral water, fluoridated salt, teas and children’s drinks can increase the risk of dental fluorosis in children. In the Brazilian context, however, fluoridated salt is not available [51]. Thus, considering that fluoridated water and the use of fluoridated toothpaste are the most efficient and cost-effective strategies to prevent caries, other forms of delivery should be indicated only for people at high risk for caries or disease activity. A recent article showed that dental caries experience in children was related with the effectiveness and frequency of oral hygiene and diet [52]. Caries prevention programs must be adjusted to individual characteristics of each child, taking into consideration oral hygiene practices, diet and total fluoride intake [52].

It is worth noting that external control is only a mechanism to detect the problem of water fluoridation and not the solution. Public policies to guarantee ideal levels of fluoridation need to be implemented, with stricter enforcement of the law. Moreover, to ensure the benefits of fluoride and minimize the risk of dental fluorosis, it is recommended that caries community prevention programs take into consideration the estimate of total fluoride exposure from water or food prepared with it, dentifrices and mouthrinses, as well as the oral hygiene and dietary habits of the studied population [52].

Many countries have policies to maximize the benefits of fluoride, but many have yet to do so. Policies were introduced to reduce excessive fluoride exposure during the period of tooth development, and these were successful in reducing dental fluorosis without compromising caries prevention [53]. In Brazil, water fluoridation has been provided by law since 1975, with varying degrees of implementation throughout the country. Historically, richer, more developed regions benefited from water fluoridation earlier, however the Brazilian Oral Health Policy of 2004 promoted fluoridation of more deprived areas, resulting in the reduction of inequalities in access to fluorides. Thus, even considering that the topical application of fluoride, especially brushing with fluoride toothpaste, is the most important preventive procedure that maintains the cariostatic concentration of fluor in the oral environment, the fluoridation of water at ideal levels of fluoride concentration represents an important public health action, since it promotes access to fluorides to all who receive water from water supply systems. The advantages of water fluoridation are that it provides substantial lifelong caries prevention, is cost-effective, and reduces health inequalities: it reaches a substantial number of people worldwide [53].

Thus, it is possible to agree with Buzalaf et al. [18] that reinforce the belief in the importance of the implementation and maintenance of external control of fluoride in water supplies to improve the consistency of water fluoridation. This measure is fundamental to achieve the maximum benefits of water fluoridation, which contributes to improve the oral health condition of people who drink water from those supplies.

This review presented some methodological limitations. Most studies selected used samples collected in different ways (sample size, frequency, storage) and with different criteria, producing high heterogeneity of results. This might have affected the analysis of fluoride concentration in the water. Thus, the high and unexplained heterogeneity observed in the analyzes is an important limitation that must be considered when interpreting our results, which prevents us from being more emphatic in our conclusions. Also, the studies included in this review cover only three of the five Brazilian regions, reflecting the lower coverage of fluoridated water in the north and midwest regions. Therefore, new studies are encouraged to monitor the amount of fluoride in public water supplies in the whole Brazilian territory, so that public policies may be developed and correctly directed to the population. Moreover, standardized studies on fluoride external control are required to produce comparable results in different locations of the country [51]. In this aspect, the implementation of the Vigiflúor system represents a major step towards the surveillance of the public water supply in Brazil [54].

Nevertheless, this study is original and contributed to the development of scientific knowledge from two main points. First, it is the first systematic review with a meta-analysis that assesses the fluoride external control concentration in Brazilian public water supplies. Second, an extensive search strategy was applied without any restriction of language or publication date and including the “grey literature” to avoid selection and publication biases.

We conclude that the fluoride levels in the public water supply in several Brazilian cities are inadequate to guarantee the anticaries benefits and safety from fluorosis.

The establishment of effective local policies of oral health surveillance is imperative to ensure that the fluoride concentration in the water supply is ideal to guarantee the effectiveness of the fluoride ion and the low risk of fluorosis, thus including the understanding of the epidemiological dynamics of dental caries in the cities.