Update on Endemic Fluorosis in a Region of Europe (Tenerife, Canary Islands)

Abstract

Fluoride is an anion that is widely distributed in nature whose main source is volcanic emissions. This anion reaches humans mainly through water and can produce beneficial effects at certain doses, with toxic effects occurring at high doses. There are regions of the world that stand out for their high concentrations of fluoride, as it has produced endemic fluorosis in their population. This is the case for Tenerife, one of the islands of the Canary archipelago of volcanic origin. The aims of this study are to perform a historical review of fluoride concentrations in this region and to assess the current levels. The determination of fluoride will be carried out by potentiometry using an ion-selective electrode. The high concentration of fluoride in the water supply continues to be a problem, since about 55% of the studied municipalities on the island of Tenerife exceed the parametric value for the concentration of fluoride in the water (1.5 mg/L). The municipalities with the highest fluoride concentrations in Tenerife are El Tanque (10.61 mg/L), Icod de los Vinos (5.28 mg/L), Tegueste (4.37 mg/L) and La Victoria de Acentejo (3.63 mg/L). The child population aged from 1 to 3 years with a daily water consumption of 1.5 L exceeds the ULs in 21 of the 31 studied municipalities of the island. Meanwhile, the child population aged from 4 to 8 years (daily water consumption of 1.5 L) greatly exceeds the ULs in 13 of 31 of these municipalities. In the case of the adult population (consumption of 2 L/day), it exceeds the ULs in 3 of the 31 studied localities. In addition, since only exposure induced from one type of food has been studied, and not in regard to overall diet, it can be concluded that there may be a risk of overexposure to fluoride for the population aged 1 to 3 years in all the municipalities studied, as well as for the population aged 4 to 14 years and the adult population in most of them. Thus, it can be determined that, currently, with the recommended water consumption, the population could be exposed to a toxicological risk depending on the municipality and age.

1. Introduction

Fluoride, due to its chemical properties, tends to always exist in its anionic form in the environment [1] The main sources of fluoride are natural, with volcanic emissions standing out above all, which can contribute between 60 and 6000 kilotonnes per year in the form of HF [2,3], followed by marine aerosols, which contribute about 20 kilotonnes. However, in industrialized territories, the main sources of fluoride will be waste products resulting from industrial activity, and the relationship between cases of fluorosis and exposure to this type of industries have been demonstrated in different studies [4,5]. In all cases, through the bio-geological cycle, inorganic fluorides will eventually reach the biota (aquatic or terrestrial) through soil, water or air, and these fluorides will be able to bioaccumulate without biomagnification in the food chain [6,7].

Thus, it is not difficult to imagine that, in geothermal regions or regions with volcanic activity, fluoride concentrations in surface soils will be elevated [8,9]. This may have repercussions on the fluoride concentration in groundwater, especially if the soil is porous and permeable, since rainwater reaching the soil seeps into the ground, leaching the hydrofluoric acid formed into the different galleries [10,11]. This can be exacerbated by emanations of volcanic gasses from deep magmatic pockets, which are rich in hydrofluoric acid and reach these waters [12]. This is the case in the Canary Islands [13], where lichens with high fluoride content have also been found, which have been used as biomarkers of contamination by this ion [6].

Among the beneficial effects that fluoride can provide, the prevention of the cariogenic process through various mechanisms stands out [7,14,15,16]. However, at certain doses, it can produce toxicity, highlighting changes in tooth enamel, nail changes, bone changes and effects on the nervous system, among other changes in the organism [7,17,18,19,20,21,22,23,24,25,26,27].

In general, fluoride exposure for humans is brought about through dietary intake [13,28]. Therefore, most studies on fluoride intake focus on the study of the fluoride composition of foods, which will depend mainly on the origin and sources of exposure.

Generally, the fluoride contributed by the dietary intake of solid foods could be considered negligible, considering the quantities consumed and the concentrations that these foods usually have (in the order of mg/kg) [29]. The contribution of fluoride made by liquid foods is much more significant, particularly water and those beverages with a high content of fluoride, such as tea or infusions, which are usually considered the main source of this ion [13,29,30,31,32,33,34,35,36,37,38,39,40].

The first studies that established the relationship between fluoride concentrations in the water supply and the symptoms of fluorosis poisoning were published in India, an area of endemic fluorosis, between the end of 1944 and the beginning of 1945 in the journal The Indian Medical Gazette [41,42,43]. In these articles, fluoride was known to be present in the environment from industries and the use of fertilizers, and it was said to be unavoidable in a person’s diet because of its presence in drinking water from fluoridated rocks, with the toxicological effects being related to long-term high exposure to this ion. Chaudhuri (1944) [41] combines knowledge and various studies, describing how, in 1902, the high prevalence of dental fluorosis in Italian immigrants to the United States from Naples began to attract attention until, in 1932, it was determined that the etiological agent was the drinking water. Chaudhuri (1944) [41] also describes how, in cases of higher exposure, the bone could be affected, observing certain examples of sclerosis in the vertebrae, ribs and coccyx with the use of radiographic techniques, as was demonstrated in 1937 with some advanced cases in patients from Madras (India) who came from areas where the drinking water contained concentrations above 6 mg/L. Even then, it was described how a change in the drinking water to water with a lower concentration of the ion could reverse the symptoms [41].

Fluoride concentrations in water supplies has become a public health problem in numerous countries, including Venezuela [44], Ethiopia [45], northern Iran [46], Jordan [47], India [48], Thailand [49], China [50], Ghana [51] and a multitude of countries in Africa [52], Western Estonia [53], Mexico [54], Argentina [55], Australia [56], Iran [57], Colombia [58] and the United States of America [59], among others, where fluoride concentration in the water supply can exceed 4.5 mg/L.

Historically, inhabitants of the village of La Guancha on the island of Tenerife were recognized for having characteristically stained teeth, sometimes even called “those with rotten teeth”, a condition similar to that observed in some areas of India [60,61]. However, it was not until the mid-1950s that Fernandez Caldas and Perez Garcia determined the origin of the problem in this population, and, in 1976, they published an article indicating that some springs in the region between San Juan de la Rambla and Icod de los Vinos had a fluoride concentration much higher than the rest, reaching 6.4 mg/L [62]. Subsequent reviews over the years confirmed that elevated fluoride levels persisted in this region, particularly in the Barranco de Vergara gallery. Although they were increasingly higher, it was not given greater importance, as this was considered to be an area of endemic fluorosis [63].

It was not until 1991 that an oral health study conducted by the General Directorate of Public Health determined that 26.4% of school children in the north and northwest of Tenerife exhibited some type of fluorosis. In a later study, in 1998, an increase in the incidence of fluorosis in schoolchildren was indicated not only in the north and northwest but also in the southern part of the island [63]. This coincided with tourism development on the island, necessitating the transfer of water from one area to another due to increased demand. However, the severity of chronic fluorosis cases also diminished due to interventions by authorities, such as restrictions on drinking water consumption or the installation of brackish water desalination plants (WDPs) in 1995 [63].

Over the years, the overexploitation of springs beyond their replenishment rates (attributable to decreased rainfall) has resulted in increased salt concentrations in the groundwater. This occurs due to the precipitation of stagnant water over the passage of time, being more concentrated in the lower zone. Moreover, the remaining water continues to be enriched by being in continuous contact with the rocky material, where microfractures are formed by the temperature and pressure, exposing a larger rocky surface [64,65,66].

For this reason, it is crucial to conduct regular monitoring of fluoride concentrations in the water supply of all municipalities of the island, particularly those in the north and northwest. Additionally, a thorough risk assessment should be performed until a satisfactory and definitive solution to the problem is found.

The aims of this study are to carry out a historical review of the fluoride concentrations determined in the water supply in each of the municipalities of the island and to make a determination of the current fluoride levels as part of a risk assessment.

2. Material and Methods

2.1. Samples

A total of 62 samples were taken on the island of Tenerife, in the archipelago of the Canary Islands, located off the northeast coast of Africa between coordinates 27°37′ and 29°25′ north latitude and 13°20′ and 18°10′ west longitude. This island represents the emerged part of a large pyramidal volcanic edifice, with its triangular base being located about 3000 m below sea level. It boasts the largest surface area (2058 km²) and the highest peak (Pico del Teide, 3718 m) of the Canary Islands archipelago [67]. The central sector of the island is occupied by the Cañadas Edifice. Its hydrographic network is configured by ravines that only carry water during rainy seasons, but they are characterized by significant depth and steep slopes. In general, they follow a radial direction from the edge of Las Cañadas towards the sea [67]. Tenerife is also the most populous island in the archipelago, with a population of 955,063 inhabitants in 2024, accounting for approximately 42% of the Canary Islands’ population [68]. The distribution of the population by municipality is as follows: Adeje (5.29%), Arafo (0.60%), Arico (0.95%), Arona (9.07%), Buenavista del Norte (0.49%), Candelaria (3.01%), Fasnia (0.32%), Garachico (0.52%), Granadilla de Abona (5.98%), La Guancha (0.59%), Guía de Isora (2.37%), Güimar (2.27%), Icod de los Vinos (2.54%), San Cristobal de La Laguna (16.78%), La Matanza de Acentejo (0.96%), La Orotava (4.46%), Puerto de la Cruz (3.29%), Los Realejos (3.93%), El Rosario (1.88%), San Juan de la Rambla (0.51%), San Miguel de Abona (2.42%), Santa Cruz de Tenerife (22.13%), Santa Úrsula (1.61%), Santiago del Teide (1.30%), El Sauzal (0.97%), Los Silos (0.49%), Tacoronte (2.59%), El Tanque (0.29%), Tegueste (1.19%), La Victoria de Acentejo (0.98%) and Vilaflor de Chasna (0.20%).

Two samples were taken from each municipality. Sampling prioritized those districts that are most representative or that have had the most problems with official consumption restrictions due to fluoride in compliance with Directive 2020/2184 and RD 3/2023, which establish a parametric value of 1.5 mg/L [69,70]. This approach aims to determine the approximate average fluoride concentration in each municipality’s water supply. All samples were collected in the month of August 2023 directly from the tap of users of the public water supply, with each sample being 100 mL. The 3 sampled regions into which the island has been divided for subsequent analysis are shown below (Figure 1); a topographic and the water supply system infrastructure map of the island is also included (Figure 2 and Figure 3):

• Metropolitan region: Santa Cruz de Tenerife, La Laguna, El Rosario and Tegueste.
• Northern region: Tacoronte, el Sauzal, la Matanza de Acentejo, la Victoria de Acentejo, Santa Úrsula, la Orotava, Puerto de la Cruz, los Realejos, San Juan de la Rambla, la Guancha, Icod de los Vinos, Garachico, el Tanque, los Silos and Buenavista.
• Southern region: Candelaria, Arafo, Güimar, Fasnia, Arico, Granadilla de Abona, San Miguel de Abona, Vilaflor, Arona, Adeje, Guía de Isora and Santiago del Teide.

2.2. Analytical Method

A fluoride analysis was performed by ion-selective potentiometric techniques using a fluoride ion-selective electrode (ISE). The potentiometer used corresponds to a HACH SensION-MM340 model (HACH, Düsseldorf, Germany) with a fluoride selective electrode model HACH ISE F-9655C (HACH, Düsseldorf, Germany).

The instrument parameters were range (0.01 to 19,000 mg/L), pH range (4 to 8), linear range (0.1 to 19,000 mg/L), slope (59 mV/pF) and operating temperature (5–50 °C). The possible interferences that can present the electrode are, in particular, Ca⁺², Fe⁺³ and Al⁺³, and it was necessary to use a conditioning solution to minimize them. Therefore, 0.75 M of orthophosphoric acid was used [40], which was prepared from 85% concentrated orthophosphoric acid (Honeywell-Fluka, Seelze, Germany).

The method precision was evaluated under reproducibility conditions using the standard addition method. After determining the initial fluoride (F-) concentration in the samples, a known quantity of fluoride was added. This procedure was replicated 15 times daily across three separate days.

Repeatability, expressed as relative standard deviation (RSD), was found to be 0.50%, while reproducibility was 2.70%. Recovery rates averaged 99%, with an RSD below 5%. These results demonstrate acceptable precision and satisfactory recovery.

Before proceeding with the measurements, a calibration curve was made with fluoride concentration solutions of 10⁻⁵ M, 10⁻⁴ M, 10⁻³ M and 10⁻² M in conditioning solution, starting from a stock solution of 10⁻¹ M concentration prepared with NaF of analytical purity (Merck, Germany).

For measurement, the samples were prepared with 5 mL of the conditioning solution and 25 mL of the sample. Three aliquots were taken from each of the samples, which were measured three times each, taking as the final value the mean of all of them in each case.

2.3. Statistical Análisis

A statistical analysis was carried out using GraphPad Prism version 8.0.1 software (GraphPad, Boston, MA, USA). The distribution of the results was studied with the Anderson–Darling, D’Agostino and Pearson, Shapiro–Wilk and Kolmogorov–Smirnov tests, which followed a non-normal distribution. Once the non-normality of the results had been demonstrated, nonparametric tests (Mann–Whitney test) were applied.

The statistical study was carried out with the aim of studying significant differences (p < 0.05) in the current fluoride content between sampling regions (metropolitan, north and south) on the island of Tenerife.

2.4. Fluoride Exposure Assessment and Risk Characterization

The European Food Safety Authority (EFSA) has established an Adequate Intake (AI) for fluoride of 0.05 mg/Kg/day, as well as Tolerable Upper Intake Levels (ULs) for different age ranges.

Table 1. Reference intake levels established by the EFSA [73,74].

Age Groups	UL (mgF/day)
0–6 months	-
7–11 months	-
1–3 years	1.5
Male 4–6 years	2.5
Female 4–6 years	2.5
Male 7–9 years	2.5
Female 7–9 years	2.5
Male 10–14 years	5
Female 10–14 years	5
Male 15–17 years	7
Female 15–17 years	7
Male > 18 years	7
Female > 18 years	7

shows these intakes by age range and sex.

The assessment of the dietary intake was based on obtaining the Estimated Daily Intake (EDI) of fluoride from the consumption of two different amounts of water supply by the adult population (over 18 years of age) and the child population (in three age intervals: 1–3 years, 4–8 years and 10–14 years) (Equation (1)). The quantities of water consumed considered were 1.5 and 2 L/day for those over 10 years of age and 1 and 1.5 L/day for children between 1 and 8 years of age, according to EFSA recommendations [75]. The equation used to obtain the EDIs is as follows (Equation (1)—Equation for Exposure Assessment):

$$\mathrm{E}\mathrm{D}\mathrm{I}\left({\displaystyle \frac{\mathrm{m}\mathrm{g}}{\mathrm{d}\mathrm{a}\mathrm{y}}}\right)=\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left({\displaystyle \frac{\mathrm{m}\mathrm{g}}{\mathrm{L}}}\right)\times \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left({\displaystyle \frac{\mathrm{L}}{\mathrm{d}\mathrm{a}\mathrm{y}}}\right)$$

(1)

Once the EDI has been obtained for each consumption scenario and for each zone studied, a risk characterization was carried out through the calculation of the contribution to the ULs established by the EFSA (Equation (2)—Equation for Risk Characterization).

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{\mathrm{E}\mathrm{x}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}\left(\mathrm{E}\mathrm{D}\mathrm{I}\right)}{\mathrm{R}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\left(\mathrm{U}\mathrm{L}\right)}}\times 100$$

(2)

3. Results and Discussion

The highest concentrations of fluoride were observed in El Tanque (10.61 mg/L), Icod de los Vinos (5.28 mg/L), Tegueste (4.37 mg/L) and La Victoria de Acentejo (3.63 mg/L). In contrast, the lowest levels were found in Adeje (0.37 mg/L) and Arona (0.45 mg/L) (Figure 4). It can be established that the highest concentrations of fluoride were found in the northern region, followed by the metropolitan and southern regions (

Table 2. The fluoride mean concentrations, standard deviation and confidence interval (CI 95%) of the water supply samples for each of the districts, municipalities and regions of the island of Tenerife.

Region	Municipality	District	[F−] mg/L District	[F−] mg/L Municipality	CI (95%) Municipality	[F−] mg/L Region	CI (95%) Region
Metropolitan	Santa Cruz de Tenerife	Casco urbano	0.16	2.08 ± 2.71	2.08 ± 3.76	2.24 ± 1.50	2.24 ± 1.47
		Sobradillo	4.00
	La Laguna	Valle Guerra	1.30	1.64 ± 0.48	1.64 ± 0.66
		Geneto	1.98
	El Rosario	Radazul	0.82	0.89 ± 0.09	0.89 ± 0.13
		Machado	0.95
	Tegueste	Casco urbano	4.29	4.37 ± 0.11	4.37 ± 0.15
		El Socorro	4.44
North	Tacoronte	Casco urbano	0.44	0.47 ± 0.05	0.47 ± 0.08	3.12 ± 2.45	3.12 ± 1.24
		Agua García	0.51
	El Sauzal	La Baranda	1.09	1.59 ± 0.71	1.59 ± 0.99
		Ravelo	2.10
	La Matanza de Acentejo	Casco urbano	2.52	4.13 ± 2.27	4.13 ± 3.15
		El Pirul	5.74
	La Victoria de Acentejo	Santo Domingo	0.93	3.63 ± 3.82	3.63 ± 5.30
		Vera-Carril	6.33
	Santa Úrsula	Casco urbano	2.19	2.00 ± 0.27	2.00 ± 0.37
		La Corujera	1.81
	La Orotava	Casco urbano	1.23	1.28 ± 0.08	1.28 ± 0.11
		Pinolere	1.33
	Puerto de la cruz	Casco urbano	0.92	1.23 ± 0.44	1.23 ± 0.61
		Las Dehesas	1.55
	Los Realejos	San Cayetano	1.75	1.49 ± 0.37	1.49 ± 0.51
		Casco urbano	1.23
	San Juan de la Rambla	San Juan	2.21	2.48 ± 0.38	2.48 ± 0.53
		Los Quevedos	2.75
	La Guancha	Las Longueras	3.41	3.32 ± 0.13	3.32 ± 0.17
		Casco urbano	3.23
	Icod de los Vinos	San Marcos	6.50	5.28 ± 1.73	5.28 ± 2.40
		El Amparo	4.06
	Garachico	Casco urbano	3.97	3.75 ± 0.31	3.75 ± 0.43
		San Juan del Reparo	3.53
	El Tanque	Casco	3.46	10.61 ± 10.12	10.61 ± 14.02
		San José de los Llanos	17.76
	Los Silos	La Caleta de Interián	3.25	3.29 ± 0.06	3.29 ± 0.08
		Casco urbano	3.33
	Buenavista	Masca	0.43	2.25 ± 2.58	2.25 ± 3.57
		Las Portelas	4.07
South	Candelaria	Casco urbano	0.57	0.94 ± 0.51	0.94 ± 0.71	1.22 ± 0.60	1.22 ± 0.34
		Las Cuevecitas	1.30
	Arafo	La Hidalga	0.94	0.94 ± 0.01	0.94 ± 0.01
		Casco urbano	0.93
	Güimar	Puertito de Güimar	1.41	1.20 ± 0.31	1.20 ± 0.43
		Casco urbano	0.98
	Fasnia	Los Roques de Fasnia	2.05	2.26 ± 0.29	2.26 ± 0.40
		Casco urbano	2.46
	Arico	Abades	0.95	1.08 ± 0.20	1.08 ± 0.27
		Casco urbano	1.22
	Granadilla de Abona	San Isidro	0.39	1.22 ± 1.18	1.22 ± 1.63
		Casco urbano	2.06
	San Miguel de Abona	Las Chafiras	0.46	1.02 ± 0.79	1.02 ± 1.09
		Casco urbano	1.58
	Vilaflor	La Escalona	2.57	2.32 ± 0.34	2.32 ± 0.48
		Casco urbano	2.08
	Arona	Las Américas	0.05	0.45 ± 0.57	0.45 ± 0.80
		Casco urbano	0.86
	Adeje	Costa Adeje	0.02	0.37 ± 0.49	0.37 ± 0.68
		Casco urbano	0.71
	Guía de Isora	Alcalá	0.91	1.62 ± 1.00	1.62 ± 1.39
		Casco urbano	2.33
	Santiago del Teide	Puerto Santiago	0.45	1.16 ± 1.01	1.16 ± 1.39
		Casco urbano	1.87

).

Our analysis revealed that 55% of municipalities on the island of Tenerife have fluoride concentrations above the parametric level established in European and Spanish legislation (1.5 mg/L) [70]. This fact confirms the permanence of this public health problem of endemic fluorosis that has been demonstrated since 1983 [76].

A statistical analysis confirmed significant differences in fluoride content between the northern and southern zones (p = 0.0015). This was not the case between the northern and metropolitan zones (p = 0.5858), nor between the metropolitan and southern zones (p = 0.3435). This aligns with the historical trend, showing a gradual increase in fluoride concentrations, with the lowest concentrations being found in the southern region, the highest in the northern region and intermediate concentrations being found in the metropolitan region (Table 2). The only region that complies with the official parameters is the south of Tenerife.

To contextualize this issue, we present a historical overview of fluoride concentrations in Tenerife’s water supply (

Table 3. Fluoride concentrations (mg/L) in the water of some galleries in the municipality of La Guancha between 1976 and 1997 [79,80].

Gallery	Year
	1976	1981	1984	1986	1995	1997
Barranco Vergara	6.41	7.80	8.00	9.40	10.20	10.00
El Frontón	4.72	5.10	-	4.40	-	-
Pinalete	4.18	-	-	4.30	-	-
Sta. Teresa	3.96	5.00	-	3.90	-	-
Los Palomos	3.25	5.98	-	4.30	-	-
El Porvenir	2.58	3.10	-	3.10	-	-
Derriscadero	2.54	2.00	-	3.30	-	-
La Esperanza	0.57	-	-	-	-	-
Fuente Pedro *	-	5.20	5.00	-	-	-
Bilbao *	-	4.10	3.50	-	-	-
El Partido *	-	2.90	2.75	-	-	-

* This gallery belongs to the San Juan de la Rambla municipality.

and

Table 4. Historical review of fluoride concentration (mg/L) in the water supply of the municipalities of the island of Tenerife.

Municipality	Years
	1983 a	1986 b	1987 c	1995 d	1997 e	1998–2000 f	2006 f	2009 g	2015 h	2020 i	2023 j
El-Tanque	0.31–4.93	-	0.65	-	-	>2.7	>4	0.35–4.38	0.34–2.95	-	2.13 ± 0.01
Garachico	0.17–5.12	-	0.35	-	-	>2.7	1.5–4	0.47–2.5	1.33–3.05	-	3.32 ± 0.78
Icod-de-los-Vinos	0.25–5.60	-	5.20–5.60	4.38 ± 2.13	-	>2.7	1.5–4	2.22–3.94	2.05–3.12	4.59 ± 2.75	2.52
Buenavista	0.43–4.53	-	0.35	-	-	>2.7	>4	1.11–1.16	1.00–6.70	-	-
Los-Silos	0.20–0.62	-	0.50	-	-	>2.7	1.5–4	1.05	0.96–3.05	-	2.41 ± 1.25
La-Guancha	2.75–8.00	3.90	4.10	5.11 ± 1.02	5.8	>2.7	1.5–4	1.52–2.07	-	5.55 ± 1.18	3.82 ± 0.08
San-Juan-de-la-Rambla	0.70–4.47	-	2.50–2.90	2.64 ± 0.20	-	>2.7	1.5–4	0.95–2.45	0.82–3.01	2.50 ± 0.35	2.73
La-Matanza	0.31–1.19	-	1.20	-	-	1.5–2.7	1.5–4	2.41–3.03	2.95–3.51	-	2.85 ± 0.35
El-Sauzal	1.31–1.38	-	1.40	-	-	1.5–2.7	1.5–4	2.34–2.76	0.67–4.5	-	7.00
La-Victoria	0.70–1.19	-	1.20	-	-	1.5–2.7	1.5–4	0.36–2.87	0.33–3.42	-	2.71
Santa-Úrsula	0.38–1.40	-	0.80–0.95	-	-	1.5–2.7	1.5–4	0.22–1.21	0.77–1.28	-	2.75 ± 0.55
La-Laguna	0.40–1.37	-	0.95	-	-	1.5	<1.5	0.15–2.14	0.96–3.06	-	1.51 ± 1.13
Tacoronte	0.31–1.25	-	1.40	-	-	1.5–2.7	<1.5	0.56–3.02	-	-	0.42 ± 0.16
Tegueste	0.45–1.35	-	0.90	-	-	1.5	<1.5	0.26	-	-	5.39 ± 1.77
Puerto-de-la-Cruz	1.10–2.03	-	0.30–4.40	-	-	1.5	<1.5	0.66–1.41	-	-	0.75
La-Orotava	0.27–0.52	-	0.28–0.30	-	-	1.5	<1.5	0.42–0.52	-	-	4.92 ± 5.30
Los-Realejos	0.00–3.35	-	0.35	-	-	>2.7	<1.5	0.54–0.88	-	-	1.63 ± 0.40
Santa-Cruz-de-Tenerife	0.13–2.31	-	0.28	-	-	<0.7	<1.5	0.07–0.32	-	-	0.73 ± 1.39
El-Rosario	0.27–0.52	-	0.43	-	-	<0.7	<1.5	0.23–0.34	-	-	1.62 ± 0.83
Candelaria	0.20–0.78	-	0.50	-	-	<0.7	<1.5	0.19–0.23	-	-	0.96 ± 0.67
Arafo	0.65	-	0.53	-	-	<0.7	<1.5	0.36–0.41	-	-	2.32 ± 0.97
Güimar	0.44–2.65	-	0.66	-	-	1.5	<1.5	0.38–0.59	-	-	0.99 ± 0.02
Arico	0.87–2.69	-	1.10–1.66	-	-	1.5	<1.5	0.23–0.53	-	-	0.84 ± 1.46
Granadilla	0.45–0.95	-	0.30–0.95	-	-	<0.7	<1.5	0.57–1.33	-	-	1.16 ± 0.68
San-Miguel	0.37–0.80	-	0.80	-	-	1.5	<1.5	0.33–0.59	-	-	0.93 ± 0.05
Vilaflor	0.40–1.12	-	0.30	-	-	<0.7	<1.5	0.21–0.28	-	-	0.57 ± 0.14
Arona	0.56–0.90	-	0.35	-	-	<0.7	<1.5	0.19–0.48	-	-	0.11 ± 0.08
Adeje	0.45–2.00	-	1.60	-	-	<0.7	>4	2.59–4.89	-	-	0.27 ± 0.40
Guia-de-Isora	0.45–1.15	-	0.30	-	-	1.5	<1.5	0.57–1.41	-	-	-
Santiago-del-Teide	0.45–0.91	-	0.25	-	-	1.5	<1.5	0.36–0.39	-	-	3.03 ± 0.07
Fasnia	-	-	0.12–0.95	-	-	1.5	<1.5	1.22–1.31	-	-	2.60 ± 0.05

^a [76]; ^b [81]; ^c [82]; ^d [79]; ^e [80,83]; ^f [84] The fluoride concentrations are given by intervals according to the sanitary levels of consumption recommendation in force at the time of the study; ^g [85]; ^h [37]; ⁱ [86]; ^j [87].

). Table 3 shows fluoride concentrations in different galleries in the municipality of La Guancha from 1976 to 1997. In the 1990s, most of these galleries ceased to be used due to loss of flow, leaving the Barranco de Vergara gallery as the main one, with a current flow of 1056 m³/h and three conduits that try to supply a large part of the island [72,77,78]. Table 4 summarizes the fluoride levels detected in various studies carried out in the island’s water supply from 1984 to the present.

While fluoride concentrations in the water supply remain high, some areas experience increasing levels. Fluctuating concentrations are observed, with some areas showing decreasing trends and eventual stabilization. This may be due to the fact that, since 1995, different water desalination plants (WDPs) have been implemented in the areas with galleries affected by high fluoride concentrations, such as WDP Altos de Icod y WDP El Reventon en Icod de los Vinos, WDP Cruz de Tarifes en la Guancha, WDP de Tamaimo en Santiago del Teide y WDP de Aripe en Guía de Isora. All of them are based on the reverse electrodialysis defluorination methodology.

In addition, the hydrological plans of Tenerife developed by the Consejo Insular de Aguas de Tenerife, in compliance with the European Water Framework Directive (Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000) [88], have attempted, since 2009, to implement new WWTPs in the different territorial areas for the improvement of water quality, which has not yet been carried out [72,77,78].

This study addresses a critical public health need, as a comprehensive assessment of fluoride levels across all municipalities on the island has not been conducted since 2009. Furthermore, based on the evidence presented, Tenerife remains an area of endemic fluorosis. Despite the development of hydrological plans, insufficient measures have been taken to address the problems derived from the presence of fluoride in the water supply, with the situation of food insecurity being maintained to this day.

A potential limitation of the study is that, while the sampling district was chosen to include a variety of concentrations, collecting data from only two consumption points may not provide a fully representative view of the municipality.

Exposure Assessment and Risk Characterization

In the population group of infants from 1 to 3 years old, with a water consumption of 1.5 L/day, the ULs are exceeded in the following 21 municipalities: Santa Cruz de Tenerife, La Laguna, Tegueste, El Sauzal, La Matanza, La Victoria, Santa Úrsula, La Orotava, Puerto de Cruz, Los Realejos, San Juan de la Rambla, La Guancha, Icod, Santa Úrsula, la Orotava, Puerto de la Cruz, Los Realejos, San Juan de la Rambla, la Guancha, Icod de los Vinos, Garachico, el Tanque, los Silos, Buenavista, Fasnia, Granadilla de Abona and Vilaflor and Guía de Isora (

Table 5. Fluoride EDI and percentage contribution to ULs from public water supply in the different municipalities on the island of Tenerife for the age ranges 1–3 years, 4–8 years, 10–14 years and >18 years.

Municipality	EDI (mg/L)			Contribution UL 1–3 Years (%)		Contribution UL 4–8 Years (%)		Contribution UL 10–14 Years (%)		Contribution UL > 18 Years (%)
	1 L	1.5 L	2 L	1 L	1.5 L	1 L	1.5 L	1.5 L	2 L	1.5 L	2 L
Santa Cruz de Tenerife	2.08	3.12	4.16	138.6	207.9	83.2	124.8	62.4	83.2	44.6	59.4
La Laguna	1.64	2.46	3.28	109.2	163.8	65.5	98.3	49.2	65.5	35.1	46.8
El Rosario	0.89	1.33	1.77	59.1	88.6	35.5	53.2	26.6	35.5	19.0	25.3
Tegueste	4.37	6.55	8.73	291.2	436.7	174.7	262.0	131.0	174.7	93.6	124.8
Tacoronte	0.47	0.71	0.95	31.6	47.5	19.0	28.5	14.2	19.0	10.2	13.6
El Sauzal	1.59	2.39	3.18	106.1	159.1	63.6	95.4	47.7	63.6	34.1	45.5
La Matanza de Acentejo	4.13	6.20	8.27	275.6	413.3	165.3	248.0	124.0	165.3	88.6	118.1
La Victoria de Acentejo	3.63	5.45	7.26	242.2	363.2	145.3	217.9	109.0	145.3	77.8	103.8
Santa Úrsula	2.00	3.00	4.00	133.4	200.1	80.0	120.1	60.0	80.0	42.9	57.2
La Orotava	1.28	1.92	2.56	85.3	128.0	51.2	76.8	38.4	51.2	27.4	36.6
Puerto de la cruz	1.23	1.85	2.46	82.1	123.2	49.3	73.9	37.0	49.3	26.4	35.2
Los Realejos	1.49	2.23	2.98	99.2	148.8	59.5	89.3	44.6	59.5	31.9	42.5
San Juan de la Rambla	2.48	3.72	4.96	165.3	248.0	99.2	148.8	74.4	99.2	53.1	70.9
La Guancha	3.32	4.98	6.64	221.3	331.9	132.8	199.1	99.6	132.8	71.1	94.8
Icod de los Vinos	5.28	7.92	10.56	352.0	528.0	211.2	316.8	158.4	211.2	113.1	150.9
Garachico	3.75	5.63	7.51	250.2	375.3	150.1	225.2	112.6	150.1	80.4	107.2
El Tanque	10.61	15.92	21.22	707.4	1061.1	424.5	636.7	318.3	424.5	227.4	303.2
Los Silos	3.29	4.94	6.58	219.4	329.0	131.6	197.4	98.7	131.6	70.5	94.0
Buenavista	2.25	3.38	4.51	150.2	225.3	90.1	135.2	67.6	90.1	48.3	64.4
Candelaria	0.94	1.41	1.88	62.6	93.8	37.5	56.3	28.2	37.5	20.1	26.8
Arafo	0.94	1.40	1.87	62.4	93.5	37.4	56.1	28.1	37.4	20.0	26.7
Güimar	1.20	1.79	2.39	79.7	119.6	47.8	71.7	35.9	47.8	25.6	34.2
Fasnia	2.26	3.38	4.51	150.4	225.7	90.3	135.4	67.7	90.3	48.4	64.5
Arico	1.08	1.63	2.17	72.3	108.4	43.4	65.0	32.5	43.4	23.2	31.0
Granadilla de Abona	1.22	1.84	2.45	81.6	122.4	49.0	73.5	36.7	49.0	26.2	35.0
San Miguel de Abona	1.02	1.53	2.04	68.0	102.1	40.8	61.2	30.6	40.8	21.9	29.2
Vilaflor	2.32	3.49	4.65	154.9	232.4	92.9	139.4	69.7	92.9	49.8	66.4
Arona	0.45	0.68	0.91	30.3	45.5	18.2	27.3	13.6	18.2	9.7	13.0
Adeje	0.37	0.55	0.73	24.5	36.7	14.7	22.0	11.0	14.7	7.9	10.5
Guía de Isora	1.62	2.43	3.24	108.1	162.2	64.9	97.3	48.7	64.9	34.8	46.3
Santiago del Teide	1.16	1.74	2.32	77.3	116.0	46.4	69.6	34.8	46.4	24.9	33.1

).

In the group of infants aged 4 to 8 years (consuming 1.5 L/day), the ULs are exceeded in 13 municipalities; Santa Cruz de Tenerife, Tegueste, La Matanza, la Victoria, San Juan de la Rambla, la Guancha, Icod de los Vinos, Garachico, el Tanque, los Silos, Buenavista and Fasnia and Vilaflor. Next, in infants aged 10 to 14 years (consuming 2 L/day), the ULs are exceeded in eight municipalities; Tegueste, La Matanza, la Victoria, la Guancha, Icod de los Vinos, Garachico, El Tanque and los Silos (Table 5). In the case of the adult population (over 18 years of age) consuming 2 L/day of water, the ULs are exceeded in only three municipalities: Tegueste, Icod de los Vinos and el Tanque (Table 5).

It is crucial to note that this risk characterization considered fluoride intake from water alone and not the total dietary exposure. Therefore, although water is the main dietary fluoride source, exceeding 40–50% of the UL is a concern as it may indicate a risk of overexposure to this anion.

Therefore, the population aged 1 to 3 years in the established consumption scenarios could be at risk of overexposure to fluoride, regardless of the municipality under study, if overall dietary exposure is taken into account. This risk also applies to the population aged from 4 to 14 years, except for the municipalities of Tacoronte, El Rosario, Santiago del Teide, Adeje and Arona. For adults, there may be a risk of overexposure in La Victoria de Acentejo, San Juan de la Rambla, La Guancha, Garachico and Los Silos in addition to the municipalities exceeding the tolerable upper intake level established by the EFSA.

Therefore, it would be recommended, always under the indications of the health authority of the region, that the population that presents a possible risk of overexposure considers limiting tap water consumption and exploring alternatives such as low-mineralization bottled water [89]. Mainly, the populations that proved to be the most susceptible to the effects of this anion were those aged from 1 to 14 years of age and the population from the northern region of the Island, where the fluoride concentrations are the highest.

4. Conclusions

The high concentration of fluoride in the water supply from certain underground sources on the island of Tenerife has been documented since the mid-1950s. As a result of this exposure, the population of certain municipalities, such as La Guancha, has suffered from “mottled tooth”, and this has led to a series of measures and restrictions that are still under consistent development.

About 55% of the municipalities on the island of Tenerife exceed the parametric value for fluoride concentration in the supply water of 1.5 mg/L, most of them being municipalities in the northern and metropolitan regions. The municipalities with the highest fluoride concentrations in Tenerife are El Tanque (10.61 mg/L), Icod de los Vinos (5.28 mg/L), Tegueste (4.37 mg/L) and La Victoria de Acentejo (3.63 mg/L). The lowest concentrations correspond to Adeje (0.37 mg/L) and Arona (0.45 mg/L).

A possible risk of fluoride overexposure through the consumption of supply water has been detected for children aged from 1 to 3 years who consume 1 and 1.5 L per day, regardless of the municipality studied. This risk also exists for the population aged between 4 and 14 years old, with the exception of some municipalities, and also for the adult population in most municipalities.

Therefore, it would be recommended that the population that presents a possible risk of overexposure should limit their consumption of tap water, using alternatives such as bottled water with low mineralization levels.

With this study, we urge that the local government and the study population become aware that the situation is worsening and that they take appropriate measures to remedy the issues. Additionally, we propose the systematic implementation of similar studies to ensure the ongoing monitoring of food safety related to water consumption on the island of Tenerife, an area with endemic fluorosis.