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Abstract

Abstract

Water fluoride levels above the World Health Organization’s guideline (1.5 mg/L), common in overexploited aquifers, represent a health hazard. Our objective was to assess the health risks posed by exposure to fluoride in different drinking water sources in a contaminated basin in Mexico. Fluoride was measured in mutual drinking water sources and in the urine of 39 children and women. Risks were estimated through hazard quotient (HQ) by drinking water source. Dental fluorosis was assessed in the children. Mean fluoride water concentrations (mg/L) were: well, 4.2; waterhole, 2.7; bottled, 2.1; rainwater, 0.4. The mean urinary fluoride concentrations (specific gravity adjusted) were 2.1 mg/L and 3.2 mg/L in children and women, respectively. Our multiple linear regression model showed children’s urinary fluoride concentrations increased 0.96 mg/L for every 1 mg/L increase in water fluoride (p < 0.001). Dental fluorosis was diagnosed in 82% of the children, and their HQ according to drinking water source was: well, 1.5; waterhole, 1.1; bottled, 0.8; harvested rainwater, 0.3. The pervasive dental fluorosis indicates a toxic past fluoride exposure; urinary fluoride levels and HQs indicate high exposure and current health risks for most children. Drinking harvested rainwater will likely prevent most of the local fluoride exposure.

*Full-text study online at https://www.mdpi.com/1660-4601/18/21/11490/htm

Figure 1

Excerpt:

4. Discussion

This study’s different results suggest past, present, and potentially future exposure to F– levels above the international drinking water guidelines, and that consequently represents a health risk for children and women whose drinking water source is the La Onza well or wells with similar F- concentrations in the CARL. Since convenience sampling is non-probabilistic, it cannot pretend to be representative of the whole population. However, we have no reasons to believe (although we have no evidence) those who were recruited in this very homogenous population were different in terms of drinking water volume or body composition from those who were not recruited.
A dental fluorosis prevalence above 80% in the 6–14-year-old children from CARL is an unequivocal sign of a pervasive early life and/or prenatal (past) F– exposure; that it is also a sign of toxicity is debatable, since some argue that dental fluorosis may be a cosmetic rather than a toxic effect. However, IQ reductions were seen in a meta-analysis of water F– exposure even at levels = 1 mg/L [37], which corresponds to the NOAEL for objectionable dental fluorosis (from mild to severe) [35] seen in 37% of this study’s children. Moreover, IQ reductions were seen in children prenatally exposed to low F– levels: an increase of 0.5 mg/L in maternal urinary F- predicted a lower IQ by 2.5 points in the offspring (95% CI -4.12, -0.59); mean maternal pregnancy urinary F– levels in those 299 pregnant women from Mexico (0.90 mg/L) were three times below what we found in this study [15]. If the previous association holds true for our population (assuming the women had similar urinary F– levels when they were pregnant = 3.1 mg/L), we could expect children from our study area now to have a 10-point reduction (2.4 to 16.5) from their potential IQ.
Additionally, a study in China of children of similar age (7–13 years old) to ours found that in children with urinary F- concentrations > 1.7 mg/L, every increase in 1 mg/L above that level was associated to a reduction of almost five potential IQ points (95% CI: -9.198, -0.732, p = 0.022); since our children’s mean urinary F– concentrations were >1.7 mg/L, we could probably expect similar results. Children in the aforementioned study who were prenatally exposed fared worse than those exposed after birth, but no increased effect was seen in children exposed both prenatally and afterwards [38] Since 95 % of the women and 100% of the children in our study had lived all their lives in the study area, we can assume a prenatal plus a continuous postnatal exposure to F–.
Even more so, a quantitative risk analysis aimed at finding a safe daily F– dose for children found that to protect against a five-point IQ loss, this dose would possibly be 0.045 mg F–/day; children in our study drinking a mean volume of approximately 1 L of water/day with a F– concentration of 4.2 mg/L, if it comes from the local well, are ingesting more than eight times this F– dose [39].
Given our result that for every 1 mg/L increase in water F–, the urinary concentration of the studied children increases by 0.96 mg/L, it is probably safe to assume that the source of drinking water plays a major role in determining these children’s exposure to F– and its potential health effects. In a cohort of Canadian children, water F– concentrations (mean = 0.35 mg /L) were significantly associated to urinary F– concentrations in a model controlling for age and sex, just as we carried out, but the magnitude of the association was much lower (B = 0.44, 95% CI: 0.30, 0.59, p < 0.001) [40].
Harvested rainwater was the only water source below the World Health Organization’s F– drinking water guideline, but it was not fluoride-free. Measurements of rainwater F– concentrations can vary considerably in one place depending on weather conditions or in different places depending on local sources. In Nigeria, rainwater F– concentrations were 0.02 mg/L during the southeast monsoon and 0.1 mg/L during the northeast monsoon [41]. The existence of nearby sources of airborne fluoride emissions has been seen to increase the rainwater F– concentrations: in Vanuatu, a volcano produced F– rainwater concentrations ranging from 0.7 to 9.5 mg/L [38], and in Wielkopolski National Park in Poland, located 10 km away from chemical plant that emits fluoride, F? rainwater concentrations ranged from 0.32 to 0.55 mg/L [41]. Our hypothesis is that in the CARL, a semi-arid, dusty, and windy area, groundwater containing high levels of F? that is used for irrigation will evaporate and might leave behind F– contaminated soil and dust; these dust particles may in turn contaminate rainwater while it falls or once it is harvested, stored, and gradually used for months during the dry season.
Even though we only measured one sample of bottled water, F– levels were similar to those found in bottled water samples from two other states of Mexico with endemic fluorosis, Durango and Jalisco (2.0 and 2.9 mg/L) [15] Bottled water F– levels from non-fluorosis endemic areas, such as Mexico City, have found inconsistent results, sometimes being below the Mexican norm [42] and sometimes having samples above the norm. It is imperative to have representative and sufficient F– level measurements in bottled water in the CARL, since it seems to be an inefficient measure in avoiding water F– exposure; the users are incurring a costly measure for a source that is still above the optimal F– concentrations.
The F– concentrations in the waterhole are consistent with what would be expected, since rainwater dilutes the underground water that fills it. However, it will be important to inform the population of its high F– contents, given that it perceives it as a very safe, although not easily accessible, drinking water option.
A present F– health risk to the CARL population that drinks well water with F– concentrations that were approximately three times the WHO’s F– drinking water guideline, especially for the children, is suggested by the HQs above 1, reflecting a mean exposure above the reference dose [43]. Additional evidence suggesting potential current F– health risks is the children’s exposure reflected in mean urinary F– concentrations, which for those who drink well water are ten-fold the urinary F– mean of the Canadian children who presented an association, albeit weaker than at younger ages, with performance IQ (B = -1.51, 95% CI: -2.90, -0.12) [44].
Apart from contaminated water, only soda seems to be another potentially significant source of fluoride intake in the CARL, where risk perception of contaminated drinking water has led to an increased consumption of soda, based on the belief that it is free from toxic elements such as F–, but this has proved erroneous, as previous studies in the state of Guanajuato [45] and Mexico [42,46] have found. It is paramount to carry out a quantitative F– assessment to inform the population in this regard, because drinking soda not only has higher F– levels than water but also implies other health risks.
Salt in Mexico is fluoridated in an attempt to prevent dental caries, but exceptions to the Mexican Official Norm mandating this are observed in places where water fluoride concentrations are naturally above 0.7 mg/L [47]; the whole state of Guanajuato is exempt from the distribution of fluoridated salt [48]. We did not apply a detailed food frequency questionnaire in this study, but we inquired about the weekly consumption of groups of foods and we were fed by community volunteers during our filed work; we can conclude that the studied population’s diet is not high in the main food group sources of F–, especially in Mexico, according to a study that included the most representative foods and beverages consumed at the national level: seafood, meats and poultry, or fast food [46].
Even though 82% of the children had a professional diagnosis of some level of dental fluorosis, this was only perceived in 25% of the children by their mothers or guardians. This is probably explained by the fact that in 45% of the cases, the children presented a very mild fluorosis (corresponding to opaque, paper-white areas involving less than one fourth of the tooth surface), which might be missed by non-experts or may not even stand out as something altered due to its high prevalence. On the other hand, the data suggest that awareness of dental health problems has probably led to a change in the drinking water source: 100% of those who now drink bottled water had dental fluorosis, whereas only 60% of those who drink well water had dental fluorosis.
Future F– health risks are to be expected in the CARL, since 15% of the families studied still drink well water. Besides, 66% of those who still drink well water give it no treatment to reduce F–. The investigated water treatment methods do not necessarily reduce water F–, but the fact that they use none probably reflects a lack of risk perception or information. Finally, groundwater FF– levels are expected to increase because of deeper drilling of wells that reach older water that is richer in F–, more urban pumping that upwells mineralized groundwater, and less rainfall recharge along with more diverse and F– contaminated recharge waters [3].
Even though the groundwater geogenic contamination problem has been studied from the hydrogeology [2,3], regulatory [49], and dental [20] perspective in the CARL, the human exposure and health risk magnitude have not been investigated in-depth. With this study, we were able to produce important diagnostic results that reduce the assumptions and increase the precision of the exposure assessment. For instance, the CARL being a semi-arid region, 6–14-year-old local children reportedly drink approximately three times more water than the mean assumed by the EPA for their age group. Still, this study was limited by the small sample size and the few drinking water samples; future studies will benefit from increasing their numbers and including follow-up measurements.

5. Conclusions

This is the first study to analyze the association between F– concentrations in different drinking water sources and urinary F– concentrations in children and women in the CARL. We observed a strong association in magnitude and significance of both the water F- levels prediction of urinary F- levels in children (? = 0.96 and p value < 0.001) and of the difference in urinary F- levels depending on drinking water source in women and children (p value < 0.05). Rain-harvested water proved the only source below the F– WHO drinking water guideline, and children who reportedly drank this water were the only ones with mean urinary F– levels (1.3 mg/L) below those associated with neurotoxicity in children exposed in different windows of early life (1.7 mg/L). It is also the first study to identify a non-probabilistic health risk of the population of women, and especially children, associated with well water F– concentrations, since the HQs of this source (1.25 and 1.5, respectively) indicate an exposure above the reference dose.
Even though our sample size was small and recruited by convenience (not guaranteeing representativeness), the results of this risk assessment are especially relevant to other CARL populations. On the one hand, we might be able to extrapolate the exposure dose and health risks to other inhabitants who share sociodemographic characteristics and drink water from the several existing wells with similar or higher F– concentrations than the well we studied. On the other hand, our results are important given a possible increase of aquifer F– concentrations in the future, as the trends suggest in some local wells, due to increased overexploitation in response to population growth and agricultural water demand expansion.
In spite of our study’s small population sample and number of sampled water sources, which will need to be built upon in future studies to decrease uncertainty, our results strongly suggest that exposure to toxic drinking water F– levels is preventable in the CARL by switching to harvested rainwater. Future studies could complement these results by quantitatively assessing F– exposure through diet, especially considering soda and confirming that local salt is indeed not fluoridated.

Funding

This research received no external funding.

Institutional Review Board Statement

The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of the National Institute of Public Health, Mexico (protocol code CT1P01, date of approval: 31 May 2018).

Informed Consent Statement

Informed assent was obtained from all the children, and informed consent was obtained from all adult subjects involved in the study.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy issues.

Acknowledgments

We are very grateful to the study participants, San Cayetano Parish’s staff and its priest, Father Juan Carlos Zesati, Caminos de Agua A.C. and its director, Dylan Terrell, and professional volunteers Priscila Rodríguez, S Ayala, and Roberto Alcántara-Rodríguez for their invaluable collaboration in making this study possible.

Conflicts of Interest

The authors declare no conflict of interest.

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