A cross-sectional study to assess the intelligence quotient (IQ) of school going children aged 10-12 years in villages of Mysore district, India with different fluoride levels.

Results

Four hundred and five 10-12-year-old children took part in this study.Dental examination revealed that 74% children in high-fluoride group demonstrated mild to severe fluorosis. In comparison, only 20% of the subjects in the normal and 8% subjects in low fluoride groups showed very mild to severe fluorosis and the remaining were unaffected [Table 1]. An increase in water fluoride content above the standard level was associated with the incidence of more severe dental fluorosis (P < 0.001).

IQ evaluation results from the three groups are shown in Table 2. Although IQ scores for children with normal and low fluoride content were significantly higher than high fluoride level (P < 0.01), there was no statistically significant difference between the IQ of children residing in normal and low fluoridated areas (P > 0.05). Furthermore, we did not observe any significant difference between the scores of girls and boys in any of the groups examined (P > 0.05). IQ scores of children living in areas with various water fluoride content (normal, medium, and high) have been demonstrated in Table 3, exhibiting that more children residing in the normal and low fluoride area showed normal IQ levels.

Binary regression model was used to adjust the relationship between IQ score and the other potential confounders in the study. The dependent variable was IQ score. The independent variables included were age, gender, parental education, family income, and water fluoride level. After adjusting the variables, it was found that the IQ scores were increasing as the education level of parent increases. However, this association was not found to be significant. Significant association was found between the IQ score and water fluoride level when the variables like age, gender, parental education, and family income were controlled [Table 4].

Discussion

Here we demonstrated that the average IQ of children living in the area with high fluoride content in the drinking water was significantly lower than children with normal and low water fluoride levels. Since all potentially confounding factors were adjusted, the difference in IQ scores may reveal the potential effect of high fluoride exposure on the intellectual development of children. These results are consistent with findings reported by Xiang et al., who confirmed a decrease in children’s neurobehavioral ability when exposed to elevated fluoride levels.[⁹]

Furthermore, the current study compared three fluoride levels to evaluate the effect of different concentrations of fluoride on children’s IQ and dental fluorosis. In addition, our results demonstrated a higher percentage of children with above the normal IQ range in the normal and low fluoride group compared to high fluoride group. This was in line with the findings of Trivedi et al., who reported that an elevated fluoride level would affect the higher levels of intelligence more vigorously than normal and low intelligence levels.[⁴]

Possible mechanisms for the neurotoxic effect of fluoride may be explained by several animal studies.[^10,11,12,13] Fluoride can pass through the placenta by maternal exposure to elevated fluoride levels during the prenatal period or it may be ingested through the child’s diet. High levels of absorbed fluoride in children (80-90%) and adults (60%) are retained in the body.[^7,14] Once absorbed in the blood, fluoride forms lipid-soluble complexes which cross the blood-brain barrier and accumulate in cerebral tissues.[^7,10,15] The penetrated fluoride complexes adversely affect the CNS development by different neurotoxic and exciotoxic mechanisms, such as free radical generation, inhibition of antioxidant and mitochondrial energy enzymes, and inhibition of glutamate transporters.[¹⁶]

Our findings showed that the prevalence and severity of dental fluorosis was greater among children with high water fluoride content. As a probable result of “the halo effect”, we found a 20% prevalence of fluorosis in children living in the village with a standard fluoride level, which was in accordance with WHO guidelines explaining that at an optimal fluoride level (1 ppm), about 20% of the population demonstrate fluorosis.[^7,17] However, in village with a higher than standard fluoride level, 83% children demonstrated dental fluorosis with a severity index of very mild to severe.

In this study, we administrated RCPM test to evaluate the child’s intellectual development, a validated test for basic cognitive abilities and widely used to evaluate the normal development of brain functions. It consists of problems containing a matrix of geometrical design, with a part removed. The child has to select the missing cell from six given alternatives. Owing to the high correlation of this test in evaluating children’s IQ (ranging from 0.7 to 0.92) compared to the conventional IQ test, RCPM is recommended for measuring the intellectual and conceptual ability of young children. Furthermore, due to the nonverbal characteristics of the test, it is successfully administrated in preschool children, as it does not require any verbalization or reading skills.[^18,19]

As it is well accepted, the neurobehavioral development may be influenced by many genetical, socioeconomical, and geographical factors. Thus, we have recruited our samples from a homogenous population in Mysore district, diminishing the effect of some environmental and inherited factors, yet it is obvious that complete exclusion of such factors is impossible. In the present study, the education level of the parent appears to play a definite role. Yongxiang et al., discovered that the IQs of children born into an “employed” household was higher than those born into a farming household, and that the IQ levels of the children increased along with the education level of their parents.[²⁰] This indicates that a positive educational influence from the family is a benefit to child’s intellectual development. Therefore, strict uniformity requirements must be enforced when selecting an area for study, determining the test subjects, etc. The present study paid special attention to this aspect; and the occupation and education levels were basically matched in all the three villages.

In a previous study, the urinary fluoride level was implemented as the basic indicator for the child’s fluoride exposure.[²¹] However, since urinary fluoride excretion may vary from one subject to the other or even in the same person, and because drinking water was typically the greatest single contributor to daily fluoride intake in this area; we have evaluated the effect of different amounts of fluoride in drinking water. In the present study, the effect of fluoride concentration on the child’s IQ was assessed; however, it is possible that other trace elements in drinking water may have some neurological side effects. Thus, further studies are required to investigate the effect of other environmental or geological contaminants.

Srikanth, in 2009, pointed out that in India about 62 million people are suffering from various levels of fluorosis, of which 6 million are children below the age of 14 years.It should not come as a surprise that excess fluoride is one of the three major hurdles according to tenth 5-year plan in India.[²²] Although the biomechanism of fluoride in reducing IQ is not clear;but on the basis of the data from this study, it is evident that excess fluoride in drinking water has neurological toxic effects. Therefore, a close monitoring of fluoride levels in local watersupplies from areas with endemic fluorosis and implementing public health measures to reduce the fluoride exposure levels in high fluoridated regions seem necessary.

Conclusion

The IQ scores of children living in areas with above the standard water fluoride levels were lower compared to children living in normal and low fluoride level regions. In the high fluoride village, the proportion of children with IQ below 90, that is, below average IQ was larger compared to normal and low fluoride village. Age, gender, parent’s educational level, and family income had no significant association with the IQ scores.