- What are the neurological or cognitive effects of community water fluoridation, compared with non-fluoridated or different fluoride levels in drinking water, in individuals less than 18 years of age?
This review included one prospective cohort study and two cross-sectional studies examining the effect of fluoride exposure on intelligence quotient (IQ) and attention deficit hyperactivity disorder (ADHD) diagnosis in children. These studies are of low quality due to high risk of bias and multiple limitations.
The prospective cohort study conducted in Canada examined fluoride exposure (determined by fluoridation status and fluoride intake from formula) over the first six months of feeding and IQ in children aged 3 to 4 years. It was found that an increase of 0.5 mg/L of tap water fluoride levels was associated with 9.3 points and 6.2 Performance IQ points decrease in formula-fed group and breastfed group, respectively. An increase in 0.5 mg fluoride intake from formula per day was associated with a statistically significant decrease in 8.76 points of Performance IQ.
The cross-sectional study conducted in China examined the association between fluoride exposure (water fluoride levels from 0.2 to 3.9 mg/L in the endemic and non-endemic areas) and IQ in children aged 7 to 13 years in relation to thyroid hormones. At water fluoride levels of 0.7 to 1.0 ppm, the study found no significant difference in IQ scores or thyroid hormone levels compared to water fluoride levels of less than 0.7 mg/L. A regression analysis including all water fluoride levels showed a statistically significant, but rather small decrease in IQ scores; 1.6 points for every 1 mg/L water fluoride, or 1.2 points for every 1 mg/L urinary fluoride.
The cross-sectional study conducted in Canada examined the association between fluoride exposure (determined by urinary fluoride, city fluoridation status and tap water fluoride levels) and ADHD in children aged 6 to 17 years. The results showed no significant association between urinary fluoride and ADHD diagnosis, or between urinary fluoride and the Strengths and Difficulties Questionnaire Hyperactivity Inattention (SDQ H/I) subscale score. Water fluoride levels and CWF status were positively associated with ADHD diagnosis and SDQ H/I subscale score. Every increase of 1 mg/L of tap water fluoride was associated with a 6.1 times higher odds of ADHD diagnosis and an increase of 0.31 points in the SDQ H/I subscale score. Similarly, children living in the fluoridated areas had 1.5 times higher odds of ADHD diagnosis and an increase of 0.11 points in the SDQ H/I subscale score. Children of older age (i.e., 14 years and over) were more likely to be affected compared to those of younger age (i.e., 9 years and under).
Considering multiple limitations of the included studies (e.g., insufficient control of confounding factors, potential misclassification of exposure, and inadequate study design), it is difficult to interpret their findings and generalize them the Canadian context. Collective evidence from the recent CADTH reports and the current review indicates there is insufficient evidence to conclusively conclude that fluoride exposure at the Canadian water fluoride levels (optimum at 0.7 mg/L) affects neurological development in children and adolescents in Canada.