Fluoride and its effect on human intelligence. A systematic review

Objectives: A systematic review was undertaken to examine if fluoride (F^-) exposure is associated with a decline in human intelligence (IQ).

Methods: Ovid MEDLINE and its allied versions, CINAHL, AMED, EMBASE, Cochrane DSR, ACP Journal Club, DARE, CCTR, CMR, HTA, and NHSEED, Health and Psychosocial Instruments, HealthSTAR, International Pharmaceutical Abstracts were searched from the earliest record until January 2008. Only original human studies examining the effect of F^- on IQ were selected. Elevated F^- in drinking water was the primary variable but studies where F^- was elevated in the urine as a result of pollution were also included. Hand-searching of the bibliographies of the selected studies, as well as a separate search strategy in Cab Direct and online Chinese databases (Chinese version of Google Scholar and several others) were performed and the relevant studies were translated into English.

Results: Of a total of 224 papers searched for relevancy from their titles, abstracts and full copy, 20 original studies met our inclusion criteria, were read in full and critiqued. Only 6 of the studies were reviewed by the 2006 US NRC Subcommittee on Fluoride in Drinking Water and 4 studies were published after 2006. The extent of F^- exposure was reported in all but one study and 9 studies reported urinary F^-. Most papers omitted important details (blinding, confounders, randomization). All but 2 reported statistically significant (t-tests) declines in IQ in children exposed to elevated levels of F^-. Only 1 study reported a bivariate analysis and another a multiregression analysis. Three studies were conducted outside of China, suggesting that this is not just a relationship that is limited to one country.

Conclusions: While the evidence is not conclusive, we identified 20 ecological studies that purport an association between high fluoride exposure and decreased human intelligence (IQ).