Following earlier work on the relevance of silica (\left(\mathrm{SiO}{2}\right)) and fluoride ((\mathrm{F})) to the biotoxicity of aluminum (Al), it is shown that relatively high risks of a measure of mental impairment are frequently associated with relatively low (\mathrm{SiO}{2}) and (\mathrm{F}) in the drinking water available in different parts of Ontario. Also, because of a negative association between (\mathrm{SiO}{2}) and (\mathrm{F}) concentrations in the treated water available in different municipalities, a low (\mathrm{SiO}{2}) concentration can account for the observed association involving high (\mathrm{F}) concentrations, and low (\mathrm{F}) concentrations can similarly account for associations involving high (\mathrm{SiO}_{2}) concentrations.
In the light of these observations, the biotoxic effects at low (\mathrm{Al}) concentrations can be explained by assuming that interactions occur with membranes. Such interactions may be less likely at higher (\mathrm{SiO}{2}) and (\mathrm{F}) concentrations, because at these concentrations, interactions between (\mathrm{Al}-, \mathrm{SiO}{2}) - and (\mathrm{F})-containing species are more important. Generally, the various associations with a measure of mental impairment can be explained on the assumption that (\mathrm{F}) exerts a protective effect against the biotoxic effects of (\mathrm{Al}), as does (\mathrm{SiO}{2}) under certain conditions, but the effects of (\mathrm{SiO}{2}) only become important at higher (\mathrm{SiO}_{2}) concentrations.
The results from this and other studies suggest that the neurotoxic effects of Al depend on other water constituents. The results also indicate that absorption through the lung or skin, rather than through the stomach or the bowel, can better explain the reported associations.