We studied the relation between silica and aluminum levels in drinking
water and the risk of cognitive impairment using data from a populati
on-based survey of 3,777 French subjects age 65 years and older. We al
so studied the effect of pH and the concentrations of calcium, magnesi
um, fluorine, zinc, copper, and iron. We used a mixed effects logistic
regression adjusting for age, sex, educational level, and occupation
of the subjects. We confirmed the inverse relation previously found be
tween calcium level and cognitive impairment. We found no important as
sociation between cognitive impairment and fluorine, magnesium, iron,
copper, or:inc. The association between cognitive impairment and alumi
num depended on the pH and the concentration of silica: high levels of
aluminum appeared to have a deleterious effect when the silica concen
tration was low, but there was a protective effect when the pH and the
silica level were high. The threshold for an aluminum effect, however
, was very low (3.5 mu g per liter) and did not support the hypothesis
of a deleterious effect for only high levels of aluminum.