Although the quantities of lead (Pb) to which individuals are exposed vary widely, susceptibility of an individual to the effects of a specific level of exposure is another highly important factor in development of lead toxicity. For example, susceptibility to lead toxicity can be modified by several dietary factors. Low dietary intakes of calcium or iron (20% of recommended levels) substantially increase the toxicity of the same level of lead exposure to rats. In the studies of calcium effect, when calcium was fed to rats at ⅕ of the recommended intake, 12 μg Pb/ml drinking water produced the same degree of toxicity as did 200 μg Pb/ml with a normal calcium diet. The maximal dose for a 10-week period that does not impair heme synthesis or renal function in the rat has been established to be 200 μg Pb/ml drinking water. The role of low calcium diet on increasing susceptibility to lead has been confirmed in several species.